All language subtitles for Harvard CS50 (2023) – Full Computer Science University Course - YouTube - English

this is a Harvard University course

taught by Dr David Meen and we are proud

to bring it to the freeo camp YouTube

channel throughout a series of lectures

Dr men will teach you how to think

algorithmically and solve problems

efficiently and make sure to check the

description for a lot of extra resources

that go along with the

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course

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all right this is cs50 Harvard

University's introduction to the

intellectual Enterprises of computer

science and the Arts of programming my

name is David man and I actually took

this course myself back in 1996 I was a

sophomore at the time I was actually

concentrating in government because a

year prior as a first year I'd come into

Harvard thinking that I liked high stre

and constitutional law and sort of

similar classes in high school and so

when I got here I rather gravitated

toward that which was familiar I figured

if I liked and if I were good at that

particular subject in high school then

that's presumably who I'm supposed to be

here but it wasn't until sophomore year

that I got up the nerve to step foot in

the cs50 classroom and even then it was

only out of curiosity like I had no

intention of studying computer science

of even taking cs50 when I got to campus

but people were talking about it and

there was a lot of like beware and it

was it perhaps for the initiated only

and I didn't really know ultimately what

computer science was but for me the sort

of light bulb went off and I found that

contrary to what I had seen in high

school where I saw friends of mine like

programming away in the computer lab

heads down sort of anti-socially just

doing whatever it was they were doing it

really wasn't that once I got to this

particular class in this particular

place it was much more about problem

solving more generally and just learning

how to express yourself in code in

different languages so that you can

actually solve problems of interest to

you even if you have no intention of

being a computer scientist or an

engineer but just want to be able to

solve problems analyze data do

interesting things in the Arts Humanity

social sciences physical sciences or

really any other field and indeed this

particular path led me to computer

science but the hope for cs50 more

generally is that indeed you just find

your way to applying principles that

you'll learn over the coming months to

whatever field is of interest to you

with that said it was definitely a lot

of work and not without its frustrations

for me but there was no better feel

then like banging your head proverbial

against the wall for some number of

hours even days trying to fix a bug a

mistake in your code and then oh my God

the rush of emotion of accomplishment of

pride of exhaustion when you finally

solve some problem that's really been

weighing on you it's just so incredibly

gratifying but also empowering because

unlike a lot of fields the computer

science was built by humans themselves

and so if a human built this surely you

another human can understand it as well

and so even though there's going to be

some distractions along the way you're

going going to see what looks incredibly

cryptic if you've never programmed

before over time and with practice

everything just starts to make more

sense and with time and with practice

you just get better at this particular

field and indeed really the key to

success in programming in general is

just to allow yourself enough time and

so at least thankfully I quickly got

into the habit of starting early in the

week for instance when writing actual

code why because you're going to like

run up against a wall you're not going

to see some bug something's not going to

jump out at you and that's fine that's

when you sort of call it a day take a

break move move on to something else and

then just come back to it and that's

what keeps programming fund for me even

all of these years later whether it's

teaching or actually applying it but

there's uh down the road a history of uh

an MIT hack and it looked a little

something like this in yester year and

there was a little sign the MIT students

when they made this hack uh on the wall

that says getting an education from MIT

is like drinking from a fire hose which

indeed they have connected to what

should have been otherwise just a water

fountain and that's going to be what it

feels like sometimes not just in

computer science per se but just in UNAM

familiar field if you're not from stem

if you're not from CS that's fine but so

much of it ultimately is going to be

absorbed by you and going to be uh

within your grasp by term Zen so just

keep in mind that's that's very much the

intent but you'll be amazed what you're

able to create to accomplish uh just

three or so months hence indeed 2third

of you contrary to what you might think

or assume have never taken a CS class

before so it's absolutely not the case

that the person to the left and to the

right surely must know more than you

indeed it's quite the opposite and as

you'll see in the coming weeks as you

write your own code and solve your own

problems what ultimately matters in this

course is not so much where you end up

relative to your classmates but where

you end up relative to yourself when you

began and it really is all about that

Delta whether you've programmed or not

just getting something out of a class

like this and and if it does take time

and if you do feel those frustrations

but you simultaneously eventually feel

that sense of accomplishment that just

means it's all working and indeed

hopefully all the more worthwhile and

gratifying ultimately as a result so

what are we going to do in the coming

week so here we are in week zero we'll

soon see why computers and computer

scientists start counting if you will

from zero but week zero is which is one

in which we explore computational

thinking sort of thinking like a

computer and starting to clean up your

thought processes getting you to think

to solve problems more methodically and

then ultimately Translating that into

code and some of you might recognize

this environment here AKA scratch

coincidentally also from MIT you might

have used it in in grade school we'll

use it today and a little bit this

weekend in the course's first homework

assignment or problem set but not so

much to kind of play around in a way

that you might have if you did use it in

yester year but to explore ideas of

computer science and programming that

we're going to use and reuse every week

Hereafter as well uh thereafter we're

going to transition just next week to

week one so to speak whereby we'll

introduce you to a more traditional

language a lower level language an older

language called C and in C you're going

to use your keyboard not so much your

mouse and pointing and clicking but

you're going to write code that soon is

going to look a little something like

this and if you programmed before can

probably glean what this is going to do

if you've never programmed before which

is the case for most of you this too

will soon make sense but this is the

most canonical program that most any

programmer ever writes called hello

world and indeed that and all of the

surrounding syntax above and below just

that sentence hello world will soon make

all the more sense you'll learn how to

use industry standard tools so to speak

pictured here something called Visual

Studio code or vs code you'll use a

cloud-based version of it initially so

you don't have to suffer with any

technical difficulties or headaches like

that it'll just work right off the bat

but we'll use that tool and others

ultimately to then explore ideas in

computer science principles that you can

apply and we'll take a look underneath

the hood so to speak of your computer at

your memory or Ram Random Access Memory

where all of the data is ultimately

going to be stored we'll also take a

look thereafter at bugs a bug is a

mistake in a program here is an actual

bug in an actual computer in yester year

but we'll teach you how to debug

programs find your own mistakes find

others mistakes and improve that code as

well we'll transition then to algorithm

step-by-step instructions for solving

some problems which we'll touch on today

too and if you picture here this is

actually a pretty representative

problems odds are you haven't had to

deal with something like this but it's

representative sorting for instance if

you think of each of these small bars as

being a small number each of the bigger

bars as being a bigger number you might

wonder well how is you could do as a

human sort all of these bars like get

all the short bars over here all the big

bars over there well odds are if you're

like me you would probably kind of

eyeball it and if you could physically

interact you might just start grabbing

the smallest elements first put them

over on the left maybe grab the biggest

elements put them over on the right but

what's your algorithm there like how

would you teach someone younger than you

who's never done that before how to do

it how would you compel your Mac or PC

or phone to do something like that you

can't just kind of wave your hand and

say oh you know figure it out move

things around you have to express

yourself more methodically so we'll

translate even ideas like this into code

to and that's what the Googles and

others of the world are doing constantly

as they sort and organize the world's

information we'll use metaphors along

the way if it helps we'll talk about

your computer's memory is kind of being

like a postal address like every mailbox

in the world has some form of postal

address Street city state country and

the like and it turns out that's how

your Mac your PC and your phone also

work you've got a whole bunch of memory

like the picture before but you can

think of it really as individual

mailboxes and you can put anything you

want in those mailboxes and you can go

to a mailbox to grab information that

from it so at the end of the day that's

really all your computer's doing with

information it's just kind of organizing

it not into mailboxes per se but a term

you probably know called bites for

instance instead we'll talk about

problems that arise even nowadays in

fact most of you are familiar with your

Mac PC even phone like spontaneously

rebooting sometimes crashing the little

Annoying spinning beach ball or

Hourglass Icon that Happ like what is

with that well those are just bugs in

programs that humans at Apple and Google

and Microsoft and others they screwed up

and they wrote buggy code and your

computer when it encounters those

mistakes doesn't know what to do and so

nine times out of 10 so to speak it just

crashes or freezes or the like but that

kind of stuff will make more sense so

even the real world will make sense and

pictured here or some lower level terms

will eventually get to Mid semester but

generally speaking when something is

going this way as per this arrow and

something is going this way as per this

Arrow like that does not end well and

that often is what happens when your

computer crashes someone's using memory

up here but someone else is using memory

down here here and then they're not

really talking left hand and right hand

so that is just a high level overview of

some of the problems we'll encounter but

we'll focus to on data ultimately so

picture here is something fairly

technical called a hash table it's an

amalgam of something we're going to soon

call an array and also something we call

a linked list and these are just fancy

terms for describing how you can

organize information even more flexibly

than just putting individual ma values

in mailboxes like how could you build

structures like actual data structures

so to speak speak two-dimensional

structures at that and so what you're

seeing here is a glimpse uh as some of

you might have recognized of some Harry

Potter Universe names but they're

organized somewhat alphabetically and

notice that anytime there's multiple

people with a name that starts with H

like hermion Harry and Hagrid well they

can't all fit in that mailbox if each of

these squares along the left is that

same mailbox so you have to kind of

chain them together well you'll learn

how to do that in code so that even if

you get more data than you expect if

your business is booming and you're some

web-based business how do you keep

adding and adding information to your

software to actually keep up with it but

this again is what codee's going to soon

look like as soon as next week in week

one this here being C but we'll

transition in a few weeks to a more uh

modern higher level language so to speak

called python indeed the course Very

deliberately back in my day and now this

introduces you first to C which funny

enough most many people don't tend to

program and certainly every day I you

see generally September October November

December when teaching C is 50 itself

but it's everywhere nonetheless in fact

even today's other languages with which

you might be familiar like Python and

Java and uh yet others still you see

this same primitive language underneath

the hood because it's so darn fast and

as you'll learn over the coming weeks it

really gives you access to and an

understanding of what's going on

conceptually down here so that

thereafter after cs50 when you're

writing code you can sort of think at a

very high level what's actually going on

so in fact in just a few weeks what

looks like this C is going to look

instead like this in Python and you'll

better understand what's going on

underneath the hood and odds are after

this class you'll reach for a language

like python more frequently than C but

you're going to benefit from that bottom

up understanding thereof thereafter and

toward terms end we'll introduce you to

a few other ideas like where do you put

large amounts of data in things called

databases not things like spreadsheets

like here but actual databases where

using those same kinds of data

structures you lay things out in an

interesting way in memory thereafter

will transition to a very familiar

environment that you and I use every day

the web like the web has become rather

the user interface or UI that we use

everywhere on the uh laptops desktops

and even mobile devices nowadays well

pictured here is a language called HTML

it's not a programming language it's a

markup language and some of you might

have made homepages or portfolios in the

past but you'll understand what's going

on here but more powerfully you'll

understand how the computer sees that

same kind of code builds up a

hierarchical family tree type structure

in memory and then you can manipulate

that tree with code to actually add more

and more information chat messages

anything on the screen that you like and

finally we'll tie all of this together

by introducing what are called

Frameworks and libraries third-party

code that makes it a lot easier to solve

problems of interest to you and so in

particular here this is the very first

web app that I myself made back in like

1997 I was part of the first year

intramural sports program not as an

athlete but as the programmer and I was

teaching myself how to build web

applications I only knew C and maybe a

little bit of something else at the time

but this became for Harvard at least the

very first website for the first year uh

intramural sports program and it wasn't

just a static website with links and

images and the like it was interactive

you could register for sports we could

input exactly who is in a tournament

bracket or the leg and it could actually

automatically keep track of this data so

there two after just three months of a

class like this you'll go from writing

quite simply this week in next hello

world to building things like this for

whether it's web mobile uh or other

platforms as well if you so choose but

we'll get you off of the courses

infrastructure by the end of the term

you won't be using any toy environments

along the way will Empower you

ultimately to write code after cs50

especially if this is the only CS class

you ever take on your own Mac or PC

using the same software but not the

cloud-based version thereof but all of

this software is itself free and can be

used by you uh powerfully after the

course's own end but along the way as

you may know uh is there this tradition

within the class particularly in healthy

times of a number of events that really

brings people get together not just

collaboratively academically but to just

solve problems and generally uh engage

with each other as well coming up first

cs50 puzzle day which is meant to be not

jigsaw puzzles but logic puzzles that

require no prior experience with

computer science or programming but it's

just an opportunity to sort of quietly

work on a packet of puzzles with some

number of friends for for prizes and

more later the semester once you tackle

your final projects the Capstone of the

course where we don't give you a

homework to write you yourself come up

with something to build we'll get

together generally around 7 a uh 700

p.m. in the evening uh wrap up around 7

a.m. if you so choose and it's an

evening a 12-hour opportunity to

collaborate with classmates on your very

own final project in a large space on

campus uh that ends if you're awake with

us at 5:00 a.m. uh we can hop on some

cs50 shuttles and go down the road for

some pancakes at IHOP around 6 uh of

course uh of course this is 6 7 a.m. at

that point but it's an opportunity

finally to lead into What's called the

cs50 fair which is an end of semester

celebration an exhibition of everything

that you'll accomplish over the coming

months and in fact pictured here are

some of your predecessors and healthy

times um the cs50 fair allows you to

come with your laptop or phone and

exhibit a students faculty and staff

across campus put together something in

person and on video that people can

Delight in seeing as you exhibit what it

is you create in what you learned over

the course of the several weeks and

ultimately a chance to just share and

Inspire others as well um and you'll all

walk home ultimately with your own I to

cs50 t-shirt saying as much as well so

with that highlevel overview of the

course I propose that we begin to take a

look at what computer science itself is

and what it is we're going to be doing

over the next several weeks at this

lower level than two so what is computer

science right if you're maybe like me or

new people like my friends and High

School you probably assume that it means

programming and that's absolutely a big

part of it for a lot of people because

with code you can write you can express

ideas and solve actual problems

especially involving data but computer

science itself is really the study of

information if you will how do you

represent it and how do you actually

process it and in that sense

computational thinking is just the

application of ideas from computer

science a course like this to problems

of interest to you again in the Arts

Humanities Sciences social sciences

whatever the domain interest is to you

so with that if computer science is all

about information and with it the

solving of problems well what does it

actually mean to solve a problem let's

see if we can't propose a model into

which all of the Lessons Learned will

ultimately follow and I'd propose that

this is problem solving you've got some

input which is like the problem you want

to solve the goal is to solve it so

that's the so-called output and then

somewhere in here the proverbial black

box is some kind of Secret Sauce that

gets the work done and in the coming

months have to decide well how are we

going to represent these inputs and

outputs and really how do we code up how

do we write solutions for what it is

that's solving the problem of interest

to us so when it comes to representation

of information like there's a lot of

ways we can do this and for instance if

the problem at hand quite simply is to

like take attendance at the beginning of

class on the first day of school well

how could we go about doing this well we

could actually use a system called unary

well what is that well that's a fancy

way of saying 1 2 3 4 5 maybe 6 7 8 9 10

and I can use my digits pun intended on

my fingers to actually count everyone up

and eventually you need toes and whatnot

if you have to count so high but unary

is a very simple system of using a

single symbol a human finger in this

case to just solve some problem like

counting the number of people in the

room let's make this slightly more

technical for a moment a little more

mathy that's just called base one where

the base under which you're operating

has one digit in it like literally a

human finger and maybe multiple such

fingers if you need to count higher but

of course of course all most of you if

not all of you generally vaguely know

that computers use something other than

unary and even you and I probably don't

use this that often they use what

language or alphabet

instead yeah so binary so binary is

indeed the system that computers somehow

use so in this case by implying two and

so computers have two digits it turns

out at their dispos one in fact if

you've ever heard the technical term bit

which is like a smaller version of a

bite more on that soon well a binary

digit is the origin of that term bit

because if you get rid of some of the

letters and you're left from binary

digit with just bit thus is a bit a bit

is just a zero and one it's two more

digits than you might have on your own

finger and of course it's fewer though

than you and I have you and I typically

use as humans the decimal system deck

meaning 10 because you and I generally

use zero through nine so on the one hand

another pun intended you've got unary

computers use binary we humans generally

think and talk in terms of De

but at the end of the day these are

fundamentally going to be the same thing

which is to say that it's all pretty

accessible to us even if you're not a

computer person I dare say you're about

to be so what is a bit well a bit then

is a zero or a one that is a so-called

binary digit but how do computers only

speak in binary how do they solve

problems represent information using

only binary well at the end of the day

if they want to represent zero and one

we need to do so physically somehow and

I dare say that maybe the simplest way

to think about a bit a zero or one is

like a light bulb and so by human

convention let's just assume that if you

are a computer be it a laptop desktop

phone or the like and you want to

represent the number zero you know what

you just keep a light switch off you

keep a light bulb off if by contrast

you're that same computer and you want

to represent the number one you take

that same switch that same light bulb

and just turn it on so a light bulb

that's on represents a one and a light

bulb that's off represents a zero so why

is this relevant into computers well at

the end of the day you and I are

charging our laptops or phones at night

so there's some like physical resource

being replenished there whether you're

on battery or some power cord and so a

inside of a computer are just thousands

millions of tiny little switches

nowadays you can think of them

metaphorically as light bulbs but they

don't yet it's actually shine light but

there are tiny tiny little switches and

those switches if you've ever heard the

term are just called transistors so like

computers have millions of transistors

that can either be flipped on to

represent ones or flipped off to

represent zeros and from that very

simple mechanism electricity is there or

it's not a one or a zero computers can

actually count obviously from zero to

one but it turns out even higher if they

use a little more electricity as well so

how might I do this well let me go ahead

and propose that I just grab one of our

own light bulbs here on stage this one's

off so for instance If This Were

miniaturized inside of your Mac PC or

phone this would be a transistor and

indeed here's the little switch on the

bottom and if your computer wants to

represent a zero it just leaves the

switch off and the light is not shining

if you want to represent a one well now

I've counted as high as one because the

switch is now on I've grabbed a little

electricity I'm holding on to it inside

of the computer and so now I see that

this is a one all right but

unfortunately with just one switch one

light bulb I can only count from zero to

one how do I count out higher might you

think

intuitively say again yeah so more light

bulbs so let me do this let me just put

grab something to put these on so I can

use a few of them at a time and let me

propose that here instead of having just

one light bulb let me give myself maybe

three in total so all of them are

initially off and if you think of this

in miniature form in your mind's eye

this is like a computer with three

transistors three switches representing

now the number you and I know as zero

why they're just all off so how does a

computer go about representing the

number one well it turns on one of these

light bulbs and how does the computer

represent the number two well you might

think if I by may you just turn on a

second light bul and if you might think

how does a computer represent three you

just turn on the third light bulb and so

as such with three bits a computer would

seem to be able to count from zero on up

to 1 two three but it turns out if I'm a

little smarter here I can actually count

higher than that why well I'm just

considering like the combination of

bulbs being on here what if I do

something like this this is still zero I

will claim but what if I propose now

that this will be how a computer

represents one on on off off this though

will be how the computer represents two

notice I didn't turn on the same two I'm

just turning on the one in the middle

this I now claim will be how a computer

represents three this is going to be in

just a second how a computer represents

the number we know as four and yet I'm

still only using three bulbs this is

going to be the number the computer

represents as five this is going to be

how the computer represents the number

six and then lastly it turns out with

three light bulbs if you're smart about

it you can count it seems as high as

seven now even if you kind of lost track

of like what I was turning on and Y I

claimed there were eight different

patterns from all of them off to all of

them on but notice that I started to

permute them I took into account which

ones were on and which ones were off why

though do these represent the numbers we

know as 0 through 7even well let me go

ahead and maybe let's let's do this

instead of just considering there to be

light bulbs let's assign some special

significance to each of them based on

where it is and maybe for this could we

get uh maybe three volunteers three

volunteer okay yeah who's you're being

volunteered okay come on up if you want

to go over to the stage there yeah you

want to come on up as well and over here

as well so there's some stairs on either

end maybe a round of applause for our

first volunteers of

term all right so you want to be you are

number one and if you want want to go

ahead and stand roughly right here how

about you want to be number two okay

come on over and right uh to the right

of here and you'll be number four it

turns out and if you want to come over

here on this end let's give you all a

moment to uh introduce yourselves uh

briefly to your classmates if you'd like

hi I'm Ellie I'm a senior nice to meet

I'm rahana and I'm a first year welcome

hi I'm Joseph and I am a first year

welcome all right so so glad to have all

three of you up here thank

you let me propose now that we'd like

you you three to represent how about the

number zero and I claim now that if each

of you now represents a switch you have

sort of fancier light bulbs now one is a

one one is a two one is a four but each

of you still just has a switch on the

bottom in fact of your plastic devices I

claim these three volunteers are

representing the number zero let me ask

you all now how might you represent the

number one how should you cooperate here

okay so so we would have on off off

which I think is matches what I did with

my three light bulbs as well if you want

to go ahead and turn yours off how might

you three represent the number

two okay so off on off now from right to

left how would you3 represent the number

three ah so that's why my two light

bulbs went on at the end how would you

three represent the number

four perfect number

five number six

and number

seven all right and give us one more how

would you represent

eight okay you can't uh how about then

one more volunteer one more volunteer

okay come on

up all right what's your name my name is

Moen if you want to say it there my name

is Moen all right Moen you're GNA be

number eight and if now you all actually

let's make this uh how would you

represent number eight all collectively

as four bits or four

switches okay eight and now lastly give

me

15 everyone's awkwardly doing arithmetic

in their head oh using

unar hey yeah is

everyone yes okay Round of Applause okay

thank you

all if you want to leave your numbers

over here we have a 650 stress ball for

you but thank you for volunteer you can

turn those numbers off and leave them

over here so thank

you so how do we go about how do we go

from there to uh creating these patterns

well even though we still had three bits

initially and three switches later four

bits and four switches ultimately we

still used it used the same approach

fundamentally to actually representing

information and now why were they those

patterns and why did I very deliberately

have our volum volunteers line up in

that way well I wanted them using base 2

AKA binary but with binary there come

certain rules and even if you're not

familiar with binary beyond that it

exists and relates somehow to computers

it's actually pretty much identical to

the system you and I use every day known

as base 10 AKA decimal so let's consider

if you will by rewinding to primary

school for just a moment like how

decimal works and you'll see that even

if you're not a computer person you

actually are you just have to tweak your

mental model ever so slightly so here is

the number that you're probably viewing

as 123 but why is that well it's not

really 123 these are just uh this is

just a pattern of three symbols on the

screen 1 2 three and your mind is sort

of rapidly assigning mathematical

meaning to them 123 but why is that well

if you're like me you probably learned

back in the day when you have a

three-digit number like this the

rightmost number is in the ones place

the middle digit is the 10 place the

leftmost digit is in the hundreds place

and why is that relevant well if you

then quickly do some mental math as you

and I just do uh instantly nowadays that

just means 100 * 1 plus 10 * 2 plus 1 *

3 of course 100 + 20 + 3 gives us the

number you and I know as

123 but beyond that how do we get to

just two digits instead of as many as

nine in the decimal system well let's

generalize this in the decimal system

you and I know if we've got three digits

represented by these hashes here yes

it's the ones place T Place hundreds

place and if we keep going thousands 10

thousands and so forth but why is that

well base terminology is now a little

more gerine that's technically the 10 to

the zero column the 10 to the 1 10 to

the two so these are powers of 10 where

10 is your base computers just simplify

things a little bit because computers at

the end of the day only have access to

electricity on or off they don't have

access to like 10 different types of

electricity just two on or off if you

will well they just use a different base

and the rightmost digit would be in the

so-called 2 to the zero then the middle

digit is 2 to the one the leftmost is 2

to the 2 AKA one's Place two's Place

Four's place and as we kept going 8 and

if we keep going 16 32 64 128 and so

forth but the idea is fundamentally the

same so why is this how the computer

represents the number you and I know is

zero well off off off from right to left

or in this case left to right is just

zero why CU that's 4 * 0 + 2 * 0 + 1

time 0 is of course 0o this is why 0 01

represents one this is why 0 1 0

represents two and three and four and

five and six and seven on up and why did

we need a fourth bit to represent eight

well we kind of needed to carry the one

so to speak using our human familiar uh

familiar human terminology but for that

we need a fourth bit another transistor

and this now represents the number eight

and that's why we ended with on from

left to right off off off so I keep

saying on and off or the light bulb is

on or off but really I just mean one or

zero and so computers and we humans

think of things digitally as just being

zeros and ones but mechanically you can

think of it indeed as these light bulbs

now a bit not very useful even three

bits four bits not that useful you can

count to seven or 15 generally speaking

bites are a more useful unit of measure

and anyone familiar how many bits is in

a bite yeah so eight bits are in a bite

can think of it as an octet equivalently

uh in some context there are nuances

there but think of a bite as just being

eight bits and that's just a more useful

measure so what does this mean in real

terms well if you've ever downloaded

like a music file or a photograph or a a

video those are measured in bytes

probably not small numbers of bytes

probably kilobytes for thousands of

bytes megabytes for millions of bytes

gigabytes for billions of bytes

especially for video that just means you

have a lot of patterns of 8 Bits some

combination of zeros and ones on your

computer's hard drive here then with a

bite of bits eight bits is how a

computer would typically represent the

number zero and if that same computer

uses all eight of its bits it's full

bite to rep uh to change it to one

anyone who's quick with math or have

seen this before how high can a computer

count with eight bits or one yeah 255

why is that well we're not going to turn

this into a constant math exercise

indeed after today we're not really

going to think about or talk about bits

at this low level but this is the ones

place 2os 4S 8 16 32 64 128 and if I do

all of that math from left to right that

indeed gives me

255 it ignores how we might represent

negative numbers but perhaps more on

those some other day but computers of

course do so much more than numbers and

math and all this lowlevel stuff we send

text messages right documents emails and

the like so how might a computer

represent something like the letter A I

claim at the end of the day your Mac

your PC your phone just has lots of

transistors lots of switches that it can

use in units of eight in units of bytes

how though if it's already using those

patterns of zeros and ones apparently to

represent numbers from zero on up how do

you go about representing letters of the

alphabet might you think yeah okay so we

could assign a number to every letter

okay so let me just conjecture well

let's just call a0 for Simplicity B1 1

C2 and now let me play Devil's Advocate

okay how do I now represent zero or one

or two well we've maybe created a

problem for oursel if now we have to

steal some numbers to represent letters

we kind of have to pick a lane but

there's a solution to that too that

we'll see and it turns out the world is

not quite as simple as a being zero a

typically is represented by computers

everywhere phones everywhere with the

number 65 the decimal number 65 using 8

Bits if we turn some of the zeros to

ones let me just stipulate you can

represent the letter A Using eight bits

by turning certain ones on and certain

ones off but we will try not to focus on

the that binary level too much so if a

is 65 it turns out that b is going to be

66 and C is going to be 67 and so forth

and so where does that get us well it

turns out there's a whole system that

Maps uh numbers to letters and here uh

as I alluded to verbally a moment ago is

the pattern of zeros and ones via which

you'd represent 65 and just quick check

here we won't constantly do math uh

one's place that's easy 2os Four's 8 16

32 64's place so 64 + 1 gives us

65 so once I do that how do I get to all

of the others well it turns out a bunch

of Americans years ago came up with this

asky the American Standard code for

information interchange now what does

that mean well it's just an acronym

describing what really you proposed a

mapping between numbers and letters not

not quite as simple as 012 starts at 65

66 67 for capital letters but here are

most of the letters in use today at

least with this system so this is just a

big chart from online and you'll see it

in the middle of this chart here here's

my 65a here's my 66 b c and let's see 72

is H 73 is I and so forth so there's a

mapping at least for English between all

of these numbers and all of these

letters and if we focus here those are

the beginning of our up case alphabet so

suppose then that today tomorrow you

receive a text message from someone and

underneath the hood now that you're a

computer person you figure out a way to

see what pattern of zeros and ones was

sent in this case it's wireless as

opposed to wired but it's still some

pattern of zeros and ones and your phone

is turning some switches of its own on

and off to represent that message from a

friend suppose that the three patterns

you received were these three btes from

left to right spelling out a

three-letter word well if if we do out

the math one's Place two's place and so

forth I'll spoil it for you suppose that

you received a text message that doesn't

literally say 72 73 33 but you've

received a pattern of 8 plus 8 plus 8 24

bits that if you do out the math

represent the decimal number 72 73 33

anyone recall what message you might

have received from the green and white

charts

yeah hi but yes hi is the message but

7273 gives H and I what's 33 any any

guesses to

33 yeah over here yeah so it's an

exclamation point how would you know

that well you really do need some kind

of cheat sheet AKA aski in this case and

if we look elsewhere let me highlight

the left of the chart you can see that

next to 33 in decimal is indeed the

exclamation point so back in the day a

bunch of humans got in a room decided

that hey when we start building PCS and

later Macs and phones we all just have

to agree on this form of representation

of letters of the the English alphabet

in this case we just need to agree on

this mapping but somewhat curiously

notice this it turns out that once you

paint yourself into this corner and

start using 65 for a 66 for B well how

do you represent 65 the number and 66

the number if you want to do math or use

Excel or something like that does anyone

kind of see the solution perhaps how do

you represent the number one in asy yeah

in the

middle yeah so this is getting a little

maybe Inception or something but you

could represent numbers with other

numbers and so if you want to represent

the number you and I know as one like

when you type it on your keyboard turns

out the computer stores that as the

decimal number 49 if you hit two on your

keyboard the computer is not storing two

per se it's storing the decimal number

50 now thankfully the the uh the uh the

the Paradox kind of stops there we just

have a mapping now of numbers to to

numbers but really at the end of the day

and you're going to learn this when we

start writing code in that other

language C next week it's just context

dependent at the end of the day inside

of your Mac PC and phone there's just

all of these permutations of bits all of

these patterns of zeros and ones and

generally speaking when you open up a

text message that you've received from

someone it's zeros and ones but

obviously if it's a text message the

whole point of text messages is to send

text and so those patterns of zeros and

ones by default will typically be

interpreted as letters of the alphabet

so you won't see zeros and ones you

won't see decimal numbers you'll see the

English message that your friend

intended by contrast if you open up

something like Excel that same pattern

of zeros and ones might indeed work out

to be 6 uh 72 73 33 you might see cells

in your spreadsheet with literally those

three numbers why because spreadsheets

are all about numbers and number

crunching and math uh in many cases if

by contrast you open up Photoshop and

try to look at that same pattern of

zeros and ones it's not going to be 727

73 33 it's not going to be zeros and

ones it's not going to be high it's

going to be some color of the rainbow

you're going to use those patterns of

zeros and ones it turns out too to

represent colors and indeed so long as

you and I just agree as humans long have

what these patterns are going to be all

of our systems many of our systems

nowadays are indeed interoperable but

I'm being very biased here and indeed

the a in ASI is very American Centric

what do you not see in this chart if you

speak any other language in English odds

are you're not seeing characters you

know and love and need every day to type

or send messages well there's a huge

character set that's not supported here

whether it's accented characters and a

lot of Asian alphabets you have many

more symbols than can fit even on this

screen here and so humans kind of

painted themselves into a corner early

on or really Americans did but on a

typical keyboard us English keyboard

yeah you have A's and B's and C's

uppercase and lowercase but you also

have accented characters here and

nowadays not sure if this is maybe

necessary but nowadays you have other

characters on your keyboard like these

and these are kind of a playful

incarnation of what's actually a

technical solution to this problem if I

claim for the moment that ASI

historically used seven bits to

represent letters and let's just round

that up to a bite eight bits to

represent letters aski can represent as

many as 255 or really

256 total characters why 256 well if you

have them all zero that's zero and the

highest number I claimed a moment AG go

was 255 so that's 256 total

possibilities that's not many letters

it's fine for English but not a lot of

human languages so what might the

intuitive solution be if you want to

represent accented characters um Asian

characters Emoji even like these which

are just keys on a keyboard

nowadays what's the intuitive solution

if a bite's too few

yeah yeah so add another digit just like

we had a fourth volunteer come on up to

give us a fourth bid let's just throw

throw Hardware at the problem and use a

few more bits so maybe instead of one

bite let's use two or heck let's use

three or four bytes even though it's

getting a little expensive we're going

from 8 to 16 to 24 or 32 bits that's how

computers do things these days and

thankfully we have so much memory inside

of our computers and phones we can

certainly spare a few to represent these

things and the solution then to aski is

what we'll call Unicode so Unicode is

also just a mapping of letters to num of

numbers to letters but in many many

different languages and indeed the

Unicode Consortium is a bunch of people

from all different companies and and uh

and a lot of different uh companies and

countries and cultures whose mission as

an organization is to capture digitally

all forms of human language in this case

and to ensure that especially smaller

demographics of humans speaking lesser

known languages are nonetheless

represented and preserve digitally using

some mapping of these zeros and ones it

turns out though if you start using 32

bits as many as 32 bits to represent

characters on a keyboard that's 4

billion possible permutations of zeros

and ones that's way more than we need

for most human languages so there's a

little bit of room in there for some of

those more playful things like those

Emoji so for instance suppose you got a

text message with this pattern of zeros

and

ones or if we do out the math suppose

you receive a text message that if you

do out the math in decimal is 4 B 36

m991 106 anyone know what emoji you're

looking

at this would be weird if you do

but what is this Well turns out that as

of this past year this is the most

popular Emoji to be sent uh by many

measures face with tears of joy so that

is the pattern that a bunch of humans in

the Unicode Consortium decided would

represent this but you'll notice some

many of you might have iPhones some of

you might have Android devices too and

sometimes these don't actually look

quite the same this happens to be the

current version of face with tears of

joy on iOS on Android it tends to look a

little something more like this and

here's kind of a curiosity even though

you and I look at these things and they

look like images they're not images

they're characters at least as we've

defined them now in Unicode and a IOS

and Android and Windows and Facebook and

other companies and uh apps nowadays

really just have different fonts if you

will so just like fonts with English and

other languages can give you different F

characters with seraps or not emoji are

themselves yes drawings that someone

made but they're really just a font and

so that same pattern of zeros and ones

might just render slightly differently

on someone's phone or another if you've

ever gotten like an icon on your phone

that's broken and you've been sent an

emoji but it's like a square or

something arbitrary and not sensical

sensible it might just mean that you

have not updated to the latest version

of iOS or Android which just updates the

font of supported Emoji because those

Folks at Unicode pretty much every year

nowadays are adding more and more Emoji

to that that particular character said

now I went down the rabbit hole figuring

out the other day just which are the

most popular Emoji these days on Twitter

specifically this past year the most

popular Emoji by contrast uh was loudly

crying face I don't know if that says

more about 20121 or about Twitter but

you'll see different Trends certainly in

how these are used but even humans

themselves didn't necessarily think two

steps ahead and now a lot of the Emoji

are the sort of default yellow color but

there's a lot of emoji that aren't sort

of these cartoon characters but they're

meant to represent humans in various

professions or gestures or the like and

nowadays too you've probably noticed on

your phone and Macs and PCs there are

different skin tones that you can assign

to certain emojis if it's supported by

the company and by Unicode you can

actually like touch and hold on a

certain emoji and then you can choose

the appropriate skin tone to represent

yourself or someone else and that then

modifies the display well let's just

think for a moment here how did Apple

and Google and Microsoft and others go

about implementing support for emoji

with different skin tones how could you

do this if you want to represent some

smiling Emoji but in five in this case

different skin tones you could come up

with what five different patterns that

are identical structurally except for

the skin tone used in places in that

image but that's a little inefficient

right to kind of just do copy paste

paste paste paste and like change the

color in Photoshop if you will that's

going to use more bits more information

than you might need to how else if you

now start to think a little bit more

like a computer scientist if at the end

of the day all you have are zeros and

ones how else could you implement skin

tones might you think

yeah okay so RGB and we'll come to that

in just a moment that stands for red

green blue that's one way in this case

though I'm seeking an alternative to

just using five different patterns of

zeros and ones to represent the same

Emoji but different skin tones so not

quite RGB

yeah okay so store one copy of the Emoji

and then store different variants of the

color that you want to assign to that

emoji yeah so this is actually an

example of do you want to

elaborate okay so you can use a loop to

actually output these things more on

that in a moment let me go down this

this road for just a moment this would

be in some sense sort of a a better

design if you will uh but why yeah

filter okay so filter if we think sort

of in the Instagram sense you can sort

of change the color of something and

that could be related here too could it

be oh interesting so maybe it could be

just a completely different font and you

have five different fonts that are

almost identical except for the various

interpretations of skin tone for those

same Emoji let me spoil I think if we go

down this one particular Road the way

the uh the Unicode folks decided to do

this some years ago were the first uh

bite or bites that you receive via text

or email just represent like the

structure of the Emoji the default

Yellow Version thereof but if it's

immediately followed by a certain

pattern of bits that these humans

standardize to represent each of these

different shades of skin tone then the

phone the Mac the PC will change that

default color yellow in most cases to

whatever the more apt human tone is so

you just use twice as many bits but you

don't use five times as many bits so

what do I mean you don't have uh five

completely distinct patterns per se uh

for each of these possible variants you

have a representation of just the Emoji

itself structurally and then uh reusable

pattern patterns for those five skin

tones un um unfortunately that wasn't

quite versatile enough for other

features that were in the pipeline and

nowadays too and there's a double

meaning now to representation emojis had

tended to focus on certain professions

and early on too were certain

professions associated with certain

genders and vice versa and you couldn't

necessarily be one gender or another in

a certain profession or another there

were these combinatorics that just

weren't possible but nowadays as you

might have seen you can have couples in

love for instance that actually look a a

little more like three emojis but just

kind of combined into one and indeed

this is just one key press on your phone

and you can combine different emoji on

the left and in the right with the Emoji

in the middle and so it turns out how

computers nowadays represent these

patterns are one set of bits for the

character on the left one set of bits

for a character on the right one set of

bits for whatever Emoji you want in the

middle and then you assemble more

complicated compositions of emoji by

just reusing those same patterns and

bits and bits Emoji doesn't the Unicode

folks don't have to come up with a whole

new representation for some very

specific Incarnation they can create one

for person for male for female for other

uh characters that you might want to

display and reuse those same patterns of

zeros and ones and so here you see sort

of the imperfection of or lack of

foresight of humans for building a

system early on that was entirely

American Centric no characters Emoji or

the like that's evolved too and so

that's an important detail in Computing

nowadays um it too is evolved in and the

languages you're about to learn in the

coming days those twoo are evolving as

well and new features are getting added

and even programming languages have

version numbers you might have a

different version of an app on your

phone programming languages too have

different versions as well questions

then thus far and how information is

represented using asy or

Unicode or anything in between yeah so

you can use a string of a good question

so to recap why can't you just um well

let me summarize that as why can't you

similarly use different patterns to

change the context of what these

patterns of bits represent whether it's

a number or a letter or a graphic in

actuality that's kind of what's

Happening underneath the hood it's not

standardized in quite the same way but

starting next week when we transition

from scratch to C you'll learn about

types data types where the onus

initially is going to be on you the

programmer to tell the program whether

or not this pattern of bit should be

interpreted as a number or as a letter

or as a color or something else nowadays

though in toward the end of the semester

you'll use languages like python where

the computer just figures it out for you

by context which makes it even easier

and faster to program as well other

questions on Unicode asky or the

like all right well how about just a few

other forms of information RGB was

called out earlier red green blue How do

images get represented in computers well

in fact it's typically an assembly of

some amount of red some amount of green

some amount of blue but there are other

representations if you're a graphic

designer you might know them but RGB is

still pretty common what does this mean

this means to represent every dot on

your phone or every dot on your TV or

your laptop or desktop there is a number

representing how much red that dot

should show a number representing how

much green and a number representing how

much blue it should show red green blue

respectively so for instance if a DOT on

your screen were using these three

numbers these three values or bytes 72

73 33 in a text message email that would

be interpreted as I claimed high but in

Photoshop or in some graphical program

that same pattern would be repres would

be interpreted as let's call it a medium

amount of red a medium amount of green

and a little bit of blue and why medium

and little turns out that each of these

are bites the smallest value you can

have in a bite we said is zero the

largest value you can have in a bite is

255 so I'm just kind of spitballing here

this is like medium medium and a low

amount of red green blue uh specifically

those three colors um like uh

wavelengths of light are combined in

such a way that you would have this dot

on the screen a sort of murky shade of

yellow or brown that is how a computer

would store precisely that color and in

fact we've seen this color when you type

in face with tears of joy generally on

your screen it looks like this typically

much smaller but let's zoom in or let's

zoom in a little more what do you

starting to see if you know the term so

pixels it's getting very pixelated a

pixel is just a DOT on the screen and if

you really zoom in on it you can

literally see all of the dots that

compose in emoji in this case on iOS in

the font that Apple's using to represent

this particular pattern of zeros and

ones so one of those yellow dots and

there's many of them all that kind of

blend together here each dot on the

screen I claim is three bytes how much

red green blue for this dot how much red

green blue for this dot how much red

green blue for this Dot and you'll

notice too that when it gets to be sort

of brownish here the dots really stand

out the three values the three bytes AKA

24 bits are just slightly different and

so underneath the hood this is why

images photographs that you take or GIFs

that you download get so darn big

potentially because you have a number

representing every dot on the screen

well if this I claim is indeed how

images are typically represented using

pattern of bits that are assigned to

some amount of red green or blue how do

you get video what is a video if at the

end of the day all we have are zeros and

ones

what's a video perhaps yeah oh let's go

here weighing back yeah pixel's really

changing values over time and do you

want to confirm or deny the hand that

went up here yeah or equivalently a

sequence of images that over time are

changing on the screen so both of those

are valid interpretations and you know

just for fun if you uh grew up with

these sort of picture books you might

remember a little something like this if

we could dim the

[Music]

lights

[Music]

so that's sort of the old school analog

way to implement a video in the sense

that um that artist wrote out like

hundreds of pieces of paper with almost

identical images but where the ink from

their pencil or pen was slightly moving

and if you digitize that such that each

of those Strokes are represented with

dots instead that's really what you're

seeing as a sequence of all these images

flying across the screen and if we dive

into the the real world if you've ever

watched a film a Hollywood movie is

typically 24 FPS frames per second that

really means you're seeing 24 images per

second or on TV or in soap operas it's

often 30 frames per second that makes

things look a little more smooth so it's

not actual motion picture if you will it

sequences of pictures and your brain and

mind are kind of interpolating that oh

this is smooth movement even though

we're just seeing a lot of pictures

really fast now that gets really big and

we'll talk later in the semester how you

can compress information so that you're

not using way more bits than you

actually need to and there's fancy

algorithms that folks have developed but

at the end of the day that's really all

a video might be is a sequence of images

conversely if you want to represent the

music that accompanies that or something

else if any of you play an instrument

and can read sheet music How Could You

digitize this like how could you

represent musical notes in a computer

you and I hear them when we play files

but what's really going on underneath

the

hood any any musicians piano players

anyone yeah Val okay so Hertz value so

some frequency so sound is some

frequency and it's kind of hitting your

eardrum and that's what makes it sound

low or high or somewhere in between so

maybe we could assign just like there's

letters a through G here maybe we could

assign specific frequency values which

are just going to be numbers measured in

something called Hertz something per

second and maybe we could have a few

other numbers for each of these notes

not just the note or the frequency maybe

we could represent the loudness of it

like how hard or how softly a human

might equivalently press it maybe a

third number like duration like how long

is their finger on the keyboard so you

could imagine quantizing something like

music that in the real world is

perfectly continuous as something more

discret by representing each note over

time as just some sequence of values and

there's so many different ways to do

this midi if you've heard uh MP3s AAC I

mean almost all the file extensions you

see on your Mac or PC if you see them at

all ultimately just mean there's a

different form of representation for in

this case something like sound so let me

just stipulate there are these and many

more ways to represent inputs and

outputs and thankfully humans have

standardized a lot of this they don't

always dis agree and this is why we have

different file formats for Apple numbers

and Microsoft Excel and Google

spreadsheets and sort of stupid

incompatibilities like that but

generally speaking humans have

standardized how we represent the inputs

and outputs to and from problems but

let's now focus on this black box so to

speak in the middle this abstraction so

abstraction is technically a term that

you'll see all over the place in

computer science and really problem

solving that just refers to the

simplification of something so that you

don't focus on the lower level

implementation details you really just

focus on the high level goals or the

process itself uh Therefore your car if

youve uh if you have a license and have

driven or have been in a car a car so

far as you're concerned is probably an

abstraction most of us if you're like me

probably don't really know or care how

the engine works and all the parts that

are moving to you it's just a way of

getting from point A to point B it's an

abstraction but someone hopefully the

mechanic does know those lower level

implementation details if you had to

understand how a car works every time

you want to go to school or to the store

it's probably going to be a pretty slow

process you just want to think and

operate at this higher level of

abstraction and we're going to do this

all the time when writing code and

solving problems

so what then is in this black box this

abstraction at the moment well generally

it's what a computer scientist would

call an algorithm stepbystep

instructions for solving some problem

now let's consider the implementation

details that is to say how you might

solve certain problems and let's take a

a sort of old school example but in

modern form this icon if you have an

iPhone is of course for your contacts

application and if you've got a whole

bunch of family members or friends or

colleagues in your phone book you have

some kind of contacts pictured here here

and it's alphabetized typically by first

name and last name and odds are you and

I are in the habit if they're not

already a favorite of like clicking on

search and then using autocomplete and

what happens when you start typing

autocomplete well if you type in the

letter H you'll see only presumably

Hagrid Harry Hermione and so forth if

you type in ha that shows you only

Hagrid and Harry and it all happens

super fast so how is that happening well

typically you could just start at the

top and look to the bottom searching for

all the hes or all of the ha but for

larger data sets that's going to get

slow for the Googles of the world that's

going to get really slow and even on our

phones when you have hundreds thousands

of contacts eventually even that kind of

approach that algorithm step by step but

it might be slow so how might we go

about searching for someone in a phone

book like this uh like say uh John

Harvard well here's an old school

incarnation of this and uh odds are you

might not have had occasion to even

physically use this thing nowadays and

in fact this is a bit of a white lie cuz

this is the Yellow Pages which means

this is a book of companies not people

uh but for this is all you can find and

at that it's even hard to find this but

this is the same thing in analog form

physical form so if I wanted to search

for someone like John Harvard how could

I do that well I could start on page one

and I could start searching for page two

page three page four page five little

hard to do physically especially since

no one's used this phone book in a lot

of years but uh is this algorithm

correct Turning Page by Page

very inelegantly is this correct will I

find John Harvard if if he's in here all

right so yes I mean this is a little

stupidly tedious because if there's like

a thousand Pages he might be a few

hundred pages into this but it's correct

at some point I will find him and if

he's on the page I'll be able to call

why because presumably the names are

alphabetized in here and there's no like

cheat sheet on the edge so I have to

search for John Harvard from left to

right searching for H if it's

alphabetized by last name well what

would be marginally better well how

about two pages at a time it's hard to

do with a 20-year-old old phone book

where the pages are kind of uh grown

together but 2 4 6 8 10 12 this

algorithm is this correct all right so

no

why yeah so I'm skipping every other

page so if I don't consider that and I

find myself in like the I section or the

J section well I might accidentally

conclude nope I haven't found John

Harvard yet just because I skipped them

because it was sandwiched between two

pages now I can fix this I think if I do

hit the I section well let me just

double back one page just in case he was

in that last page so it's recoverable

but it's almost twice as fast minus that

that hiccup there but what most of us

would do and what your phones are doing

albeit digitally is they open up roughly

to the middle of the phone book and they

look down and they say oh I'm in roughly

the M section so I'm roughly halfway

through this thousand page phone book

but what do I now know about John

Harvard where is he to my left or to my

right all right so alphabetically he's

presumably to my left and so here I get

can both uh met uh metaphorically and

physically tear the problem in

half you don't need to be impressed it's

really easy down the the spine that way

but uh I know that John Harvard is to

the left here but now I can throw

unnecessarily dramatically half and page

one out of the way and what do I now

know I've gone from a thousand pages to

like 500 I can kind of repeat roughly

the same algorithm go to the half of

this and so this time I went back a

little too far I'm in now the um e

section so what do I know is John

Harvard to my left or to my

right to my right so I can again tear

the problem in half throw this half away

and now I'm really flying I'm toing it

verbally slowly but that went from a

th000 pages to 500 to now 250 and now I

can do it again 125 i' do it again

roughly like 67 and keep doing it again

and again and again until I get left

with hopefully just one single page or

in this case an ad for ironically a

mechanic okay so

what is the implication for our

performance well let's just do this sort

of in the abstract if you will if that

first algorithm were to be plotted just

quickly on a chart without even numbers

here's my x-axis size of problem on the

x-axis so the bigger the problem the

farther out that way time to solve the

problem the tire you go up on the y-

axis the uh more time you're taking to

solve it how would we draw the running

time The amount of time taken to run

that first algorithm well it's going to

be a straight line why cuz if you add

one more page next year because more

people move to Cambridge you're going to

add one more page turn potentially so

one more second one more unit of time so

it's a straight line and we'll abstract

it away as n if there's n pages in the

phone book the slope of this line is

essentially n the second algorithm

wherein I was doing two pages at a time

was twice as fast but it's still a

straight line and in fact let me just

draw some dotted lines here if the phone

book is this big with my first algorithm

it might take this many step this many

units of time this many steps this many

page turns but with that that second

algorithm notice that the intersection

is with much lower on the yellow line

than on the red so n/2 means there's

half as many pages here if n is the

number of pages so indeed that algorithm

the second one is twice as fast minus

the little hiccup that I have to double

back one page but that's not a big deal

if I'm still doing things twice as fast

but the third algorithm looks

fundamentally different it looks like

this logarithms if you recall from high

school or prior if you don't that's fine

too it's just a fundamentally different

function a different shape and notice

that the green line is going up and up

and up but a much slower rate of

increase which means crazy things are

possible if two towns in Massachusetts

like Cambridge and Austin across the

river merge next year for instance in

terms of their phone book their phone

book just got twice as big for the first

algorithm that's going to take me twice

as many steps to go through the second

algorithm almost Twi it's going to take

me 50% more steps to go through two at a

time but the third algorithm that I

ended with tearing things again and

again divid and conquering if you will

in half and in half and in half how many

more steps will my third algorithm take

if Cambridge and Austin merge into a

phone book that's twice as

big just one more step right no big deal

you just take a really big bite out of

the problem once you decide if John

Harvard is to the left or to the right

and so you've made much faster progress

and so this in essence is what your

computer your phone is probably doing

underneath the hood when searching for

Harry or Hermione or Hagrid or anyone

else because it's that much faster

especially when you have large data if

you don't have that many contacts

probably doesn't matter if you search

from top to bottom or more uh more in

the form of this divide and conquer

algorithm but if you're the Googles of

the world or you're analyzing large data

sets indeed this is going to add up

quite quickly so where do we go with

this well we're going to introduce next

something called pseudo code how can I

translate what I did verbally there sort

of intuitively to actual code well this

won't be scratch this won't be C or

python just yet it's just going to be an

english-like syntax and this is how many

programmers would start solving a

problem they don't start typing out code

in C or python or the like they use

English or whatever their human language

is to jot down an outline for their

ideas my first step really was picking

up the phone book my second step was

opening to the middle of the phone book

my third step was somewhat different

look at the page because why my fourth

step was if person I'm looking for is on

the page I then do what never happened

in my example but I call the person so

I'm done else if the person is earlier

in the book alphabetically as John

Harvard was in the case of my H then I

should search to the middle of the left

of the phone book and then I should go

back to step three step three is look at

the page thereby repeating the same

process again and again step nine though

might be else if the person is later in

the book then let's go ahead and open to

the middle of the right half of the book

and then go back to line three else

there's a fourth scenario we should

probably consider lest my search process

freeze or crash or give me one of those

spinning beach balls with a bug

yeah yeah what if John Harvard isn't in

the phone book I'd prefer that my

algorithm my phone not just reboot or

freeze I should handle that with some

kind of catchall else so to speak let's

just quit the program so there's

welldefined behavior for every possible

scenario of the four now let's call out

a few of these Salient terms it turns

out if I highlight in yellow here

there's a pattern to what I've been

doing here these are all of my English

verbs and we're in a moment we're going

to start calling those verbs functions

when you program or write code and you

want the program or the computer to do

something for you some action or verb

we're going to refer to those actions or

verbs as these things called functions

like those here by contrast I've just

highlighted instead my if my El if my

Els if and Els this is going to

represent what we're going to start

calling a conditional a proverbial fork

in the road where you can either go this

way or that way do this thing or this

other thing and you're going to decide

which of those things to do based on

what I've now highlighted here which are

going to be called Boolean Expressions

bull referring to a mathematician last

named bull a Boolean expression is just

a question with a yes no a true false a

one or a zero answer if you will and it

governs whether you do this thing or

this thing or this thing or that the

indentation in this case is important

the fact that I've indented line five

implies by convention in programming

that I should only do line five if the

answer to line four is a yes or true and

same for these other indented lines as

well and the last characteristic here is

this here uh someone called this out

earlier in fact these lines eight and 11

are now highlighted and represent

what what might we call these in code if

you've done that yeah so these are Loops

some kind of cycles that result in my

doing the same thing again and again but

there's a key detail with this algorithm

in pseudo code even though it's telling

me to go back to line three why is this

algorithm event going to stop why do I

not constantly keep looking for John

Harvard Forever by nature of these Loops

telling me to keep going back to line

three good eventually he'll be on the

page or or to your point earlier he

won't be at all and we're out of pages

and so we just quit and that's the key

about going to the left half or the

right half it doesn't matter if you do

the same thing again and again you're

not going to get stuck in a so-called

infinite Loop so long as you keep

dividing the problem and shrinking it

into something small smaller smaller

eventually there's going to be no

problem left to solve so even if you

don't think of yourself as a computer

person even if you've never written code

what you'll find in the coming days is

that these ideas that we've just kind of

harnessed from real life are at your

fingertips already and a lot of the

process of learning to code is yes it's

going to be a bumpy Ro uh some bumps in

the road because you can't quite see the

new syntax in a familiar way but you'll

find that the ideas in fact are going to

be more familiar than you might

otherwise think and so we'll see in a

bit and we'll take a break in a moment

uh to take a breather that you will see

these same ideas in a moment in the

context of scratch an actual programming

language via which we drag and drop

puzzle pieces to make actual code work

we'll see some variant of these ideas

things called arguments and return

values and variables but we'll

ultimately convert it into this somehow

anyone want to wager what this program

will do if fed to your Mac or PC or

phone here's just a massive pattern of

zeros and

ones it will indeed say rather

disappoint ly apparently just hello

world and indeed baked into all of these

zeros and ones are not just the h l l o

but also the verbs the action of

printing something to the screen and

there's other stuff too so that the

program knows how to start and how to

stop a lot of stuff that we won't have

to worry about that whoever designed the

computer or the language did but at the

end of the day you're never going to be

writing these zeros and ones yourselves

though our ancestors Once Upon a Time

did in some form we'll be using a much

higher l level language like this in C

or better yet in just a moment like in

scratch like this and indeed this is why

today we focus with focus on and begin

with scratch this graphical programming

language so we have a way of expressing

ourselves with functions conditionals

loops and more but in a way that doesn't

have stupid parentheses and curly braces

and all these visual distractions in the

way and we'll translate that thereafter

to this lower level language but for now

that was a lot that was definitely a

fire hose let's go ahead and take a

10-minute break feel free to get up or

stay here and we'll resume in a bit with

some actual

code uh so this then is scratch a

graphical programming language from a

friends down the road at mit's media lab

that indeed some of you might have used

in grade score the like for playing and

writing code and the like but you maybe

didn't necessarily think about how some

of these Primitives ultimately worked

and in fact everything you've done if

you've used scratch before and

everything you'll see today is going to

apply to all of the weeks to come as we

explore these things called functions

and loops and conditionals Boolean

expressions and more with scratch

because it's so graphical and animated

can create can you create animations

like this one interactive art and

software more generally but you'll do so

by dragging and dropping puzzle pieces

that only lock together if it makes

logical sense to do so and what you

won't have to deal with in this first

week of class is Curly braces

parentheses all of the weird symbology

that you might recall seeing when we

just wanted to say hello world now this

particular program um riging men was

written by a former cs50 teaching fellow

Andrew bar who's actually now the

general manager of the Cleveland Browns

the American football team and so these

are just some of the programs that some

of your predecessors in the class have

created and you'll see in the remainder

of class here a couple of others as well

and more in the course's first

assignment namely problem set zero so

how do we get there well first a quick

tour of what it is we're going to do

this in scratch is perhaps the simplest

program you can write and even if you've

never seen scratch or any programming

language before can probably guess that

this just says on the screen somehow

hello world but what you don't have to

do is type esoteric commands and weird

syntax those curly braces and

parentheses I keep alluding to you just

drag this yellow puzzle piece you drag

this purple puzzle piece let them

magnetically lock together so to speak

click a button and boom with those same

building blocks and several others can

you make what exactly the sorts of

things that Andrew brought to life as

well so here's what we're about to see

at scratch.mit.edu is a cloud-based

programming environment on MIT servers

you can also download it offline on your

own Mac or PC and it gives you an

interface like this on the left hand

side of the screen you'll see a blocks

pette these puzzle pieces AKA blocks

come in different colors which rather

categorize them so pictured here for

instance in blue are a whole bunch of

motion related blocks so Andrew used a

whole bunch of those to have the singer

and the men moving around on the screen

um in synchronicity with the song that

was playing in the background meanwhile

in the middle of this interface is going

to be the code area and this is where

Andrew and sunu will drag and drop some

of those puzzle pieces and other colors

as well and lock them together to get

your character soon to be invented to do

something on the screen indeed at the

bottom right here will you see

ultimately a Sprite area where a Sprite

is a technical term for like a character

in a video game or a programming

environment like this by default

historically scratch uh is the cat the

mascot if you will for this programming

environment and so here we see by

default just one Sprite selected because

on the top right of the screen is the

stage for that Sprite and you can click

in Click and zoom in to make it full

screen but this is the world in which

Scratch by default the cat will live but

you can change scratch's costume so that

it looks like a singer or a man falling

from the sky or the like or anything

else either creating the art yourself or

importing some of the things that come

with it or elsewhere online so what is

this world that scratch rather lives in

well generally speaking we won't have to

care too much about numbers because

we'll be able to ask questions like

intera active ones like is scratch the

cat or any character otherwise touching

the edge of the screen touching

something else but scratch does exist in

this

two-dimensional uh coordinate system

world so when the cat or any character

is dead center in the middle that would

be XY location 0 comma 0 if you will

meanwhile over here is 240 pixels or

dots all the way to the right so this

would be 240 comma 0 where Y is z

because it's right on that midline so

it's neither up or below

over here to the left of course would be

240 and 0 above the cat would be xal 0

cuz it's right on that vertical midline

and 180 and then down here as you might

guess would be 0 comma netive 180

generally speaking we don't have to care

about those precise pixel coordinates

but it's helpful ultimately if you do

want the cat to move up down left or

right having some sense of direction

according to the x- axis and y AIS as

well can help you express your ideas

ultimately so what might some of those

ideas be well let's do this I'm going to

go ahead and create on

scratch.mit.edu just an empty screen

like this one here and so this is the

exact same interface but now I'm in my

browser uh full screen so that I can

start writing some code and let's get

that cat to say something actually on

the screen now this takes a little bit

of practice but honestly just by

scrolling through these puzzle pieces

can you quickly get a sense of what's

possible not just categorically but

specifically and I'll jump around

because I've done this of course but I'm

going to go to events in yellow first

and I'm going to drag and drop this

first block called when green flag

clicked and I've zoomed in there just to

make it a little more legible and notice

that the shape of this green flag just

so happens to mirror this green flag

here at top next to this red stop sign

of sorts and the green flag is going to

mean go and the red stop sign's going to

mean stop to start or stop our program

next week you're going to be writing a

textual command at your keyboard to do

the exact same idea but for now it's a

button so when green flag clicked what

do I want scratch to do well how about

we have scratch just initially say

something like Hello World which indeed

historically is the first program that

most any programmer might write so

anything related to what the cat looks

like it's doing is actually going to be

under looks here in purple so I'm going

to drag over say hello and you'll notice

something curious and different about

this purple block it says of course say

in purple but then there's this white

oval and some text that by default is

hello cuz MIT just decided that by

default the placeholder will be hello

but anytime you see this white oval it's

an opportunity to provide an input into

the function called say and so here I'm

borrowing terminology from before

problem solving again is all about

inputs producing outputs and in between

there is some algorithm in a moment

we're going to start referring to

algorithms quite frequently as functions

why because it's the implementation of

some algorithm so let me override the

default with hello comma World I'll zoom

out and now if I go to the top right of

the screen and click the green flag

we'll see hopefully my very first

program in code now it wasn't a huge

lift right it only was a matter of

dragging and dropping puzzle pieces but

what has now happened well it turns out

that two things have happened when I the

human clicked on that green flag I

triggered what we're going to start

calling now an event an event is

generally something graphical or

interactive that just happens in a

computer program you and I trigger

events on our phones all day long

whenever you tap or drag or long press

or pinch or any of those gestures in

Vogue nowadays on phones you are

triggering events and people at Apple

and Google and elsewhere have written

code that listen for those events and do

something when that event happens that's

what I just did when green flag is

clicked I want something to happen

namely I want this purple function this

verb this action called say to do

something what do I want it to do I want

it to say what this input is and I'm

going to introduce another vocabulary

term the white ovals here are yes inputs

very generically but in the programmer's

terminology they're called arguments

otherwise known as parameters and that

just means an input to a function that

modifies Its Behavior in some way when I

click stop that's just another event and

that one is just built into scratch

scratch knows that when you click the

green stop sign uh everything should

just stop automatically I don't have to

write code to support that feature so

that's all fine and good hello world but

if I keep doing stop and start and stop

and start it's going to do the same

thing again and again and it's really

not that interesting at the end of the

day maybe gratifying once but it'd be

nice if this were a little more

interactive so it turns out that we can

do that too but we need a different

mental model instead so in this case

here when we think about this function

say in this input hello world this

actually Maps pretty cleanly to this

model earlier that I propose is problem

solving is computer science if you will

the input to the current problem is

going to be in white here hello world

the algorithm is the say algorithm now I

don't know how MIT got it to print out

the little pretty speech bubble on the

screen but they wrote those underlying

low-level implementation details and

they gave me and you a purple function

called say that just does that for you

you and I don't have to reinvent that

wheel the output of SE is another

technical term now called a side effect

a side effect is usually something

visual that happens like as a side

effect of you calling a function and so

the side effect here is that the cat has

this speech bubble magically appear

inside of which is hello world so we

have an input we have an output we have

an algorithm but now we're talking about

these ideas in the context of

programming so now the input is an

argument the algorithm is a function and

the output in this case is a side effect

terminology that you'll just hear more

and more and it'll eventually sink in

but not to worry if the terminology

doesn't come naturally early on so what

more might I do with this let me go back

to scratch here and make this maybe

perhaps more interactive and actually

get the cat to say something a little

more dynamically so instead of hello

world why don't I get it to say hello to

me or to you or anyone else so let me do

this let me go under say uh let me get

rid of this first and you'll notice this

neat trick as soon as you start dragging

on a block if it gets close to it it

kind of goes gray and it can be

magnetically snapped together you don't

have to do it very precisely conversely

if I want to get rid of a puzzle piece I

just drag it anywhere on the left let go

and that deletes it or you can right

click or control click in a little menu

will let you delete it as well well let

me do this instead under sensing which I

know is there because I've done this

before are a whole bunch of things

related to sensing whereby the cat can

kind of feel out its World in some sense

it can do things like ask this question

am I touching the mouse pointer like the

user's cursor am I touching a specific

color that you can override to be

anything you want is the distance to the

mouse pointer some specific value but

for now I'm going to focus this on this

this blue puzzle piece that asks a

question which itself is this white oval

that I can apparently change and then

it's going to wait for a response but

this puzzle piece is a little different

it's a little special it comes with a

freebie it comes with what we're going

to call technically a return value so

some functions don't just do something

on the screen they hand you back so to

speak a value that you can do anything

that you want with nothing happens

immediately unless you do something with

that so-called return value so let me go

ahead and drag this thing over here ask

what's your name and I'll use the

default question that seems a reasonable

place to start I'm not going to override

that default and now let me go ahead and

zoom out let me go back to looks let me

go to say and let me just form the

English sentence I want so let me zoom

in here and type in hello maybe comma

space I could do David but that's that's

obviously not right because I'm asking

for a name and then I'm like in advance

hardcoding my name that's not what I

want I just want hello comma and now let

me zoom out and grab one more say block

Let Me Maybe say here okay I don't want

to say hello hello I I don't want to

just type in my own name CU again then

what's the point of asking the user for

their name but notice this if I go back

to the sensing block this is where that

oval that's blue called answer is useful

this will be the so-called return value

of that function so I'm just going to go

ahead and do this and drag and drop even

though it's not the right size it is the

right shape and so scratch will be smart

about it and grow to fill that puzzle

piece for you let me zoom out now and

now let me click the green flag you'll

see that scratch is indeed prompting me

with the speech bubble what's your name

notice the little text box below the cat

is asking what's your name so I'm going

to type in da a v d and hit enter or I

can click the blue check

enter okay it's a little weird I wanted

him to say hello not just my name so let

me stop let me start it again all right

hello what's your name da a v

enter huh kind of rude uh why is there

this bug like I wanted to say hello

David not just David and and yet twice

it has failed to do so uh

yeah yeah the computer is processing my

directions my actions really quickly and

so it actually is doing it it's just you

and I in the room are just way too slow

to notice that it said hello David it

just seems to have just said David so

all right how can I fix this well here's

where you start to poke around and think

about how you might solve this let me go

back under looks maybe there's a smarter

way to do this maybe I could do okay I

could do this how about instead of just

say hello there's apparently another

puzzle piece where I can time it so I

can maybe slow things down a little bit

so let me do this let me throw away all

of this let me drag a say hello for two

seconds let me drag another say hello

for two seconds let me change the first

one to indeed hello comma and then let

me go back to sensing let me grab that

same answer because I threw it away a

second ago and I'll just change it I

don't even have to delete hello I can

just overwrite it like this so now I

think we'll kind of pump the brakes and

see things more slowly let me stop let

me start da ID enter

hello David okay so it's better like it

seems to be working I think your

hypothesis was right just looks kind of

stupid right like the fact that it's

saying

hello David like we can do better and

like literally every piece of software

on your phone or Mac or PC is better

than that it adds words together in the

user interfaces you and I are familiar

with so let's go a little more fishing

here let me throw away these let me go

back to look and just get the simpler

say I want this to say hello comma name

where name comes from that answer return

value so how can I do this well let me

go under operations which we haven't

been before there's a lot of stuff in

here some of it's mathematically related

adding subtracting and so forth you can

Generate random numbers which might be

useful and if I keep scrolling down

there's this join apple and banana but

that's just placeholder text you can

join one piece of text with another

piece of text by default apple and

banana but let's change it to hello and

my name name so this to wrong size but

right shape so let me let it snap into

place let me go ahead now and do hello

comma and now I think I just want to go

grab that answer return value let me

drag the same oval as before clobber

that is overwrite banana so now I'm kind

of composing functions the output of one

function join is going to be the input

of another function say so let's see

what happens now that they're kind of

stacked on top of each other or nested

so to speak click the Green Arrow a

green flag DAV ID enter hello David all

right that was pretty fast let's just

just do it once more stop start here we

go daavid enter okay right it's not the

most exciting program in the world but

it's more correct it's better design

just because that's what you would kind

of expect the software to do and not be

some kind of lame user interface that's

just inserting random delays to just

make it kind of work like that's a

workaround a hack if you will but

there's some cool things you can do with

scratch and we won't really go down the

rabbit hole of all of the fun and

familyfriendly features that it has but

there is one that's kind of cool here

let me go into the extensions button at

the bottom left of my screen and this

one's kind of cool let me go to text to

speech and you'll notice that this one

requires internet because it's

cloud-based but this just gave me some

new puzzle pieces in a new category text

to speech and these green ones do

exactly what they say so let me do this

let me zoom out again let me keep the

join block and I'm just going to

temporarily toss it over here it's not

going to delete itself cuz I didn't drag

it over to the other side but I'm going

to get rid of the say block in purple

I'm going to do the speak block here in

green and let it snap into place and

then I'm going to drag and drop this

onto the input to speak and now perhaps

a little more adorably let's try this

green flag what's your name d ID enter

and hello

David okay it's a little it's a little

robotic but at least now it has

synthesized speech and I've kind of got

my own like Siri or Google assistant or

Alexa thing going on here now where it's

now recognized whatever text it is and

it's played it well let's make this an

actual cat that doesn't talk in that

weird human voice let me go ahead and

get rid of most of this stuff and let's

get the cat to actually meow like a cat

tends to and let me go under the sounds

block now MIT gives you a few sounds for

free because it's designed around a cap

by default and I'm going to go ahead and

grab this one play sound meow until done

and now and we saw I heard a teaser for

this earlier in the

crowd

it's a little piercing admittedly we can

lower the volume a little bit there but

notice if I want the cat to meow a

second time I'll just click it

again okay and over there too I

hear okay all right so it's kind of cute

now right so it's just meow okay yes

echo echo so it's meowing now every time

I hit the green flag now that's great

but even a kid is probably going to like

would prefer that it just meow perhaps

like again and again without having to

keep hitting the button so well how

might we do this all right well if I

want it to meow multiple times why don't

I just like grab it another time and

another time alternatively you can right

click or control click a puzzle piece

and just duplicate it from a little menu

that drops down so here we go three

meows all right that's not really a

happy cat it sounds maybe hungry so can

we slow that down well maybe in fact if

I poke around let me go under control

looks like there's a weight block wait 1

second by default and notice scratch

will be pretty accommodating if you just

hover in between blocks it will grow to

fill that too so I could change it to

one or two or anything seconds I'll just

leave it at the default for now one and

now I'll go ahead and do

this okay so cuter and less hungry and

just more friendly but this isn't the

best design it is correct and let's use

that as a term of art correct means the

code does what you want it to do I want

the cat to meow three times slow slly

and it did so iager this is correct but

it's not the best design and this is

where things get more subjective right

like you could write accurate sentences

in an essay for an English class but

otherwise just it's just completely a

mess like your arguments here and there

and you don't say anything wrong but you

don't say it well in the context of code

we can do better than this and copy

paste or repeating yourself again and

again tends to be bad practice why

suppose that you want to change the

weight to two seconds instead of one

it's aditt not a big deal F I click

there I change it to two I click there I

change it to two but what if you Ma five

times 10 times now I have to change the

weight like in five 10 different places

like that's just stupid it's taking

unnecessary human time and you're going

to screw up eventually especially if

your program is getting longer you're

going to miss one of the inputs you're

going to leave the number wrong and

you're that's a bug so just based on

what you've seen already or if you've

programmed before which a few of you

have what's the term of art here that

will solve this how can we design this

better I heard it here here yeah so a

loop a loop some kind of cycle that says

do that again do that again not

infinitely many times necessarily but

some finite number well you can perhaps

see a spoiler on the screen under the

same uh orange control category is a

repeat block and by default it's

proposing 10 but we can change that so

let me do this I'm going to throw away

most of this copy paste as redundant I'm

going to detach this temporarily just to

make room for something else and I'm

going to drag a repeat block over here

and let that snap into place and I'm

going to change it for now just to be

three for consistency and this is the

correct shape even though it's too small

but scratch will accommodate that for us

and now same uh same output but arguably

better designed why because if I want to

change the number of meows I change it

in one place no copy paste messiness if

I want to change the waiting one place I

don't have to change it in multiple

places and not screw up so let me hit

the green

flag all right so nice now it would have

been nice if MIT had just given us a

meow block that just automates all of

this for us let me wager they gave us

the low-level implementation details

they gave us the play sound meow but I

had to implement like a decent number of

blocks just to get a cat to meow again

and again I feel like we should have

gotten that for free from MIT well they

don't have to be the only ones that

invent blocks for us to use you can

write your own functions your own verbs

or actions so how can we do this let's

make our own puzzle piece called now

that uses this code but creates it in

such a way that it's reusable elsewhere

so let me do this under my blocks in

pink here I'm going to go ahead and

click literally make a block now here's

an interface via which I can give the

block a name Meo W will be the name of

this block and I'm just going to go

ahead and quickly click okay that just

gives me a very generic pink puzzle

piece that starts with the word Define

because scratch is asking me to Define

that is implement or create this new

puzzle piece for me well what does it

mean to meow I'm going to claim that it

means to do these two steps to play the

sound meow and then just wait for one

second but what's powerful about this

idea is look at this up top now that

I've made a block it exists in scratch D

MIT didn't need to create this for me I

created it for myself and even you if we

end up sharing code so I can now drag

meow up in here and what's nice about

meow is that itself is yes a function

but it's also an abstraction like never

again do I or even you need to worry or

care about what it means to meow or

implement it I can sort of drag it out

of the way I didn't delete it drag it

out of the way out of sight out of mind

why because my code is now even better

designed in some sense because it's more

readable what is it doing when the green

flag is clicked repeat three times meow

it just says what it means and so it's a

lot easier to read it and it's a lot

easier to think about it especially if

you're using Meow in other uh projects

too now let me go ahead and right hit

click play same thing so it's not really

fundamentally any different but I can

make this custom puzzle piece this own

function of M meow even more powerful

let me kind of rewind a bit and go to my

meow puzzle piece and I am going to

control click or right click on my pink

puzzle piece and I'm going to edit it so

I kind of regret making meow so simple

wouldn't it be nice if meow took an

input AKA an argument that tells meow

how many times to meow then I can get

rid of that Loop and just tell meow how

many meows I actually want so I'm going

to click on another button here called

literally add an input and it's going to

have placeholder here so I'm just going

to put a placeholder there I keep using

n for number which is a go-to in

computer scientist terms um and I'm

going to add some descriptive text just

so that it's a little more

self-explanatory I'm just going to say

meow n times but there's only one oval

times is just going to be explanatory

text and now notice what has happened

now my puzzle piece takes an input AKA

an argument that will tell that function

to meow some number of times but it's

not just going to going to work

magically I need to implement that lower

level detail so let me zoom out I have

to remind myself what this function was

so I'm going to drag it higher up just

so they're on the screen at the same

time I'm going to go ahead now and

temporarily move this over here I'm

going to temporarily detach this over

here why because what I think I want to

do is move my Loop into the function

itself move the play and the weight into

the loop but I don't want to hardcode

three notice that n here is its own oval

I can drag a copy of N and just let it

go there so now I have a new version of

meow that takes an argument in that

tells meow how many times to meow and

now let me again drag this out of sight

out of mind because who cares how I

implemented it once it's implemented

it's sort of done now my program is even

better designed in some sense why

because now it really just says what it

means there's no Loop there's no repeat

no like implementation details when

green flag clicked meow three times and

so functions indeed let you implement

algorithms like they're just code that

do something for you but they're also

themselves abstractions why because once

a function exists it has a name and you

can think about it in that term and you

can use it by its name you don't have to

care or remember how the function itself

was built whether it's by you or even

MIT so again here I'll click the green

flag it's the same

thing so still correct but better and

better designed and so anytime here and

out with scratch or soon C and

eventually python when you find yourself

doing anything resembling copy paste or

again and again grabbing the same code

probably an opportunity to say wait a

minute let me refactor this so to speak

that is rip out the code that seems to

be repeated again and again and put it

in its own function so you can give it a

descriptive name and use and reuse it

any questions just yet on now saying or

these Loops or these functions that

we're using yeah

[Music]

how did I make it so it meows three

times so I originally only had a puzzle

piece called meow and I decided to

improve it so I held down control and I

right clicked or control clicked on the

pink puzzle piece at top left and I

clicked edit and that brought back the

original interface that lets me add some

arguments to the puzzle piece itself and

I clicked add an input on the left here

and then I clicked on add a label over

here so that just lets you customize it

even further all right so we've done

this let's add one of those other

Primitives too to do something

optionally so how about we make the cat

meow only if it's being petted by a

human as by moving the mouse to hover

over the cat like a human would pet a

cat well let me go ahead and throw uh

away the meowing uh for now and let me

simplify it by just using a sound I'm

going to go ahead and do this I'm going

to go ahead and have a control block

that says if because I want to implement

the idea of if the cursor is touching

the Cal

then play sound meow or I could use my

same pink puzzle piece but I'm going to

throw that away and focus only now on

the sounds and I'm going to do this uh

if uh touching Mouse pointer so I need

to sense something about the world and

we saw this earlier so if touching Mouse

pointer so notice this shape here way

too big but it is the right shape so if

I hover just right it'll snap into place

and this now in blue is my Boolean

expression a yes no question true false

uh if is a conditional and what do I

want to do well if the Cat is being uh

is touching the mouse pointer I want to

go ahead and play sound meow until done

so let's do this I'm going to hit green

flag click now nothing's happened yet

because it's a conditional right it's

only supposed to do something if I'm

touching the cat let me move the cursor

over to the cat and and and wait for

it h another bug why is the cat not

meowing even though I very explicitly

said if touch in Mouse pointer meow yeah

in the

middle yeah this is again my computers

are so darn fast like yours I click the

green flag it asked the question am I

touching the mouse pointer well no cuz

my cursor was up there not touching the

cat it's too late the cat's out of the

bag and so we have to instead solve this

some other mean by some other means how

can we fix this how do we fix that sort

of race yeah yeah so why don't we just

keep asking the question until I

eventually am I'm not actually petting

the cat so let me detach this

temporarily let me go under control let

me go under instead of repeat some

finite number of times let's just do it

forever so sometimes Loops that do work

forever are a good thing like the clock

on your phone that's in a loop forever

because you want it to always tell time

and not stop at the end of the day so

sometimes you do want code to Loop

forever as in this case so let me go

ahead and drag and drop it there let me

again click the green flag nothing's

happening yet but notice the program's

still running and so if if I move my

cursor move my cursor move my cursor

and okay so maybe we could add some

waiting but the cat does not want to be

pet in this case but it's indeed

conditional so there we have an

incarnation in scratch of doing

something conditionally now we can make

this really cool really fast if you will

let me stop this version let me go ahead

and do this uh let me go ahead and throw

all of this away let me go into my

little uh extensions Bucket over here

and let me do video sensing since most

uh laptops or phones these days have

cameras and there indeed I am with

Sanders behind me and let me do this um

when video motion and let me get out of

the way when video motion is greater

than some value so 10 is the default

this is just a number that measures how

much motion there is or isn't so small

number is like no motion big number is

lots of motion so I'm going to choose 50

somewhat arbitrarily here so 50 this is

not normal to program off to the side

but I'm now going to say this when video

motion is 50 go ahead and play sound

meow like this so the cat is still in

that world I'm going to stop the program

and rerun it so here we go green flag

and

now here come all right this is a little

creepy the way I'm petting the cat

but

and ah

okay there we go okay so 50 was too big

of a number I have to pet the cat

faster whereas this if I don't

know yeah

so okay so you can make things even more

interactive in this way by just

assembling different puzzle pieces and

honestly there are so many different

puzzle pieces in here we're not going to

even scratch the surface of a lot of

them but they generally just do what

they say and indeed when you see on the

screen here um this pallet of puzzle

pieces really a lot of programming

especially early on when learning a

language is just trying different things

and try and fail and if it doesn't work

quite look for doesn't work quite right

look for an alternative solution there

too as even I just had to do a moment

ago well let's go ahead and use actually

how about another example of something a

predecessor of yours made let me go

ahead and grab a program I opened in

advance here uh called wacka um might we

get a brave volunteer to come up who is

willing to whack a mole with their head

virtually maybe okay let's see how about

in way back you want to come on down all

right come on down and in just a sure

Round of Applause for our

volunteer all

right so here we

have come on down there what's your name

I'm Josh oh actually say it into the

microphone hi I'm Josh

okay nice welcome Josh come on

over all right so same idea here I'll

take the mic back you can you'll have to

stand in front of the camera in just a

moment you're going to have to position

your head in a box that your classmate

from yester year

created and we'll start with beginner

okay so line your head up in the Box in

a moment all right all

[Music]

[Applause]

right

[Music]

nice 12

seconds 5 Seconds notice the score is up

to 18 already pretty

good all right a round of applause for

Josh if you

can so notice how using some fairly

simple Primitives things do get

interesting pretty fast and how was that

implemented well there were probably at

least four Sprites so you're not re

confined to just one cat you can create

more and more Sprites change what they

look like so they actually look like a

mole in this case there's probably some

conditionals in there Some Loops for 30

seconds that's checking if Josh's head's

movement is exceeding some value over

this way or over this way then increment

something called a variable we'll see

those two just like in algebra you might

have X and Y and Z storing values like

numbers so can computer programs have

variables called X or Y or Z or more

descriptively called score as in this

case at top right or another variable

called countdown typically one word in

code but in this case two words that

just store some value so there's

probably some math going on in there

whereby the author of this program just

is incrementing that is adding one in

one every time it detected that uh a

mole had been whacked in this case with

movement so back in the day I myself

actually implemented my very first

program in scratch when I was at a

graduate student actually at MIT um cost

registered at MIT taking a class from

mit's media lab specifically the

lifelong kindergarten Group which is the

group that created scratch itself and

the program I wrote all those years ago

and still rather cling to is a little

something here called Oscar time that I

thought I'd play just a quick excerpt of

my myself here so in this case consider

as the music starts playing how this

program which is much more sophisticated

certainly than the earliest say hello

examples we just did might also be

implemented let me go ahead now and

click the green

flag so some trash is moving presumably

in some kind of loop from the

Top If I'm touching the mouse cursor it

follows

me if I hover over the trash can it

responds if I let

go in some kind of loop Oscar pops out

creates a variable with the current

score and it happens

again pretty easy at first

but I don't need to keep playing this up

on stage in front of everyone so my

score is already now up to some six or

so but in a moment two you'll see that

it's going to escalate so I'm taking

into account some time apparently so

now so more and more Sprites are

suddenly appearing and notice that each

time they're appearing from a different

part of the screen that's an illusion

perhaps do that pick a random number

between X and Y so you can actually pick

some range of values to have the game

constantly changing and indeed I'm going

to go ahead and click stop since i'

spent like 8 hours plus years ago making

this and I can never listen to the song

again not that I should be anyway at

this point in my life but this song is

uh synchronized in with a lot of the

actions that's happening and ultimately

there's just a lot of building blocks

but I didn't sit down and Implement

Oscar time as I called it all at once I

really did take baby steps so to speak

and I figured out well how could I

decompose this Vision I had at the time

to create this game ultimately and how

do I bite off maybe the easiest Parts

first and honestly the first thing I did

was I found this image and I just like

dragged and dropped it into scratch okay

done like lamp post is installed it

doesn't do anything it's not interactive

but I at least set the stage so to speak

for the program then what else might I

have done well let me do this let me go

ahead and open up uh in another editor

here a early incarnation of Oscar Time

by doing this let me go into Oscar time

here let me full screen this and here

you have let me hide the trash for just

a moment is what I might call the second

uh version of my program wherein at the

top right of the stage here I had the

lamp post which I just dragged and

dropped and got going but then I added

an actual Sprite and it has to be a

Sprite if you wanted to do things

interactively the lamp post not a Sprite

it's just an image a costume if you will

for the whole stage itself a backdrop

but this thing is indeed a Sprite

because it needs to respond to code and

events like dragging and dropping so

what might I have done early on with

that code well maybe the first version

would have been something like this

where by my very first version of Oscar

time might have said something like oh

this how about let me control the

program as before or rather events when

the green flag is clicked what do I want

to do well I want to go ahead and

forever do something like this uh

forever so I want the lid to open up if

I touch it so if the cursor gets near

the lid I want the lid to open up and

then if I move away I want it to close

so how can I do that I want an if but I

just don't want one question I really

want two a fork in the road that goes

left or right so to speak and let me

grab this puzzle piece here as I did

long ago so notice it grows to fill

what's the question I want to ask well

under sensing I'm going to go ahead here

and say if this trash can is touching

the mouse pointer what do I want to do

well I want to change what the trash can

looks like and this part I did in

advance of class if you go up here to

costumes this is where all the graphical

stuff happens and you'll see that I

imported a whole bunch of different

costumes that effectively much like a

video when you play them quickly creates

the illusion of movement some animation

but it's really just dot dot dot dot dot

different images showing on the screen

well some of these costumes are called

like Oscar 1 Oscar 2 Oscar 1 is closed

Oscar 2 is open so let's just deal with

those first so if I'm touching the mouse

pointer let me go under how about looks

and we didn't use this before but

there's this block switch costume to

something else I'm going to drag and

drop this inside of the if and notice

it's a little bit indented I'm going to

change it not to Oscar 8 but Oscar 2

otherwise if not touching the mouse

pointer this is the other direction in

the fork and the road let's go ahead and

switch the costume back to what I

described as Oscar 1 so let me run this

program and not much of interest is

happening yet but notice if I move the

cursor up down but how is that working

it's just changing the costume that's

being overlaid on the Sprite so it looks

like interactivity but you're really

just changing the Aesthetics and we

humans are just kind of you know

assuming oh it's opening up well no it's

just changing a costume so here's the

difference the high level abstraction

trash can opening the lower level

implementation detail

costume changing creating that illusion

and if I wanted to look prettier I could

just have many other costumes and go

boom boom boom boom boom to create more

frames per second if you will so I need

to do um one other thing maybe if I

accidentally leave the trash can open

let me make one change here let me make

sure that the very first thing I do when

the green flag is clicked is always

start with the trash can closed because

otherwise you might accidentally leave

it open so this gets me into some

default state so now it's always closed

until I manually hover over it instead

well what might I have done next well if

I wanted to introduce something like the

trash I need a second Sprite and here in

advance I grabb the image already let me

pretend that this never happened let me

drag this away here and now I have

nothing in my code area for this piece

of trash but it is a second Sprite and

all I did was I clicked on the little

cat plus icon here created a second

Sprite I named it trash I added an

costume for it sort of the aesthetic

stuff I did in advance but here I'll do

now the code how do I want to do this

well how about when the green flag is

clicked for the trash can I want the

trash can in parallel to do or I want

the trash the piece of trash to do its

own thing so what I wanted to do is

maybe let's do motion how about and

let's go to a specific coordinate now

there's a lot of options here there's

turning go to a random position go to x

comma y Glide more elegantly there's a

lot of different ways to implement

movement I just wanted to go to a very

specific location first so I'm just

going to go to x comma y first and I'm

going to say x how about will be um uh

let's not hardcode this let's just have

it be well let's do it at zero initially

and then 240 so whoops let's do 0 comma

240 so that this piece of trash always

starts at the top middle of the screen

if you think back to that coordinate

system 0 0 is in the middle 240 is

straight above it all right now after I

do that what do I want to do well how

about I control this thing by forever

falling now how do I make the trash move

we haven't seen this puzzle piece yet

but under motion the very first thing is

called move some number of steps by

default it's 10 but we'll do it more

simply let me go ahead and

move uh oh sorry move is going to move

it uh in whatever Direction it's facing

I only want it to move down so here even

I'm getting confused as to how many

different ways there are to do things

what I think I want to do is this let me

only change my y AIS as follows so

here's another puzzle piece called

change y so again Y is the vertical so

let me just change y by one pixel

downward at a time so Nega one one pixel

at a time so it's kind of slow and I

think now I think that's it let me hit

stop notice that my trash can is still

going to be interactive I haven't

changed or deleted that code I've just

added now code for my trash piece of

trash if I click the green flag notice

that after I enable it let me start that

again I had it hidden for before class

but let me enable it now green flag

notice it starts dead center at xal 0 y

= 240 and it's dropping one pixel at a

time if that seems a little boring we

can change it to negative 10 pixels at a

time and boom it's done so that's how

you might change the speed of a program

but I'm going to leave it more simply as

negative 1 and honestly it would be nice

if it doesn't always start from the top

otherwise this game is not going to be

very interactive I'm literally going to

be grabbing the trash from the same

place every time so why don't I instead

stop this let me go under operators and

let's pick our random number so let me

change the hardcoded the manually

inputed zero and let's make X be

somewhere between zero so in the middle

and all the way over to uh what was it

one oh I got my numbers wrong 240 and my

y will be 180 sorry I got my X and my my

y confused so let me play this again and

now we have a game that's more like

games you might have played growing up

or even now like there's some Randomness

to it so the CPU so to speak is doing

something more interesting let me run it

again now it's a little to the left let

me run it again now it's a little more

to the left again now it's back to the

right so Randomness just makes games

more interesting and this is why when

you play any video game if different

things are happening there's probably

just some Randomness and it's quantized

is just a simple number now I think I

just need one final flourish here if I

may let me go ahead and add this how

about uh events or rather yes events

when green flag is clicked I can do

multiple things within the same Sprite

they don't all have to be attached to

the same one let me go ahead and forever

go ahead and do something else how about

whenever the trash is how about touching

the trash can so forever if let's see I

need a sensing block so how about is

touching uh not the mouse pointer this

time but touching Oscar himself there

now let's see what happens all right so

let's go ahead and click the green flag

now I go down over here and let

go okay that's I kind of want it to go

into the trash can how do I make it go

into the trash can how can we take this

high level idea put trash into the trash

can and make it seem to

disappear logically what could we do

yeah okay so when it touches it let's

have it disappear so I could hide it or

honestly if the game's going to be

ongoing like it was letting me Mo drop

more and more trash let me just have it

go ahead and pick a new random location

so let me do this let me go ahead and

copy this puzzle piece up here and

duplicate and I don't want the whole

thing sorry let me get rid of this let

me just do this let me go back to some

random location at the top so now notice

what happens if I click and drag on it

here it goes and I let go it looks like

it's going into the trash can because it

snaps back up to some random location

now the only thing I'm not doing really

is keeping track of any kind of score

and it turns out if I full screen this

it's not going to be draggable by

default so just as a corner case so to

speak something that you might trip over

otherwise let me go ahead and under uh

let's see uh sensing it turns out I also

need this for the piece of trash there's

this way of setting and scratch a puzzle

a Sprite to be draggable or not

draggable I need to explicitly make it

draggable so that when I do full screen

this thing now it Still Remains

draggable and someone like myself can

play it again and again well how about

we supplement this with one final

flourish why don't we keep track now of

the user score so how about when the

user actually drags the piece of trash

to the trash can let me go under

variables here where in advance I've

already made myself a variable called

score I could have called it X or Y or Z

or ABC but that's not very descriptive

in programming you typically give things

a more descriptive English or some other

language name so I called this one score

so how do I want to do this in my score

well let me go ahead and initially set

this game score to zero at the very top

of one of these Scripts

or one of these programs up here and

then anytime my cursor my piece of trash

is touching Oscar let's not just jump to

the top let's change the score by one up

here so now notice if touching Oscar

change the score that is add one to the

score and then pick a new random

location and now green flag let's do

this slowly here it goes it's the trash

can opens I let go and now notice at the

top left of my program notice the score

is now two notice the score if I do this

again is about to become three and so

here we have building blocks literally

of making this program better and better

and better and so indeed that's how you

generally approach solving any large

program uh any problem with code be it

in scratch or C or python or some other

you take this Vision you might have or

some Vision you've been assigned in a

homework assignment and try to break it

down into these constituent parts and

just pluck off the easy ones first put

the lamp post there first and at least

feel like you're making some progress

then pluck off something like the trash

can and just make it do a little thing

and it doesn't have to be in some same

order here I could have done this in a

million different ways but figure out

what the small pieces are that

ultimately like a few of the problems

we've solved today assemble into a

greater solution there too uh so that

you have now a mental model for these

types of blocks and others let's return

for a moment to this we saw a moment ago

that when I started saying hello David

and nesting those puzzle pieces we had a

whole different Paradigm Al together my

input for that second version of hello

world was to now pass in for instance

what's your name into my function called

ask that gave me not a side effect but

what I called again a return value

called answer by default in scratch and

now notice and recall when I had that

same output become the input to my next

block it looked a little something like

this say so how does this type of block

and this nesting the stacking of blocks

fit into the same mental model well same

idea my input for that part of the story

is now taking in not one input but two

two arguments hello and the answer from

before the function in this case is that

new block called join the output thereof

is hello David which itself became if we

sort of animate this the input to my

final function which indeed was still

say and this is only to say no pun

intended that almost everything that

that you do with these puzzle pieces be

in the context of Oscar time or the mole

whacking or even just something simple

like hello world will ultimately fit

into that relatively simple mental model

there now I thought we'd End by taking a

look at just a couple final examples

these ones two made by some of your

predecessors and for this I thought we

would not write code together but read

it instead and so allow me to open up

one other example here that will show us

a few different versions of a program

that a predecessor made give me just a

moment here and we'll see how we might

build up to something even more

interactive and in just a moment we'll

see something they called Ivy's uh

hardest game focused here on these

particular mechanics so here is version

zero so to speak of this program where

in the goal is to create a game where

you have to like get out of some kind of

Maze and you have to get out in this

case the Harvard Crest from this maze

let me go ahead and just hit play on

this green flag so you can see what what

the first building block for this

program might have been notice that my

hand here is actually on the arrow keys

of my keyboard and it seems that by

moving up down left or right this little

Crest on the screen responds in exactly

that way now let's hypothesize for just

a moment even though we've not done

anything quite like this before how

might this code be implemented how do

you get a Sprite be it a cat or a Crest

to respond to keys on a keyboard might

you think intuitively yeah there could

be something that's sensing what key

you're pressing on yeah there could be

something sensing what key you're

pressing on and if you do it again in a

forever Loop you'll just constantly be

listening for keystrokes and this is how

like every piece of software nowadays

works it's constantly waiting for your

phone to be tapped or something to be

typed on the screen so let me go ahead

and look inside of this existing program

here and there's more going on but we'll

take a quick glance what's actually

going on well up here at top left notice

we just have go to x equals 0 and Y

equals 0 that means put the Harvard

Crest dead center in the middle of the

stage then we have forever two functions

that we made in advance as custom

functions uh listen for keyboard feel

for walls so it's doing two things at

once it's forever listening for the

keyboard up down left right and feeling

for the walls in the sense that if I get

too far to the left I don't want it to

keep moving past that black wall and if

it moves too far to the right I don't

want it to blow through that wall either

so it's going to do two things

constantly listening for keyboard and

feeling for walls so to speak and how

are those implemented well this one's a

bit long but on the left here is listen

for keyboard so this pink puzzle piece

listen for keyboard first checks if the

key up arrow is pressed question mark

Boolean expression in a conditional

change y by one that means move it up

one else if the key down arrow is

pressed then change y by negative one

and similar for left Arrow similar for

right arrow and even though there's not

a loop in this pink function there is

where I'm using it so it's constantly

being asked again and again how about

feeling for walls well over here to the

right it's a little cut off but here you

have if touching left wall change X by

one so if you hit the wall it's too late

you're kind of blowing through it

already so I want to move it wh one

pixel so it's no longer touching that

wall similarly if it's touching the

right wall I want to back it up one

pixel so it's no longer touching that

wall so it's kind of like bouncing off

ever so slightly so that it doesn't slip

through that actual wall and what are

those walls well noce down here it's

just a simple Sprite with a black line

that I've oriented hor vertically

instead of horizontally and that's just

so that I can ask questions of these

other two Sprites now that gives me that

form of interactivity what more can I

now do well what if we make things a

little more interactive here let me go

ahead and see inside version one our

second and let me propose what's going

to happen here well how might we add a

little something like Yale into the mix

well what's Yale going to do when I I

hit the green flag now based on this

code any

hunches here is the code for my Yale

Sprite yeah yeah it's kind of got to be

an adversary by blocking my path

theoretically if I keep writing more

code so why it to goes to the middle of

the screen it points in Direction 90 de

so similarly there's a whole degree

system as well and it forever asks this

if touching the left wall or notice the

green block touching the right wall then

just turn around 180° and in if you

think this through logically that just

means you're bouncing this way and this

way by just flipping yourself around

180° for just this Yale Sprite so if I

go ahead and zoom in on this and click

the green flag I can still move up and

down but Yale is just kind of doing this

all day long back and forth and back and

forth forever nothing bad happens if I

try to go through it but we could add

that certainly to the mix in fact let's

add one final feature before we play

this particular game and let me go ahead

and open up the final version of these

building blocks that adds MIT to the mix

so here's

MIT someone want to explain what this

code does and this is what we're doing

this itself is a skill reading someone

else's code and understanding it is half

of the part of programming besides

writing yeah yeah it's chasing down the

Harvard logo outline so this is

apparently the name of the costume that

this student made Harvard logo outline

outline and apparently it goes to a

random position first but then it

forever points to Har so no matter where

I'm moving it up down left or right MIT

is being a little more strategic than

Yale bouncing back and forth like this

so let's go ahead and play this one in

full screen and here we have a green

flag so if I move up

MIT rather strategically is following me

no matter where I go all right so still

nothing bad happens but and now it's

struggling right it's going up down up

down it's trying to uh follow me even

though I'm not moving so we need some

final flourishes and so I think for this

we need perhaps one final volunteer

after this cake awaits for everyone

outside as is an end of first lecture

cs50 tradition would you like to come up

and be our our vinyl of

volunteer all right and so this will be

the actual version bit written by one of

your predecessors that I'll full screen

here it's going to stitch together all

of these same Primitives in more but add

the notion of score and lives so that

there's actually a goal which in this

case is to move the Harvard Crest to

constantly pursue the character on the

right hand side so that your Sprite

touches that one would you like to

introduce yourself uh hi my name is

Muhammad all right wonderful welcome

aboard and here we come with some

instructions and final flourish if we

want to keep the lights up but perhaps

increase the

music

[Music]

e

[Music]

[Music]

you

[Music]

all right this is cs50 and this is week

one wherein we continue programming but

we do it in a different language because

recall last time we focused on this

graphical language called scratch but we

use scratch uh not only because it's

sort of fun and accessible but because

it allows us to explore a lot of these

Concepts here Nam functions and

conditionals Boolean Expressions Loops

variables and more and so indeed even if

today's syntax as we transition to this

new language called C feels a little bit

cryptic maybe a little intimidating at

first and you don't quite see all of the

meaning of the symbols beyond the syntax

itself realized that the ideas are

ultimately going to be the same in fact

as we transition from what was last week

a hello world program that looked a

little something like this this week of

course it's going to now look a little

more cryptic it's going to look a little

something like this and now even if you

can't quite distinguish what all of the

various symbols mean in this code turns

out that at the end of the day it's

indeed going to do what you expect it's

just going to say hello world on the

screen just like we did in scratch so

let's start to apply some terminology to

these to these uh tokens first so what

we're about to see what we're about to

write henceforth we're going to start

calling source code code that you the

human programmer write is just

henceforth called source code doesn't

matter if it's scratch doesn't matter if

it's C doesn't matter if it's python

before

source code is the general term for

really what you and I as human

programmers will ultimately write of

course computers don't understand source

code it turns out computers don't

understand scratch and puzzle pieces per

se or C code like we're about to see

they only understand this which we

called what last week yeah so this is

binary zeros and ones but really it's

just information represented in binary

and in fact the technical term now for

patterns of zeros and ones that a

computer not only understands

understands how to interpret as letters

or numbers or colors or images or more

but knows how to execute as well

henceforth is going to be called machine

code to contrast it with source code so

whereas you and I the humans write

source code it's the computer that

ultimately only understands machine code

and even though we won't get into the

details of exactly what pattern of

symbols means what you'll see that in

this kind of pattern of zeros and ones

there's going to be numbers there's

going to be letters but there's also

going to be instructions because indeed

computers are really good at doing

things addition subtraction moving

things in and out of memory and suffice

it to say that the Macs the PCS the

other computers of the world have just

decided as a society what certain

patterns of zeros and ones mean when it

comes to operations as well so not just

data but instructions but those patterns

are not something we're going to focus

on in a class like this we're going to

focus on the higher level software side

of things simply assuming that we need

to somehow output machine code so it

turns out then that this problem we have

to solve getting from source code to

machine code actually fits into the same

Paradigm as last time but the input in

this case is going to be source code on

the one hand like that's what you and I

ideally will write so that we don't have

to write zeros and ones but we need to

somehow output machine code because

that's what your Macs PCS phones are

actually going to understand well it

turns out there's special programs in

life whose purpose is to do exactly this

conversion convert the source code you

and I write to the machine code that our

phones and computers understand and that

type of program is going to be called a

compiler so indeed today we'll introduce

you to another piece of software and

these come in many forms we'll use a

popular one here that allows you to

convert source code in C to machine code

in uh zeros and ones now you didn't have

to do this with scratch in the world of

scratch it was as simple as clicking the

green flag because essentially MIT did

all of the heavy lifting there figuring

out how to convert these graphical

puzzle pieces to the underlying machine

code but now starting today as we begin

to study programming and computer

science proper now that power moves to

you and it's up to you now to do that

kind of conversion but thankfully the

fact that these compilers exist means

that you and I don't have to program in

machine code like our ancestors Once

Upon a Time did be it virtually or with

physical Punch Cards like pieces of

paper with holes in them you and I get

to focus uh on our keyboard as such but

it's not just going to be a matter today

of like writing code it's going to be a

matter ultimately today onward of good

code as well and this is the kind of

thing that you don't just learn

overnight it takes time it takes

practice just like writing an essay in

any subject might take time and practice

and iteration over time but in a

programming class like cs50 we're going

to Aspire to evaluate the quality of

code along these three axes generally is

it correct first and foremost like does

the code do what it's supposed to do

after all if it doesn't well what was

the point of writing it in the first

place so it sort of goes without saying

that you want code you write to be

correct and it's obviously not always

again anytime you're Mac or PC or phone

has crashed some human somewhere wrote

buggy that is code with mistakes but C

correctness is going to be the first and

foremost goal but then there's a more

subjective goal we'll see in time a

matter of design and we saw a little bit

of this last week when I proposed that

we could design even scratch programs

better maybe by using Loops instead of

just by copying and pasting the same

blocks again and again so design is more

subjective it's more of a a learned art

whereby two people might ultimately

disagree as to which version of a

program is better designed but we'll

give you building blocks and principles

over the coming weeks so that you can

have a better sense for yourself if your

own code is well designed and why is

that valuable well the better design

your code is often the faster it's going

to run the more maintainable it's going

to be by you or colleagues if you're

working with others in the real world so

good design is a good thing it helps you

communicate your ideas just like an

typical English essay and then lastly

we'll talk this week onward about style

and this is really just the Aesthetics

of your code it turns out that computers

often don't care how sloppy your actual

code is um where uh in the world of code

it turns out that you don't really need

to indent things in a beautiful way you

don't need to paginate things like you

might in an essay the computer generally

does not care but the human does the

teaching assistant does you will care

the next day when you're just trying to

understand what your code does so we'll

focus lastly on Style the Aesthetics of

the code that you're writing so where

are we going to write code where are we

going to compile code so for this class

not only with C but the other languages

we use later in the term we're going to

use a free text editor that is program

called Visual Studio code AKA vs code

it's super popular nowadays not just for

C but for C++ and Python and Java and

any number of other languages it's a

text editor in the sense that it lets

you edit text and that all that's all

code is going to be now strictly

speaking you could write code on paper

pencil in fact in high school if you

took a class you might have done that

one or more times as sort of an in-class

exercise you can't run it on paper of

course but you could write it certainly

you could use something like Microsoft

Word or notepad.exe or text edit on the

Mac but none of those programs are

really designed to format the code in

the best way for you nor are they

designed to let you compile and run the

code so VSS code is going to be a tool

via which you can do all that and more

write the code compile the code run the

code so that you all don't have to

wrestle with stupid technical support

headaches at the beginning of the course

by installing this software and that on

your Macs or PCS we'll use a cloud based

version of VSS code at code. cs50. and

that's going to be the exact same tool

and the goal then is by the end of the

semester to sort of uh migrate you off

of that cloud-based environment to your

own Mac and PC so that even if cs50 is

the only CS class you ever take you're

100% equipped to continue writing code

after the class using not something

that's even cs50 specific but a de facto

industry standard at least for some time

so what's this program VSS code going to

look like be it on your Mac PC or

initially in your browser and it's going

to look a little something like this and

there's going to be several different

regions to the screen and picture here

is that very same code I keep proposing

is the simplest program you can write in

C and what are these different regions

of the screen well there's essentially

these four here so first highlighted up

top is going to be one or more tabs

where you're going to actually write

code so much like in Google Docs or

Microsoft Word you can have tabs open

with files similarly in VSS code or

really any programming environment do

generally nowadays have tabs of some

sort and this is going to be a tab

containing a file it seems called

hello.c and that's going to be the very

first file we write in just a moment uh

down here though is going to be an

interface that many of you might not

know this is what's called a terminal

window and a terminal window provides

what's generally called a commandline

interface or CLI and this is in contrast

with a graphical user interface or guey

now you and I every day are using guies

on our phones on our PCS and a guy is

literally graphical so menus and buttons

and icons and you generally use your

finger or a trackpad or a mouse or

something like that to interact with it

but it turns out that many programmers

dare say most programmers at least over

time come to prefer not a guey but a CLI

a command line interface where you

actually do everything somewhat uh

somewhat arcely via keyboard alone why

well it turns out there's just more

features built in to most computers if

you can access them with a keyboard

turns out you can most of us can type

faster than you can point and click and

so that ends up being an efficient gain

over time so in time will you get

comfortable using this terminal window

to do things like compile your code or

make your program as well as run it so

you won't be in the habit initially of

just double clicking icons like we do in

our typical real world you'll do it sort

of the programmer's way but it's not to

the exclusion of adding icons and

clickability and more on the left hand

side of vs code there's going to be a

somewhat familiar file explorer some

kind of hierarchical tree like on your

Mac or PC where you can see all of the

files in your account pictured here for

instance is just hello.c which I'll

create myself in a moment and then far

away on the left is the so-called

activity bar and this is where you just

get a lot of traditional menus and

buttons so VSS code itself gives you

both a guey and a CLI but it's within

the CLI the terminal window the bottom

region of the screen that we're actually

going to type most of our commands and

in general in class I'm going to hide

all of the graphical stuff that's just

not of all that uh that much interest so

with that said let me actually change

over to a live version of vs code and

I've indeed HD in the activity bar I've

indeed hid in the file explorer so what

I have here for visibility sake is a

really big area for writing code and a

really big terminal window at the bottom

you'll see in the terminal window

there's a dollar sign and this doesn't

mean any form of currency this is just

the standard symbol that represents type

commands here so the fact that there's

just a dollar sign in a cursor means

eventually that's where I'm going to

type commands but first i'm going to

actually create some code so how might I

program using vs code be it on my Mac PC

or in this cloud-based environment that

you'll get set up for problem set one go

about writing my first file well perhaps

the easiest way is this literally run

the command code and then the name of

the file you want to create notice that

I deliberately end the file with C in

lowercase notice that I've deliberately

lowercased the whole file name and these

are just conventions you could use a

capital H you kind of could use a

Capital C but just don't do that follow

best practices so that it's consistent

with what most everyone else would do

when I hit enter I just get an empty tab

just like the screenshot a moment ago

and it's in this tab where I can now

write my very first program in C

unfortunately it's not quite as user

friendly as scratch where you drag and

drop a couple of puzzle pieces and boom

it's done so I'm going to do this for

memory but this too will become familiar

to you over time I'm going to include

something called standard io. I'm going

to type int main void and parentheses on

a new line I'm going to insert some

curly braces as we'll call them and then

I'm going to type print F and then some

parentheses and then in quotes hello

comma world then a backslash then a

lowercase n then a close quote and then

a semicolon at the very end of the line

so all I've done is recreate just from

memory that very first program in a

little bit we'll make clear what most of

this does but for now let's just

actually run this thing and just like I

click the green flag last week for the

first time let's actually compile and

run this program if it were your Mac PC

and Google or Microsoft or someone else

had made the software at this point in

the story we'd be double clicking an

icon but we can't do that yet this is

still source code so I'm going to click

back down in my terminal window notice I

have a second dollar sign below the

first which just means it's ready for a

second command and now the command via

which to make this an actual program to

compile it from source code to machine

code is going to be quite simply make

and then the name of the program I want

to make slight subtlety I'm omitting

deliberately. C because the program I

want to make I just want to call hello

so don't write make hello. C just write

make hello and this program make is

essentially our compiler technically

speaking it's a program that automates

the compilation of my program for me but

it is going to see that I've typed the

word hello it's going to automatically

look now for a file on the hard drive

called hello.c and convert it from

source code in C to machine code in

zeros and ones so if I didn't make any

typos

enter nothing seems to happen and that's

a good thing almost always if nothing

gets outputed on the screen like you did

good like you didn't make any mistakes

you didn't get yelled at there's no

error messages so this is actually a

good thing how do I now run this program

well notice I've got a third dollar sign

which just means I'm ready for a third

command and now I'm going to go ahead

and run dot slash hello and this is

admittedly a little weird that you have

to do dot slash but for now just take on

faith that this is how you run a program

called hello in your current folder in

your current directory in this

cloud-based environment all right

crossing my fingers again hitting enter

and voila my very first program in see

hello world and now let me go ahead and

reveal the file explorer that I proposed

exist earlier I'm just going to use a

keyboard shortcut to reveal that and

generally I keep it close because I

don't really need to know constantly

what files are in my account but you'll

see now in the file explorer similar

INSP to a Mac or PC but graphically a

little different here's my file hello.c

it's highlighted because I have that tab

open but now there's a second file here

called just hello that's the name of my

program so if you were on a Mac or PC

you would ideally double click that

thing you can't do that in a command

line environment you have to run it down

here but that's all we've done we've

created a file called hello.c and then

my compiler made the program from that

let me pause here and see if there's any

questions because it's a lot of magical

phrases

yeah yeah so if you're currently

following along uh playing along at home

and you're getting some kind of error

message part of today will be for me to

deliberately induce some of those error

messages for now let me just propose

that if you literally did what I did you

must have made a typo somewhere and

notice that it's indeed standard IO

stdio.h uh maybe you type

studio. okay super common mistake if I

don't if I could call you out um like it

is not studio. it is standard io. so

common but this is exactly

representative of like the kind of

stupid headaches you're going to run

into this week probably for a few weeks

probably honestly for a few years but

you start to see past these sort of

stupid mistakes over time and it just

gets easier and easier because the

computer is going to be so regimented

like you it will only do what you tell

it to do and if you say because it's

verbally sounds like studio. it's not

going to know what the file is so

actually thank you for tripping over

that so early that's super common to

happen yeah have two hello so I why do I

have two hello files so why do I have

two hello files one is the one I created

as the human called hello.c and it's

pictured right here but then when I ran

make hello that process compiled my

source code into machine code so this

second file just called hello is the

file that contains all of those zeros

and ones that the server actually

understands all right so yeah

question if you try clicking on the

Hello file you'll see in this

environment of vs code quote unquote the

file is not displayed in the editor

because it is either binary AKA zeros

and ones or uses an unsupported text

encoding in this case it's binary it's

zeros and ones now you could use

software to see those zeros and ones it

won't be intellectually enlightening to

most any human so VSS code just takes

the choice of not showing it to you at

all so that would be a common mistake

too clicking on a file you don't intend

but the source code is indeed going to

be editable by us all right all right so

I've written this program it seems to

sort of magically work at least with

some effort if you get every single

keystroke right well what is it that's

going on and how is this working well

first of all notice that even without my

highlighting things or choosing buttons

from menus notice that it's already kind

of colorcoded and yet I wasn't

highlighting along the way in sort of

Google doc style changing the color

certainly well it turns out what VSS

code and most programming environments

nowadays do for you automatically is

syntax highlighting so syntax

highlighting is just this feature of

typical text editors nowadays that

analyzes the code that you've typed and

when it notices certain types of

keystrokes things that represent

functions or conditionals or Loops or

variables a lot of the vocab from last

week it just highlights it ever so

differently for you so main for instance

which we'll soon see is in purple here

int and void and include are in red

hello world is in blue my parentheses

are in green this will totally vary by

programmer too in fact if you do want to

change these colors for problem set one

for your own environment you can poke

around vs code settings via the gear

icon you can change a different a change

to a different color theme syntax

highlighting isn't some specific color

scheme like it is in scratch it just

generally is to each human their own

preference but that's all that's

happening here is this notion of syntax

highlighting at the moment well what

more is going on in this code before I

run it but rather write it well it looks

a little something like this if I take

away all of the colors and then just for

discussion sake let me ahead and color

it a little more like scratch recall

that our very first scratch program that

just said hello world on the screen had

a when green flag clicked icon uh puzzle

piece roughly an orange and then a

purple uh sa block beneath it so whereas

this is the C version if we rewind to

last week this was the same program in

scratch but what's happening now is

exactly the same so if you think back to

last week and you've got some function

like the say function in purple that

might take one or more arguments like

inputs that influences what it says on

the screen and then functions recall can

sometimes have side effects right like

the speech bubble appears on the screen

so last week when we used the sa block

and we passed in an argument of hello

world at left we got this visual side

effect on the screen that says now hello

world in the speech bubble and that's

exactly what just happened in VSS code

but much much more textually and let's

look a little closer now at the code

itself let me wave my hand at the

equivalent of the when green flag

clicked part of my code and let's focus

only on the say block in scratch and the

corresponding function in C so if I step

through this and I wanted to convert

what we did last week with the say block

to C I would first use the print

function although that's actually a bit

of a white lie it's actually the print F

function print f means formatted and

it's just a function that allows you to

format text on the screen There Is No Sa

function in C there's a print F function

what MIT did down the road years ago go

was they took what existed historically

as print F and they simplified it for a

broader audience by just calling it

essentially say instead but notice that

now if I want to convert the scratch

code at left to C code at right it's

sort of the same shape so MIT

deliberately used this white oval if

only because it kind of conjures these

uh this idea of having parentheses too

so on the right if I want to pass an

argument or an input to the print F

function I use an open parenthesis and a

CL parenthesis in those parentheses I

then type whatever it is I want to print

on the screen in this case hello comma

world but notice I've deliberately left

some room because you need some extra

keystrokes in the world of C anytime you

type out some text otherwise known as a

string of text to use computer science

jargon you need to quote it in this case

with double quotes double quote at the

left double quote at the right and

notice to I'm going to include some

slightly cryptic symbol here too back

sln which I also typed and said verbally

earlier and then one last nuisance at

the end of this which is a semicolon so

suffice it to say this is why we start

with scratch this drag and drop you're

good to go in a language like C print F

parenthesis double quotes the text you

want backs slash and semicolon at the

end there's just so much syntactic

overhead but at the end of the day it's

just a function and you'll get used to

these sort of nuisances like the

parentheses the quotes the semicolon and

the like but things can very easily go

wrong and it's very easy to make

mistakes even with lines of code like

this so let me do this let me go back to

vs code where I have the exact same code

notice that on line five is exactly that

line of code so this is the equivalent

of the sa block and let's consider what

mistakes I may make early on or even now

20 years later after learning this that

are quite common um in general suppose I

forget the semicolon there so easy to do

you will do this eventually let's see

what happens now when I go back to my

terminal window and try to compile my

code again just to keep things tidy I'm

going to clear my screen but that's just

for lecture sake so that we can focus

only on the most recent command but I'm

going to go ahead now and rerun make

Hello this will ensure that my program

is recompiled and this is a manual

process I changed my code the zeros and

ones on the hard drive have not changed

I need to recompile it to Output the

latest machine code so here we go I'm

going to hit enter crossing my fingers

as before but again I remove the

semicolon by

accident oh my God there's like more

lines of Errors now than there are of

actual code and this too takes them

getting used to um the programs we're

using were not necessarily written with

the least comfortable audience in mind

but really professional programmers back

in the day but through practice and

through experience and through mistakes

you'll start to notice patterns here too

so here's what I typed make hello after

the dollar sign prompt now I get yelled

at with as follows hello.c colon

529 well what's that referring to I've

screwed up somewhere on line five on the

29th character on that line generally

the specific character is not that

useful unless you actually want to count

it out but line five is a good clue why

it means I screwed up somewhere on line

five here all right well what is the

error expected a semicolon after

expression and this error is actually

pretty obvious now that I see it and I

realize oh wait a minute all right

here's my line of code here in sort of

asky art so to speak textual uh text

representing Graphics it wants me to put

in green here this semicolon at the end

of that line one error generated built

in so some esoteric stuff there but my

program did not compile when you see an

error like this it means it did not work

so what's the fix well obviously the fix

is to go back up here put the semicolon

there and now if I recompile my code

with make hello I won't clear my screen

just yet just to show you the difference

now it just worked so we're back in

business as before all right let me

pause here though and ask if there's any

questions about what I just did

[Music]

these error messages will become

frequent initially

yeah really good question do you need a

semicolon after every line or just some

it turns out just some uh this is

something you'll learn through practice

through demonstrations and examples

today generally you put a semicolon

after a statement so to speak and this

is the technical term for this line of

code it's a statement and think of it is

it's kind of the code equivalent of like

an English sentence so the semi colon

and code is sort of like a period in

English when you're done with that

particular thought you don't need

semicolons for now anywhere else and

we'll see examples of where else you put

them but it usually is at the end of a

line of code that hasn't that isn't

purely syntactic like uh curly braces

instead other questions on the mistake I

just fixed and created for myself

[Music]

yeah uh correct so line five is where

the error is most likely character 29

means it's sort of 29 characters that

way and then it's actually in this case

giving me a suggestion the compiler

won't always know how to advise me

especially if I've made a real mess of

my code but often it will do its best to

give you the answer like this

yeah ah so how come I first typed code

space hello.c and now I'm typing make

hello two different processes so when I

typed code space hello.c that was

because I wanted to open vs code and

create a new new file called hello.c

it's like going to file new in in a a

Mac or PC thereafter though once the

file exists and is actually open here

and it does autosave you don't need to

hit command s or control s all the time

I can now compile it with make hello

again and again so theoretically I

should never need to type code space

hello.c again unless I want to create a

brand new file called the same thing all

right so what about this other piece of

syntax here let me clear my terminal

window here you can also hit control L

just to throw everything away just to

clean it up aesthetically suppose that I

omit whatever this sequence of symbols

is back sln since I'm not really sure at

first glance why that's even there does

anyone want to conjecture especially if

you've never programmed before what

might happen now if I recompile and

rerun this version of the program I left

the semicolon but I took away the backs

slash

n any

instincts all right well yeah

will the next dollar sign appear

straight after your it will the next

dollar sign will appear right after my

hello world but what makes you think

that back creates a line exactly back

sln is actually a special sequence of

symbols that creates a new line and so

to your point if I recompile this

program make hello enter no syntax error

so it did compile this time so you don't

need the backs slash in you do need the

semicolon but if you don't have the back

sln watch what happens when I do/ hello

this time now indeed I see hello comma

world and then a weird dollar sign and

this is still a prompt I can still type

commands at it like clear and everything

gets cleaned up but it just looks kind

of stupid if I run it again here with

hello you know it's just not very user

friendly it is convention that when

you're done running your program you

should ideally clean things up move the

cursor to the next line for the user and

so the backs slash n is simply the

special symbol otherwise known as an

escape

sequence that c knows means move the

cursor to the next line in other

languages python among them uses the

same symbology as well now if I go back

to the code here and for instance I try

to do this differently like suppose I

don't put the backs slash N I just hit

enter like a normal person would in

Google Docs or Microsoft Word let me go

ahead and try compiling this program and

this you would hope would work right you

would hope this would print out hello

world and then a blank line because I

move the cursor to the next line but no

if I run make hello now and try to

compile that c does not like this now I

get a different error still on line five

this time starting at character 12 uh

error missing terminating double quote

character and then some other esoteric

stuff and then this does not sound good

fatal error this time too many errors

admitted stopping now so I really

screwed up here so why can't I do this

just because like the humans who

designed C decided that if you have a

string of text it must stay on the same

line it can get really long it can soft

wrap that is without you hitting enter

but you can't hit enter to create a new

line if you deliberately want a new line

you have to indeed use this back slash

and Escape character so let me go ahead

and do this let me put it back let me go

back to my terminal window I'll clear

the screen again let me go ahead now and

do make Hello to recompile to that

version do/ hello and voila we're back

in business with uh hello all right so

now let's tease apart some some other

aspects of this code because there's a

lot going on just to get us to say hello

world on the screen for today we're

largely going to ignore this int main

void and these curly braces here we'll

come back to that before long as to why

it's there but for now just think of int

main void and these curly braces here as

really being the C equivalent of when

green flag clicked like why you just

need it there that's how you get your

program going and Maine is indeed going

to be some special function but more on

that another time but why do I have this

line of code here the cor spelling is

indeed standard io.

stdio.h and they're angled brackets this

time so that's a little new there's a

hash and then an include keyword you

know if you don't know what something is

you know there's not really that much

harm in just getting rid of it and see

what happens so let me delete that line

let me go back to my terminal window

clear the screen and then run make hello

again and let's try compiling this

program now without that first line why

I don't understand it so let's see what

happens all right here's yet another

error but let's see how hello. C line

five character 5 so it's pretty early on

error implicitly declaring Library

function printf with type int and then

dot dot dot so implicitly declaring

Library function print F so this is very

cryptic sounding you'll get better at

understanding phrases like these but

apparently I do need the include line

for standard

i.h but why based on this symptom what

might Your Instinct be for what that

first line of code is doing for us in

the first place why intuitively must it

be

there exactly it's like importing a

library so that you can do things like

print things out on the screen now in

scratch you didn't have to do this for

most of the puzzle pieces but you might

recall that partway in through week zero

I went to the extensions button at the

bottom left of the scratch screen and I

imported some extra puzzle pieces for

text to speech that gave us the sort of

creepy uh humanized voice that actually

came out of the cat's mouth well that

was like adding a library code that

someone else wrote in that case it was a

third party then but I gave myself

access to it same here turns out that

you don't really get printf

automatically in C you have to include a

so-called header file that declares that

function to exist now the reason for

this historically is just efficiency

back in the day when computers were

really slower and resource constrained

you don't want to just give yourself

access to the entire kitchen sync of

functionality you only want to include

only the functions you actually care

about nowadays it's sort of a sort of a

a copy paste step because you almost

always want to print something out on

the screen at least when writing

programs like these but these so-called

header files contain enough information

about all of the functions in What's

called the standard IO library and

standard IO just means standard input in

output and that's appropriate right

because printing is pretty basic output

turns out there's other functions for

getting input from the human's keyboard

more on that in a bit but anytime you

want to print something on the screen

and see you indeed need to include this

header file at the top of your code and

that's going to essentially inform the

compiler hey compiler I want to use

functionality from the standard IO

Library including printf in this case

and if you omit the header file by

accident it's just not going to work

because it doesn't know what print f is

it's sort of some unrecognized symbol in

that case all right questions then about

this line of

code this line of code here or what

these header files

are all right you might wonder well how

do you know what functions exist how do

you know what files you might indeed

want to include well it turns out that c

is a many-year old language and it has

ample documentation a caveat is that its

documentation isn't necessarily all that

userfriendly but what we have for the

course is a simplified version of the

official documentation for C at this URL

here manual. cs50. so in the world of c

and other languages too there are what

are called manual pages and these are

just like text-based documentation that

honestly is typically written in a voice

that you kind of have to be an

experienced programmer to understand

some of it so what we've done it this

version of the same documentation is

we've imported all of the original

official documentation but we've added

sort of less comfortable translations in

English for a lot of the functionality

that you might use in class just to help

onboard you so at the end of the day you

don't need this documentation long term

but just to get started we'll translate

it into terminology uh that you might

appreciate from a teaching assistant for

instance as opposed to the original

author of these documents and so for

instance if you were interested in

reading up on what functions exist in

the standard io. um header file well you

could go to a URL like this or you could

search for it at manual. cs50. that

would show you a list of all of the

available functions in that library and

print F indeed would be one of them and

then you could click further on that

reaching a URL like this that's just

going to give you all of the

documentation for how to use print def

it turns out you can do even more than

it uh than just printing out hello world

and we'll scratch the surface of that

today but it turns out that the

documentation will always be your

authoritative Source ultimately for

questions like what can I do and how can

I do it meanwhile it turns out that cs50

has its own library in A Accessible via

header file called cs50.h it turns out

in C that output is actually pretty easy

relatively speaking once you get used to

all the curly braces parentheses quote

marks and the like but input is a little

more difficult and if you have

programmed before input's not that hard

to do in Python it's not that hard to do

in Java it's more difficult to do in C

and we'll see why in a couple of weeks

but for the first couple of weeks of the

class we actually provide you with some

training wheels of sorts whereby we have

a number of functions that are declared

in this file cs50.h it lives its

documentation at a URL like this this

and in a moment we'll use a few of these

you'll see that cs50 provides you with

some functions like get Char forget a

single character from the user's

keyboard uh get int to get an integer

from the user's keyboard uh get string

to get a sequence of text from the

user's keyboard and a bunch of others as

well so let's actually use some of these

functions how about by revisiting really

the second program we wrote in scratch

last time which adds some input to the

output so first version of scratch was

just hello world said the same thing

every time you click the green flag

version two recall though did this it

asked the user what's your name and then

that somehow gave it back a a return

variable return value we called it and

we then joined hello in that name to say

something a little more interesting on

the screen so what did that model look

like same thing as before we've got a

function in the Middle where function is

like the code implementation of our

algorithm that takes in one or more

arguments like what is it you want to uh

say on the the screen ultimately and

return value in this case is going to be

actually a value that comes back so in

the case of getting input we can

consider this ask block again like last

week the input to it is whatever words

of English you want to ask the user and

then it returns a value and this was

called by default in mit's World answer

that we'll see and see you can call

these return values anything you want

ultimately in variables but this is

different from a side effect a side

effect is just something visual often

that happens on the screen like the

speech bubble or hello world a return

value is actually a value you get back

from a function that you can use or

reuse so how do we convert this scratch

block from last week to C code this week

well if you want to ask the user for

something like their name you can do

this you use a cs50 function called get

string and you use the parentheses to

represent here comes the inputs there

too you can then put the sentence you

want to ask the user quote unquote

what's your name but you do indeed need

the quotes literally in C so I'll go

ahead and add those as well well subtle

but I've deliberately included a space

after the question mark but before the

double quote just so that the cursor

moves one step over because in this case

we're not going to get a special speech

box like we did in scratch it's just

going to leave the cursor where it is so

we'll see that aesthetically this just

moves the blinking cursor one space

after the sentence on the screen all

right but the catch is with scratch we

just automatically got back the answer

from the user in a special variable

called answer in C you're going to have

to be a little more specific in C if you

want to get back a return value from a

function like get string you have to use

an equal sign and then the name of a

variable on the left the choice of

variables is up to you I could have

called this anything X YZ I'm going to

more descriptively call it answer for

parody with what MIT did with scratch

but notice that this doesn't represent

equality per se this is assignment in

this case so in C when you use a single

equal sign that means means copy the

value on the right over to the value on

the left from right to left so what does

this do for us well if get string is a

function that prompts the user with

quote unquote what's your name and it

has I claim a return value that means it

kind of hands me back some value but

it's up to me in C to do something with

that value so if I want to copy that

value into a variable that I can use and

reuse I use an equal sign and I invent

on the left hand side of that equal sign

any variable name I want there's certain

rules certain conventions but generally

if you use a single word with all

lowercase you're in good shape but C is

a little more pedantic than that and

those of you who have have programmed

before might not be used to this for

instance in Python which is a world

we'll get to in a few weeks you also

have to tell see what type of value

you're storing so if I do want a string

of text from the user so not an integer

not a single character I want a whole

string of text like a phrase a sentence

a name in this this case I have to tell

C that this variable is of type string

so it's a little wordy but you get used

to it and you just have to be precise

you're informing the computer what type

of value is going in this

variable all right it's so close to

being correct but I have omitted

something that's annoyingly important

still what's missing still yeah so

semicolon this is a statement this is

like a a a full thought if you will in

code I do need to end It ultimately with

the semicolon at the end there all right

so this was more of a mouthful but let's

try using this in now my code let me go

back to vs code where I have version

zero of my code here let me go ahead and

include one other file at the top of

hello.c namely include cs50.h so that I

have access now to get string and

anything else I might want now let me go

ahead and add a line of cod here inside

of these curly braces and let me go

ahead and do this string answer equals

get string quote unquote what's your

name question mark I'm going to add an

extra space before the double quote I'm

going to indeed end my thought with a

semicolon and now let me deliberately

make a mistake just to make a point here

let me now try changing hello world to

hello comma answer all right now perhaps

even though this is some new lines of

code you can see where I've aired

already but let me try making this

program now so far so good so no error

messages so that's a good thing let me

go ahead and run /hello and you'll see

the prompt what's your name question

mark and notice the cursor indeed one

space to the right just because I

thought it would look prettier to put a

little Blank Space there as opposed to

leaving it right after the question mark

let me type my name but even if you've

never programmed before I have screwed

up here what are we going to see on the

screen when I hit

enter yeah hello answer most likely why

cuz the computer is going to take me

literally and if I say quote unquote

hello answer that is the string of text

followed by a new line that's going to

be outputed to the screen so we need

some way of actually plugging answer

into this line of code it's not quite as

simple as scratch where you could just

grab like a second say block and drag

and drop the variable there we actually

need a new syntax and it's going to look

weird at first but it is everywhere in

software nowadays especially in the

world of c and certain other languages

so let me go ahead and propose that I

solve it as follows well back when we

did this in scratch remember that the

most elegant solution was this here we

used the say block still which is going

to be analogous to print F today but I

use the join puzzle piece in scratch to

combine hello comma space and then the

name of the human so how do we translate

this code to C well it's going to look a

little different now I'm going to start

with print f with some parth C's and a

semicolon representing the say block but

how do I now do this joining this is

where the puzzle pieces don't quite

translate perfectly this would be the

way to do this you put hello comma and

then a placeholder so this is what's

known as a format code in C specifically

for printf and it just means this is a

placeholder for a string again a string

is just text so this means hey computer

print out literally hello comma space

and then not literally percent s percent

s is uh treated specially to mean plug

in some value here all right so what

else do I still need well this is still

some text so I'm still going going to

surround the whole thing with double

quotes I'm still going to include my

back sln just to keep things tidy and

move the cursor to the next line so the

last step here in C is to somehow join

the answer with that word hello and the

way you do this is with printf passing

it not one argument which is what I keep

doing I keep passing it one string of

text quote unquote I'm going to now add

a comma and then the name of the value

that I want want printf to go back and

plug into that percent s and printf is

just smart about this if you have 1% s

and one additional argument after a

comma it just does from right to left it

plugs it in if you have two percent S's

and two variables after the comma that's

okay too if you separate them with

commas it'll plug the first into the

first percent s and the second variable

into the second percent s so it's just

left to right order of operations it's

not as pretty or as uh simple as this

but this is how it's done in C all right

let me pause because this is a lot of

symbology any questions on this

technique here

[Music]

yeah yeah really good question why did I

exclude the backslash n a moment ago

really just my sense of Aesthetics if

you will No Good Reason beyond that so

if I look back at my code you quite

rightly noticed that I didn't have a

backslash in there that's just because

for whatever sense of style that I have

I wanted the name to be typed right

after the question I totally could have

added a back sln there instead of a

space that would have just allowed me to

type down here totally fine just wanted

to show you something different good

good catch

yeah can I show an example with 2% s's

surely so let me uh in vs code do this

let me clear my terminal window to clean

things up and let me do this instead of

calling the variable answer all over the

place let me call it uh first and I'll

ask two questions what's your first name

and now let me do string last equals get

string quot whoops capitalization

matters so let me fix my capital S there

quote unquote what's your last name

question mark semicolon and now we'll

plug in one percent s and a second

percent s and now I'm going to plug in

first first and last last coincidentally

and now I'm going to go back to the

terminal window make hello crossing my

fingers

all good/ hello here's my first question

David here's my second question me and

again hello David Ma so it just inserts

them left to right all I was doing for

parody with scratch though and let me go

ahead and undo this again I'll go back

to answer like this I'll go back to just

asking for the person's name I'm going

to delete mention of last I'm going to

delete mention of the second perc s and

now if I recompile this simpler

version I did screw up didn't intend it

what did I do

wrong yeah so just newbie mistakes so I

changed my variable back to answer just

to be consistent with week zero but I

didn't change it here so I have an use

of Undeclared identifier first it's

Undeclared in the sense that I declared

answer align prior I didn't declare

first so indeed intuitively I want to

just change that to that let me now do

make hello again /hello type in just my

first name this time and there it is

hello David questions on this then

syntax with print F

yeah uh the placeholder I'll zoom in is

just a single percent and then an S so

inside of my string here is percent s

and then I have a comma outside the

quotes and then the name of the variable

whose value I want to plug in for that

percent s and now notice there's

technically two commas in side of these

parentheses on line seven and yet I

claim that printf at the moment is only

taking in two

arguments why is there then two commas

but only two arguments if there were two

commas you would think there would be

three arguments

right exactly the comma in between the

quotes is just an English thing it's

separating the hello from the name so

that's why indeed it's not only in

quotes that that's also why programs

like VSS code tend to syntax highlight

it a little differently just so that it

sort of jumps out as different to you

even though in this case it's a little

subtle a light blue versus white but

indeed it's trying its best other

questions now on this placeholder

yeah ah good question if I wanted to add

an exclamation point after the name

would I have to add another placeholder

and so forth I could actually do that

much more simply I can just put the

exclamation point right after the

percent sign I don't need an additional

placeholder per se if I zoom out now and

run make hello again/ hello and type in

just my name no exclamation point now

you'll see more excitedly hello comma

David So print f is smart it will figure

out where the percent s is and then go

and replace it now let me propose that a

common thing in programming is that as

soon as we make a decision as to how to

design something we often paint

ourselves into a corner and sort of

regret a decision can anyone think of a

problem that arises from using percent S

as a

placeholder in this string to print F

what could go wrong if we're using

percent in this special

way yeah if you literally want to say

for whatever weird reason percent s on

the screen or honestly even just a

single percent it turns out that a

percent sign is treated specially inside

of of print F strings so what's the

solution here there's different uh

patterns of solutions to problems like

these but suppose you wanted to say uh I

got 100% for instance let me let me go

ahead and change this completely so I

got 100% on your test or whatever all

right let me go ahead and run make hello

enter all right so invalid conversion

specifier I mean I have no idea what

this means but it's underlining the

percent sign is problematic well it

turns out that humans years ago decided

all right damn it we already Ed percent

well 2% signs will mean 1% literally so

now if I rerun make hello aha hello I

got 100% so there's going to be things

like that honestly that you have to ask

someone you have to Google you have to

look it up in the documentation but

there's always a solution to those kinds

of problems and thankfully they don't

come up all that often yeah oh just

poting other questions

[Music]

yeah if you have multiple variables it

is in the Left Right order so print F

will analyze the first string of text

that you pass in between quotes and

whatever the first percent is the next

the first variable that's passed in

after a comma gets plugged in there and

then the second gets plugged into the

second third and to the third and so

forth so it's just based on left to

right

yeah it's just a placeholder it's called

a format code and it just means

colloquially plug in some value here and

printf the like the humans who wrote

printf decades ago decided to treat

percent s special why just because they

needed some placeholder they decided

that eh no one's ever going to really

want to type percent s and if they do

they can just do percent percent s so

they decided to implement printf in such

a way that they have code that look

analyzes whatever text comes in looks

for percent s and then somehow plugs in

the subsequent values into that

placeholder and just this H question

[Music]

sorry ah so what if you wanted to do

single characters like initials like DM

or djm for first middle last absolutely

and that too is a perfect segue from the

two of you to what in general are going

to be called data types in C so it turns

out in C there's not only strings as

text and we'll see in more detail in

over the next couple of weeks what a

string really is underneath the hood but

strings of text are not the only thing

that programs can output they can indeed

output single characters as for initials

they can output integers as well turns

out that printf has different format

codes for all sorts of different data

types and just some of the data types

we'll see in the coming weeks will be

this list here which you'll notice it

almost perfectly lines up with the cs50

functions that I rattled off earlier

like get Char get int get string the

reason we called those functions that is

because each of them is designed to

return to you a different type of value

we've used get string already in this

example here we'll soon see get in and

we'll see opportunities to use others

but these indeed are the menu of

available data types plus others dot dot

dot that you can use when writing a

program in C the onus therefore is on

you to decide in advance do I want to

store an INT in this variable or a

string or heck when writing fancier code

an image or a sound or a video even

those can all be different data types

dot dot dot but for now we'll focus

really on just these Primitives that was

a lot let's go ahead and take a 5minute

break here no cookies yet but in 5

minutes we'll come back dive into more

detail and our second break today we'll

have

cookies all right we are back and so if

you have been playing along at home but

hitting some bumps in the road that's

totally normal and indeed the goals of

lecture generally will be to give you a

sense uh conceptually of where we'll be

going during the course of the week but

it's indeed through the Hands-On labs

and problem sets that you'll really have

an opportunity at your own pace to work

through uh some of those same bumps in

the road but for today let me give you a

few more building blocks and these two

will translate from scratch initially

namely like conditionals like how now

and see after knowing now how we can use

functions at least uh get string and

print F and we can use variables like

the string I created earlier how can I

now add to the mix things like

decisionmaking and conditionals at that

well with conditionals in scratch we had

this kind of syntax on the left here in

scratch is how you might Express if two

variables X and Y have this relationship

if x is less than y then say on the

screen X is less than y well let me

translate that to the right now in C

code so in C the corresponding code is

going to look like this assuming X and Y

already exist more on that later and

notice a pattern we're going to see

again and again there is going to be

parentheses around the X and less than y

so parentheses around the Boolean

expression recall the Boolean expression

is the true false the yes no answer that

a question that you're trying to ask in

order to decide whether or not to do

something so you use parentheses there

so similar in functions where we Ed

parentheses for printf and parentheses

for get string and this is just a weird

inconsistency stylistically when using

the keyword if you should as a matter of

better best practice put a space after

the word if when using a function like

print F or get string you shouldn't both

will work but you'll find that these are

conventions stylistically that most

people adhere to so space when using an

if here all right now inside of the

curly braces is where the actual code

goes that you want to execute

conditionally so if you want to print

out X is less than y only if x is

actually less than y in C you use this

open curly brace which up until now

you've probably rarely used on your

keyboard and the Clos curly brace down

here and those are kind of hugging if

you will the one or more lines of code

underneath the if very similar in spirit

to how the orange block here kind of

hugs the purple puzzle piece here so

there's no Graphics in C it's all text

so you can think of those curly braces

as really representing the same idea as

a side note if you only have one line of

code inside of the if condition if you

will you strictly speaking don't need

the curly braces but as a matter of good

style do include them it will make more

obvious what your intent is how about in

scratch if you wanted to express this

two ways in the road that you might go

left or right so to speak well if x is

less than y I want to say x is less than

y else I want want to say the opposite X

is not less than y in this case so I'm

making a decision based on that Boolean

expression in C it's almost the same but

you're adding to the mix the keyword Els

so MIT borrowed for scratch the same

keyword there and a second pair of curly

braces open and close respectively and

you might guess now what goes inside of

those well you print out X's less than y

or you print out X's not less than y all

right what if there's a three-way fork

in the road in scratch this actually

gets a little unwieldy graphically if

you will but notice that in scratch this

is how we could express if x is less

than y say x is less than y else if x is

greater than y say x is greater than y

else if x equals y then say x is equal

to Y now minor inconsistency here just a

little bit ago I claimed in C that a

equal sign represents what

operation assignment from right to left

left in so far as scratch is really

meant for kids and they didn't really

want to get into the weeds of this kind

of uh semantic equal sign in scratch

means equality however we're going to

need to fix this in C in just a moment

in c equal sign means assignment right

to left in scratch it literally means

what you would expect all right let's

translate this code then to C on the

right this code would correspond really

to this and you can perhaps see somewhat

goofy what the solution was not unlike

the percent percent solution earlier

when hum painted themselves into one

other corner you say if you say else if

and you say else if and how did we

resolve the use of a single equal sign

already in C when you want to repr when

you want to express equality is the

thing on the left equal to the thing on

the right you literally use two equal

signs right next to each other no space

in between them but now this code would

be correct on both the left and the

right whether you're doing this in

scratch or C respectively but now we can

kind of nitpick our code specifically

the the design thereof logically can

anyone critique the design of this code

either in scratch or C like I feel like

we could do better how about in

back perfect logically it's got to be

the case that X is less than y or X is

greater than y or by conclusion it's got

to be equal to Y so why are you wasting

my time or the computer's time asking a

third question you don't need to ask

this final else if because logically as

you knowe it should go without saying so

it's a minor tweak like you're doing

extra work potentially in cases where x

equals y so we can just refine that and

just like in scratch you could just use

an else block similarly in C could we

simplify this code to just an else a

sort of catchall logically that just

handles the reality that of course

that's going to be the final situation

instead all right so we have this

ability now to express conditionals with

Boolean expressions let's actually do

something with this next here so let me

go back to vs code I've closed hello.c

and I want to create a second file for

the sake of some demos now recall that

you can create new files by typing code

space and then the name of the file you

want to create uh for instance I might

do compare. c I want to write a program

that's going to start comparing some

values for demonstration sake but before

I do that let me just show you by

opening the file explorer at right this

is similar in spirit to a Mac or PC like

you can go up here and click on an icon

and you can click on the plus icon and

you'll get a blue box and I can type in

compare. C and I can just manually

create it that way notice that opens the

tab even without my having typed code so

again on the left you have a guey a

graphical user interface albeit a

simplistic one on the right and at the

bottom here you have a command line

interface but they're one and the same

what's nice though is that if I close

this file accidentally intentionally

whatnot I can reopen it without creating

a new one by just running that same

command code space compare. c so code is

a vs code thing it's just a userfriendly

shortcut but it's just creating a file

or opening an existing file like that

I'm going to hide the file explorer just

to make more room for code here and

let's go ahead and do this let's write a

program that compares two values that

the human inputs but not strings this

time let's use some actual integers all

right I'm going to go ahead and include

the cs-50 librar header file at top

cs50.h I'm going to also include

standard io. why one gives me

userfriendly input via get string get

int and so forth one gives me user

friendly output via printf in the case

of standard i.h now I'm just going to

kind of blindly type this line of code

which we'll come back to in future weeks

but for now that's analogous to the when

green flag clicked code in scratch and

now let's go ahead and do this let me go

ahead and get an INT from the user and

ask the user what's X question mark I'm

not going to bother with a new line I

want to keep it all in one line just for

Aesthetics sake but when I get back an

INT just like I get back a string I get

back a return value so if I want to

store the result of get int somewhere I

had better put it in a variable and I

can call the variable anything I want

previously I used answer or first or

last now I'm going to use x but there's

still two things left to do here

logically even though we haven't

technically done this yet what do I

still need to

do so I need the semicolon at the

end and the in at the beginning you the

programmer starting today kind of need

to decide what you're going to be

storing in your variables and you just

need to tell the computer that so that

it knows now as a teaser for languages

like python more modern languages turns

out humans realize well gee this is

stupid like why can't the computer just

figure out that I'm putting an INT there

why do I have to tell it proactively so

in some languages nowadays like python

we'll get rid of some of this syntax

we'll get rid of the semicolons but for

now we're looking at really the origins

of how this all worked all right so I've

done this one line ending with semicolon

let me do one other and let me get a

second int asking the user what's why

question mark so almost identical but

different responses from the user

hopefully and let me just ask simply if

x is less than y in parentheses then

some curly braces let me go ahead and

print out quote unquote X is less than y

back sln and now just as a side note I I

seem to be typing kind of fast some of

that is because vs code is helping me

let me go back to this

first line with the if hit enter and now

I'm only on my keyboard going to type

the open curly brace this is a feature

of many text editors nowadays it

finishes part of your thought why just

to save yourself a keystroke to make

sure you don't accidentally forget the

closing one so you'll notice sometimes

that things are happening that you

didn't type it's just vs code or future

programs you use trying to be helpful

for you I'll go ahead and manually type

out now printf uh X is less than y back

slend close quote semicolon so let me go

ahead now and try to run this and we'll

see let's see so make not hello but make

compare because this file is called

compare. C hitting enter okay no output

is good because it means I haven't

messed up let me do dot SL compare

instead of/ hello enter what's X how

about one what's Y how about two x is

less than y well let's try it again and

here I'll save you some keystrokes too

let me clear my screen instead of

constantly typing dot slash this and Dot

slash that you can also use your

keyboard's arrow keys in vs code to

scroll back through time so if I hit up

once there's the last command I wrote If

I write uh do it up twice there's the

second to last command I wrote so

sometimes if you see me doing things

fast it's just because I'm kind of

cheating and going through my history

like that all right let me go ahead

though and

rerun compare enter let's reverse it

this time two for x one for y and now of

course there's no no output all right

well that's logically to be expected

because we didn't have an else here so

let's add that else now let's open my

curly braces letting vs code do one of

them for me print F quote unquote X is

not less than y back sln semicolon let

me go ahead and try this again/ compare

enter again two for x one for y and we

should see

huh what did I do wrong why am I not

seeing any else output

yeah exactly you got to get into the

Habit after you change your code of

recompiling it otherwise the zeros and

ones in the server are the old ones

until you manually compile so let's fix

this make compare enter no error

messages that's good do/ compare 2 1 and

now I get back the output so X is not

less than y how about if I go and add in

the third condition well we can do this

either efficiently or inefficiently let

me go ahead head and refine this so else

if x is greater than y let's literally

say x is greater than y and now I could

do X else if x equals equals y but I

think we already claim that that's

unnecessarily inefficient so let's just

have our catchall and here I'm going to

say quote unquote X is equal 2 y back

sln close quote there so I think now

with this code we've handled all three

scenarios let me go ahead and recompile

it properly compare do/ compare and now

1 and 2 x is less than y let me run it

again 2 and One X is greater than y and

lastly one and one and X is equal to Y

so for the most part our code's getting

longer we're up to like 21 lines of code

though some of them are just single

characters on the screen almost

everything else is the same I'm using

the cs50 libraries header file for my

get int function standard i.h for my

print F function and the rest of this is

just now new syntax for conditionals as

well questions then on this C

implementation of just some basic

comparisons like

this any questions

[Music]

yeah good question do the opening

brackets need to be on a separate line

in cs50 yes what you'll see is that as

part of the submission process We

compare your code against a style guide

which is the norm in Industry a company

would have its own sense of style and

how its code should look and there's

generally automated tools within a

company that help uh give feedback on

the code or stylize it as such there are

alternative Styles than what we use in

the class we deliberately keep and ask

that you keep the curly braces on their

own line if only because it rather

resembles like the hugging nature of

scratches blocks and just makes clear

that they're balanced open and closed

however another common Paradigm in some

languages and with some programmers is

to do something like this uh on each of

them so you have the opening curly brace

on the same line as here we do not

recommend this this is invogue in the

JavaScript world and some others um but

ultimately in the real world it's up to

each individual programmer Andor the

company they're working for if

applicable to decide on those

things all right so beyond then these

conditionals what if we want to do

something that's maybe pretty common so

almost every piece of software or

website nowadays that you use has you

like agree to some terms and conditions

by typing like yes or no or just y for

yes and N for no so how could we

Implement some kind of Agreement System

well let me do this let me create a new

program a third one called agree. C so

I'm going to write code agree. C just to

give myself a new tab I'm going to start

as always now include cs50.h let's

include standard i.h and then let me do

my int main void which again for today's

purposes we'll take it face value is

just copy paste and if I just want to

get y or n for instance instead of yes

or no we can just use a simpler variable

here how about just a Char a character A

Single Character so I can use get Char

to ask the user for instance do you

agree question mark but before as before

I need to store this somewhere so I

don't want a string because it's a

single Char I don't want an INT I just

want a Char and it's literally CH h a r

and then I could call this thing

anything I want it's conventional if you

have a simple program with just a single

variable and it's of type Char call it C

if it's an INT call it I if it's a

string call it s for now I'm just going

to keep it simple and call it C and now

I'm going to ask a question so if C

equals equals how about quote unquote

y then let me go ahead and print out uh

agreed back sln as though they agreed to

my terms and conditions uh otherwise

let's see else if the character equals

equals quote unquote n

then let me go ahead and print out uh

say not agreed as though they didn't

quote unquote and let's leave it at that

I think here initially now you'll notice

one curiosity one inconsistency perhaps

does anyone want to call it out though

it's somewhat

subtle I've done something ever so

slightly differently without explaining

it yet do you see it single single yeah

so I've suddenly used single quotation

marks for my single characters and

double quotes for my actual strings of

text this is a necessity in C when

you're dealing with strings like strings

of text like someone's name a sentence a

paragraph anything really more than one

character you typically use uh double

quotes and indeed you must when dealing

with deliberately single characters like

I am here for y or n you must use single

quotes instead why because that makes

sure that the computer knows that it's

indeed a Char and not a string so double

quotes are for Strings single quotes are

for chars so with that said let me go

ahead and zoom out let me go ahead in my

terminal window run make agree enter

seems to work okay so let me go ahead

and do/ agree uh let me go ahead now and

type in y here we go

enter huh uh let me try that again rerun

do/ agree how about no

enter why is it not behaving as I would

have expected uh because you the capital

Y yeah I kind of cheated there and I hit

the caps lock key just as I started

typing in input why because I

deliberately wanted to type in uppercase

instead of lowercase which is kind of

reasonable right it's a little obnoxious

if you force the user to toggle their

caps lock key on or off when you just

need a simple answer that's not the best

user experience or ux but it would work

if I cooperated let me run this again

without caps lock on y lowercase for yes

ah that worked and uh lowercase for no

that work worked but how could I get it

to work for both well how about this let

me go ahead and just add two

possibilities so else if C equals equals

quote unquote capital Y then also do

printf agreed back sln and down here

else if C equals equals uh single quote

uh capital N then go ahead and print out

again not agreed okay this I will claim

now is correct and I'll do make agree

real fast do slash agree and I'll use

Capital it now works I'll use uh Capital

it again works but this is perhaps not

the best design let me hide the terminal

window and pull this up on the screen

all at

once why might this arguably not be the

best design even though it's

correct there's another term of art we

can toss here like something smells kind

of funky about this code this is an

actual term of art like there's code

smell here like something smells a

little off why what do you

[Music]

[Music]

think yeah there's the same output again

and again I mean I manually typed it but

honestly I might as well have just

copied and pasted most of my original

code to do it again and again for the C

two capital letters so if line 10 and 14

are the same and line 18 and 22 are the

same and then the rest of these if and

Els ifs are almost the same like like

there's some code smell there like it's

not well-designed why because if I want

to change things now just like last week

in scratch I might have to change my

code in multiple places or copy paste is

never a good thing and go God forbid I

want to add support for yes and no as

full words it's really going to get long

so how can we solve this well it turns

out like we can combine some of these

thoughts so let me try to improve the

yeses first it turns out if I delete

that Clause I can actually or things

together in scratch there's a couple

puzzle pieces if you didn't discover

them that literally have the word or and

the word and on them which allow you to

combine Boolean Expressions so that

either this or this is true or this and

this is true in C you can't just say the

word or you instead use two vertical

bars and vertical bars together mean or

logically and so I can say C equals

equals quote unquote capital Y agreed

and now I can get rid of this code down

here and let me go ahead and say

vertical vertical bar twice C equals uh

quote unquote n in all caps and now my

program's like you know roughly a third

uh smaller which is good there's less

redundancy and if I reopen my terminal

window rerun make of agree SL agree now

I can type little y or Big Y and same

thing for lowercase and uppercase n any

questions then on this syntax whereby

now you can combine thoughts and just

kind of tighten things up

and there'll be other such tricks too

yeah a really good question is there not

a function to just ignore the case short

answer there is and we'll see how to do

that in actually just about a week's

time and in other languages there's even

more ways to just canonicalize the

user's input throwing away any space

characters they might have accidentally

hit forcing everything to lower case in

C it's going to be a little more work on

our part to do that but in fact as early

as next week we'll see how we can do

that but for now we're comparing indeed

just these literal values other

[Music]

questions really good question so we are

assuming with this program in all of my

last ones that the human's cooperating

when I asked for their name they typed

in David and not one two three or in

this case they typed in a single

character and not a full word so this is

one of the features often of using a

library so for instance if I run agree

again

and I say something like sure enter it

rejects it alt together why because s e

is a string of characters it's not a

single character now I could just say

something like X which is neither y nor

n of course but it tolerates that

because it's a single character but

built into cs50's library is some

built-in rejections of inputs that's not

expected so if you use get int and the

user types in not the number one or two

but cat C A it will just prompt them

again prompt them again and this is

where too if you were to do this

manually in C you end up writing this

much code just to check for all of these

errors that's why we use these training

wheels for a few weeks just to make the

code more robust but in a few weeks time

we'll take the library away and you'll

see and understand how it's doing all

it's indeed doing all that all right so

how about this let's now transition to

something a little more scratch like

literally uh by creating how about

another program here called meow so

meow. C we won't have any audio

capabilities for this one will just rely

on print and suppose that I wanted to

write a program in C that just simulates

like a cat meowing so I don't need any

user input just yet so I'm just going to

use standard i.h I'm going to do my

usual int main void up here and then I'm

just going to go ahead and do printf

meow back sln and let's have this C meow

three times like last week so I'm going

to do meow meow meow notice as an aside

whenever you highlight the lines you'll

see little dots appear this is just a

visual cue to you to let you figure out

how many spaces you've indented vs code

like a lot of editors will automatically

indent your code for you I've not been

hitting the space bar four times every

time I've not even been hitting tab

however in C the convention is indeed to

indent lines where appropriate by four

spaces so not three not five and these

dots help you see things so that they

just line up as a matter of good style

all right so this program I'm just going

to stipulate right now is indeed going

to work make meow which is kind of cute

and now meow there three times correct

it's meowing three times but of course

this is not well designed it wasn't well

designed in scratch last week why what

should I be doing

differently

yeah

yeah it's a perfect like opportunity for

a loop why because if you wanted to

change maybe the capitalization of these

words you wanted to change the sound to

like woof or a dog or something like

you'd have to change it one two three

places and that's just kind of stupid

right in code you should ideally change

things in one place so how might I do

that well we could introduce a loop yes

but we're going to need another building

block as well that we had in scratch

namely those things called variables so

we're call that a variable like in

algebra x y z whatever can store a value

for you and a variable in scratch might

have looked like this you use this

orange puzzle piece to set a variable of

any name not just X Y or Z but you could

call it something more descriptive like

counter and you can set it equal to some

value in C the way to do this is similar

to to Spirit to some of the syntax we've

seen thus far you start by saying the

name of the variable you want a single

equal sign and then the value you want

to initialize it to copying therefore

from right to left why because the equal

sign denotes again assignment from right

to left this isn't enough though you

might have the intuition already what's

missing probably from this line of code

just to create a variable so we need int

to make sure the computer knows that

this is indeed an INT and then lastly

semicolon as well and that now completes

the thought so a little more annoying

than scratch but we're starting to see

patterns here so not every piece of

syntax will be new all right if you

wanted to increment the counter by one

scratch uses the verb change and they

mean add the value to counter so if I

want to increment an existing variable

called counter this syntax is a little

more interesting it turns out the code

looks like this which almost seems like

a paradox like how can counter equal

counter plus one like that's not how

math works but again a single equal sign

is assignment from right to left so this

is saying take whatever the value of

counter is add one to it and copy that

value from right to left into counter

itself you still need the semicolon but

I claim you do not need to mention the

keyword int when updating an existing

variable so only when you create a

variable in C do you use the word string

or the word int or any of the others

will eventually see only when creating

it or initializing It For the First Time

thereafter if you want to change it it

just exists it's the word you gave it

the computer's smart enough to at least

remember what type it is so this line is

now complete turns out in code as we'll

see it's pretty common to want to add

things together increase increment

Things by One so there's actually

different Syntax for the same idea the

term of art here is syntactic sugar like

there's often in code many ways to do

the same thing even though at the end of

the day they do exactly the same

functionality so for instance if after a

few days of cs50 you find this a little

tedious to keep typing and some program

you can simplify it to just this this is

the syntactic sugar you can use plus

equals and only mention the variable

name once on the left and it just knows

that that means the previous thing it's

just slightly uh more uh more succinct

this to is such a common thing to add

one to a value and it doesn't have to be

one but in this case it is but if it is

indeed one you can further tighten the

code up to just do this counter Plus+ so

anytime in C you see plus plus it means

literally adding one to that particular

variable there's other ways to do this

in the other direction if you want to

subtract one from a variable you can use

any of the previous syntax using a minus

sign instead of Plus or you can more

succinctly do counter minus minus this

is the way a typical C programmer would

do this all right so if we have now

variables let's go and solve the meowing

with loops so in scratch we saw Loops

like this this of course had the cat

meow three times how do we do this in C

now this this is where things get a

little more uh involved codewise but if

you understand each and every line it

will follow logically what's going on so

here I claim is one way to implement a

loop that iterates three times in C and

this is kind of ridiculous right like we

went from two super simple puzzle pieces

like this to my God like it's 1 2 3 four

five six lines of code all of which are

pretty involved so like that escalated

quickly but what's each line doing and

we'll see other ways to do this more

simply so we're izing a variable called

counter to three just like before why

well what does it mean to Loop or to

repeat something three times well it's

kind of like doing something three times

and then do it and then count down and

then do it and then count down and then

do it until you're all out of counts so

this is declaring a variable called

counter setting it equal to three then

I'm inducing a loop in C which is

similar in spirit to repeat three but

you have to do more of the math yourself

so I'm asking the question in

parentheses

while counter is greater than zero what

do I want to do well Pur the indentation

inside the curly braces I want to meow

one time and then to be clear what's

this last line of code doing if counter

starts off at three this makes it two by

subtracting one from it then what

happens by nature of a loop just like in

scratch it kind of knows to go back and

forth even though there's nice pretty

arrow in scratch and there isn't here C

knows to do this again and again and

again con stantly asking this question

and then updating this value at the end

so if I highlight just a few of these

steps the variable starts off at three

and actually let me simplify two I

claimed earlier that uh when using

single variables people very often just

call it I for INT or c for Char or s for

string unless you have multiple

variables so let me tighten the code up

and this already makes it look a little

more tolerable let me actually tighten

it up further and one more step so now

this is about as tight as succinct as

you can make this code at the moment so

what's actually going to happen here

well the first line of code executes and

that initializes I to 3 then we check

the condition while I is greater than

zero is I greater than zero we'll per my

three fingers obviously so we print out

meow on the screen then we subtract one

from I at which point now we have two as

the value of I then the code goes back

to the condition and notice the

condition there is in parentheses that's

another Boolean expression so Loops can

use Boolean Expressions just like

conditionals use Boolean Expressions to

make decis ision the loop though is

deciding not whether to do this thing or

that but whether to do the same thing

again and again and again and as it

ticks through the code one line after

the other it's ultimately going to get

down to uh one and then zero and then

stop so put another way came with some

props here so suppose uh this bowl here

is your variable and you initialize it

to three with like three stress balls

you can do something three times right

if I want to give out three stress balls

here's your chance for free stress ball

without having to answer any questions

any okay there we go so here we go

subtracting one from my variable I'm

left with two uh oh my God all right uh

don't tell

Sounders oh I'm sorry oh okay that ended

poorly apologies all right but now the

educational point though is that my

variable has been uh decremented further

to just have I'm not throwing that far

again I can't do this in here we go all

right here we go and one final

subtraction and now our variable is left

so we have three stress balls there and

that's all a variable is right it's some

kind of storage it's actually of course

implemented in the computer's memory but

metaphorically it's really just like a

bow with some values and every time you

add or in this case subtract you're just

changing the value of that variable and

then the code meanwhile of course in

parenthesis is just checking is the bow

empty is the bowl empty is the bowl

empty AKA is I greater than zero or not

any questions on how we've implemented

Loops in this way

and I I'll you a stress ball after class

questions on Loops all right so it turns

out this is kind of ugly and like this

really starts to take the fun out of

programming uh when you have to like

write out this uh sequence of steps so

it turns out there's other ways to do

this but first let's see logically how

else you might Express this because it's

a little weird that we keep using zero

so the one other way to do this would be

to invert the logic you could absolutely

start with your variable call it I equal

to one and then you ask the question is

I less than or equal to three and notice

a bit of new syntax here on your uh

typical keyboard there is no less than

or equal sign or greater than or equal

sign like you would write in math class

with one over the other and so in C you

use two characters less than followed by

an equal sign or if appropriate greater

than followed by an equal sign and that

logically captures that idea so notice

that I'm kind of changing my questions

I'm initializing I to one and then I'm

going to increment It ultimately to two

and then three but because I'm doing

less than or equal to it's still going

to go from 1 to three so that works too

we could similarly do this yet another

way we could initialize I to zero and

then we could say well I is less than

three and keep incrementing it and I

showed this last form is actually the

most canonical like it might be the most

humanlike to think in terms of 1 to

three it might be the most stress ball

like to think in terms of three to zero

counting down but Ty typically the go-to

Syntax for most programmers once you get

comfortable counting from zero is to

always start counting from zero and

count up to less than the value you're

counting up to so it would be incorrect

why to change this to less than or equal

to three here what would happen if I

Chang the less than to less than or

equal to it'll me out twice yeah it'll

out an extra a fourth time in fact total

right because you'll start at zero then

one then two then three and less than or

equal to three sorry three will give you

the fourth time so we do want it indeed

to be just a single less than all right

so now that we have those options let me

just give you one other and this one

takes a little more getting used to as

well but it's probably the more common

way to write this let me go ahead and

propose that we implement this as

follows let me go back to my code here

let me go into my several printfs

getting rid of all but one of them

ultimately and let's implement this in

code so let's do in I get that's zero

how about then while I is less than

three then let's go ahead and say print

F quote unquote meow uh mellow meow back

sln and then we have to do I minus minus

or plus

plus so plus plus because we're starting

at zero and going up two but not through

three so let me go ahead now and make

meow after clearing my terminal SL meow

and it's still just as correct but it's

a little more uh it's a little better

designed why because now if I want to

change it from three to 30 times for

instance I can change it there I can

recompile my code I can do/ meow and

done I don't have to copy and paste it

27 more times to get that effect and I

can even change what the word is by

changing it in just one location but it

turns out there's other ways to do this

too and let me propose that we introduce

you to what's called a for loop as well

so if you want to repeat something three

times you can absolutely take the while

Loop approach that we just saw or you

can do this and this one's a little

takes a little more getting used to but

it kind of consolidates into one line

all of the same logic so notice we have

the keyword four here and four is just a

a preposition in this case that

generally implies here comes a loop

inside of parenthesis here is not just a

Boolean expression and this where things

get a little weird there's three things

to the left of the semicolon in the

middle of the two semicolons and to the

right of the semicolon this is really

the only other context we'll see

semicolons and it's weird normally it's

been at the end of the line now it's two

of them in the middle of the line but

this is the way humans decided years ago

to do it so what is this doing almost

the same thing it is going to initialize

a variable called I to zero it's going

to then check if it's less than three

it's then going to do what's ever in the

curly braces is and it's lastly going to

increment I and repeat so just

highlighting in turn at first I is

initialized to zero just like before

then this condition is checked this is a

Boolean expression yes or no true or

false will be its answer and if I is

less than three which it should be once

it starts at zero well then we're going

to go ahead and print out meow then I is

going to get incremented so it starts at

zero it goes now to one at that point

the Boolean expression is checked again

so you don't keep changing I back to

zero that first step happens only once

but now you repeat through those three

other highlights I check if I is less

than three it is so I print out meow it

then increments I I check if I now two

is less than three it is I print out

meow I gets incremented I now check is I

less than three no it's not because

three is not less than three and so the

whole thing stops and whatever code is

below this curly brace if any starts

executing instead just like in scratch

you break out of the loop and the the

puzzle piece being hugged questions then

about this alternative Syntax

for Loops AKA a four

Loop sorry say

again yeah can I explain again why it

doesn't reset to zero honestly just

because like this was the syntax they

chose this first part before the first

semicolon is only executed once just

because that's how it's designed

everything else Cycles again and again

and this is just an alternative syntax

to to using the slightly more lines of

code it was like six lines of code using

the while loop logically it's the same

thing programmers once they get more

comfortable tend to prefer this because

it just expresses all your same thoughts

more succinctly that's all yeah okay so

let's just work this into my meow

example let me go back to the code here

and notice indeed if I highlight all

these lines I think we can tighten this

up let me get rid of all of those and

instead do four in I equals 0 and I'm

saying equals most programmers would say

gets so in I gets zero means assignment

the word get now I'm going to do I is

less than three i+ plus now in here I'm

going to do my print F quote unquote

meow back sln and so it's indeed a

little tighter I mean two of the lines

are just curly braces there's really

only two juicy lines of code now let me

go ahead and do make

meow meow and again we're back in

business with three of them printing

only all right there's one last

structure we should explore just cuz

it's sometimes useful this was a forever

block and this would be a little weird

in scratch to just say meow Forever at

least without waiting but there is

indeed a forever Block in scratch which

means do the following forever and I

proposed I think verbally last week's at

least one example where this is useful

meowing forever little annoying but can

you think of common cases where you

might want to write code or use a

program that Loops forever

yeah yeah playing music like Spotify

playlist just repeating again and again

would be some kind of loop for

collisions checking for collisions and

scratch so seeing if something's

bouncing off the wall or another Sprite

yeah checking for input so yeah get

string is essentially just waiting there

forever for me to type in some input

until I do the time checking the time

and actually maintaining like human time

like a wall clock behind you was that

the

same okay checking the time and one more

detecting a key press too like in

scratch just waiting for some kind of

event to happen just like on a phone or

a browser and so there's so many

examples where you might want to do

something forever just so you've seen

the corresponding C building block it's

a little weird but this is probably the

most canonical way to do it in C if you

want to print meow forever which would

be a little crazy because it literally

print and take over your computer

printing forever meow you would

generally do it like this why well a

while loop expects in parentheses a

Boolean expression and a Boolean

expression is again a yes no a true

false question but if you want the

answer to that question always to be yes

or really always to be true turns out in

C in a lot of languages well then just

say true because true T R is never going

to change magically to false I mean it's

just a special word in the programming

language so by saying while true it just

means do the following forever another

common Paradigm before true and false

became common place would be to do uh

this instead change uh while one you

might see in online examples and texts

and the like while one is really the

same thing any value that is zero is

generally interpreted as false by a

computer any value that is one or any

other nonzero value is generally

interpreted as true and so this too

would have the same effect saying while

true or while one generally speaking

while true is perhaps a little clearer

these days now meowing forever is not a

good thing but suppose I did that by

intent or by accident well let's try

this so here I'll go into my code I'm

going to get rid of my for Loop and

change my while loop to uh how about

true um and in this case here well we'll

keep it let's do this make meow enter

and you'll see this use of Undeclared

identifier or true this is actually

hinting at my mention that the old way

was zero and one nowadays you could say

true or false but true and false are

themselves special words that you have

to include and it turns out if you want

to use special Boolean values like this

there's another header file we haven't

seen called standard bu that essentially

creates true and false as keywords

alternatively cs50 includes that same

file so it's more common in cs50 to see

it like this now if I clear my terminal

window and do make meow and then do/

meow and hit enter well unfortunately

this isn't the best thing to do uh

infinitely when you're in the cloud

using a browser this is indeed a a

browser full screened here um this means

I'm sending millions of meows over the

internet to my computer here uh so this

will happen to you at some point

probably not with meow but you'll lose

control over your terminal window why

because you screwed up and like you have

an infinite Loop you didn't really

intend it or maybe you did you were

curious to see what happens what do you

do like when when does the meowing stop

what recourse do we have here all right

well control c will be your friend

sometimes you have to hit it a bunch in

a cloud environment but control C for

cancel with will interrupt a program

that's running and I promise that almost

all of you will at some point

accidentally introduce an infinite Loop

because you're math is slightly off when

in doubt click in the terminal window

and hit contrl C sometimes multiple

times and that will indeed cancel

whatever is happening there in this case

I might have intended it but sometimes

it's not in fact intended all right so

we've been taking for granted this whole

graphical user interface for some time

and indeed uh the uh commands that I'm

typing and the buttons I'm clicking and

let me just give you a better sense of

what it is we are using underneath the

hood this whole time um namely an

operating system called Linux so I keep

alluding verbally of course to Macs and

PCs because almost all of us are running

Mac OS or Windows on our desktops or

laptops nowadays but there's lots of

other operating systems out there and

one of the most popular one is called

Linux and Linux is very often used on

servers nowadays companies that host

email companies that host websites or

apps more generally um certain computer

scientists or computer science students

often like to brag that they run Linux

just because that's a thing um but it is

really just an alternative to Mac OS or

windows that provides you with both a

guey if you want it but also and

especially a command line environment

now fun fact Windows and Mac OS do have

terminal windows or the equivalent

thereof and eventually you might uh use

it on your own Mac or PC to solve some

problem but Linux is really known for

along with other operating systems it's

command line environment which again I

distinguished earlier from Guy as a

command line interface or CLI and that

refers really to the terminal window so

if I go back to VSS code here and let me

in fact go ahead and close my tab and

focus entirely on the terminal window

this terminal window is really just your

command line interface to your very own

server in the cloud the term of art here

is you each will have your own container

in the cloud which is like your own

computer running somewhere on the

internet with your own username and

password to which you have access and

your own hard drive if you will your own

home folder that has all of your files

for the class and it's only accessible

to you unless you enable live sharing

thereof so when you're typing commands

here it looks like you're typing them of

course on your own Mac or PC but they're

actually being sent over the browser to

uh some server in the cloud where you

are controlling really your own account

therein so it turns out that there are

other commands that are worth knowing

and we'll give you just a few of these

today and over the coming weeks will you

have opport ities to play with others as

well but these are kind of some of the

basics and they're all incredibly

succinct because indeed for things

you're typing at the command line humans

generally have not wanted to type out

long commands so a lot of these are

abbreviations here now perhaps the most

common one I'll start with first is LS a

lowercase l and a lowercase s that

stands for succinctly list so if I go to

my terminal window now where up until

now I've only typed code which is a vs

code thing for creating and opening

files and make which triggers the compil

ation of my code what if I now type LS

this will list all of the files in my

current folder my hard drive in the

cloud if you will so if I hit enter

you'll see a whole bunch of results now

they're color-coded too the white ones

here and in. c those are the source code

files I've written during class today

agree. C compare. C hello.c and meow. C

and you can perhaps guess the green ones

here that just by convention have an

asterisk on the end to denote that

they're special represent what

what are the four others

yeah yeah the machine code so those are

my actual programs that are identically

named minus the C extension and the

asterisk means that they're executable

that is in the world of Mac OS or

Windows you would double click but in

the world of a command line environment

that means you do dot slash and then the

name without the asterisk to execute or

run the code therein so if I open up my

file explorer and I'm hitting command B

on my computer here just as a keyboard

shortcut you'll see the exact same thing

so LS is the command line interface for

listing the files in your account but

here because I'm using vs code or any

program like it I also get a graphical

user interface as well so it's just two

different places to be you're welcome to

use whatever you're comfortable with but

over time will you naturally get more

comfortable and capable with the

terminal window alone well what else is

on this this list here well during the

break I saw that at least one of you for

instance had created a file called hello

instead of hello. so you were in a

situation where you did this

accidentally and hit enter and then you

went ahead and typed in all of your code

like this and then down in your terminal

window you were trying to do make hello

enter and this now didn't actually do

anything like I can't I'm hitting um I'm

trying to run the command I got

permission denied as at least one of you

did now why is that well let's just do a

quick check if I do LS I see now hello

but hello has no asteris next to it

which means it's not executable that's

my code why well notice the top of my

tab confirms oh I screwed up I didn't

name my file hello.c which it just has

to be so what do you do well you could

very hackish like copy this create a new

file paste it in or no no no like we

know how to rename things now here

because that's one of our options let me

do this let me do MV for move hello and

then hello.c and hit enter you'll see

the tab closes cuz hello no longer

exists but if I now now type LS you'll

see ah there is hello.c and if I open

that file now there's all of my same

code and now if I do make hello make

hello now I do get an executable file

wherein the world is restored so MV is

just a command not just for renaming but

it also turns out eventually for moving

files as well you can also create

directories or folders so for instance

if I go into vs code again and suppose I

hover over here and click not on the

plus file icon but plus folder I can

create a folder called for instance like

pset one for problem set one in the

class and you'll see now that it's empty

CU all of my other files are in the

default folder of my account but I could

also go in there like this and I could

click on file and now I can create a new

file called like mario.c which is uh one

of the first problems for instance but

you'll notice now that

mario.c is inside of the pet one folder

so if I zoom out and I type LS at my

terminal window

I won't see mario. C anywhere but I do

see a pet one folder and it's in light

blue followed by a slash which you don't

have to type it just indicates that's a

folder now I can visually at top left

obviously see pet one contains mario.c

but if I try to do something like make

Mario here no rule to make Target Mario

like it just doesn't seem to exist and

that's because you're in the wrong

directory so in a command line interface

it's not quite as simple as just

clicking on a folder and voila it opens

you have to change into the directory or

folder and CD is going to be the command

there so if I want to actually change

into that directory I can do CD space

pet one enter and now you'll see my

prompt changes and this is just a common

convention but it's not the only one out

there now I still have a dollar sign

which indicates where I can type

commands but before it I see a reminder

constantly what folder I'm in and we put

that there deliberately like a lot of

Linux users do just to remind themselves

where they are cuz unlike Mac OS where

you or Windows where you have a nice big

window telling you where you are at the

command line you kind of need to be

reminded textually but now if I type LS

and hit enter what should I

see yeah mario.c and now if I want to

open it uh if I want to uh actually

compile it I can run make Mario in this

directory once I actually type out all

the code rest assured that in problem

sets in Labs we almost always certainly

in the first weeks of the class give you

exactly the commands to type odds are

because it's new to many of you view you

will accidentally type the wrong

commands no big deal just remember that

you have different ways to solve these

problems you've got like the graphical

file explorer which should feel a little

more familiar but in time you'll start

to know and honestly probably prefer

commands like these so CD for change

directory CP for copy a file uh LS for

list MK dirt to make a directory create

a new folder at the command line instead

of with the button MV for move or rename

RM

for remove so be careful with that one

RM dur remove directory and there's

dozens hundreds of other commands you

won't need many of them but we'll start

to scratch the surface all the more over

time but ultimately this command line

interface is going to be a more powerful

mechanism a more capable mechanism and

ultimately a more efficient mechanism

for writing code running commands uh

solving problems analyzing data more

generally even though no there's going

to be some uh some Growing Pains early

on just because it's probably so new for

many of you so with that said we have

some problems still to solve but we

promised cookies today so let's go ahead

and take a 10-minute break cookies are

now served in the transcept and we'll be

back here in

10 all right we are back and up until

now each of the code examples in C we've

done if been kind of designed to show

one specific topic but we thought we'd

try to take a step back and solve a more

General problem and give you a sense of

when given a problem set for instance or

just a programming problem more

generally like where you even begin and

how you go about approaching it when

it's not obvious what the point of the

exercise is so one of my favorite games

from yester year is this one here Super

Mario Brothers that has come in so many

different forms since but in this

original uh two-dimensional Sid scroller

game there was a lot of artwork like

this so for instance up here in the sky

were four question marks and we'll find

that in C in a lot of programming

languages initially it's a lot easier a

lot more accessible to focus really on

black and white type interactive

programs textually as opposed to

full-fledged graphics and the like but

more on the more Gra graphical acoustic

uh type of programs before long but for

now let me go over and propose that we

try to just Implement an asy art aski

again being the code that Maps numbers

to letters at least for English um into

a textual version of these four question

marks in the sky so for this let me go

over to vs code I'll create my own

version of mario.c that will be

different from the one you're challenged

with in problem set one indeed in

problem set one you'll be challenged to

build a little something like this

albeit with hashtags uh for asky art

instead of graphics and in mario.c I

want to just solve this simple problem

first so it's all involving output so

I'll do include standard i.h so I can

use printf I'll do my int main void more

on why we keep doing that in future

weeks and I'm just going to do something

simple initially like 1 2 3 4 back sln

this is about the simplest way I can

Implement four question marks in the sky

like these here using pure text like

this so let me go ahead and do make

Mario / Mario and voila we have those

four question marks but we've seen of

course that there are better ways to do

this and if you wanted to generalize

this to be five question marks six 60

different question marks you know Loop

was always the answer for not repeating

ourselves so maybe I should rewrite this

a little bit more flexibly and say

something like this four in I gets zero

I less than four I plus plus and then

inside of the for Loop now I can just do

a single question mark But I don't think

what I've just done is correct anyone

spot the aesthetic bug

already yeah why is this why is this

wrong if I want to print the same thing

[Music]

yeah yeah so I don't think I want to

backs slash in after every question mark

because the goal is again this like row

of question marks in the sky so if I now

recompile this make

Mario Mario okay it's almost there but

now I have that regression to where the

dollar sign's not on its own line so I

think I need a new line but I don't

think I want it here cuz that was not

going to end well where do I want it

instead any Instinct

yeah yeah so outside the for Loop so

indeed I can just go below line eight

and above line nine creating a new one

and now there's it's totally fine to

just print a new line like that you

don't have to print anything else with

it it's indeed a character unto itself

so let's do make Mario one last time/

Mario okay so now we're back in business

there well what if we wanted to do some

other scene from Mario uh such as this

one here where there's a lot of vertical

obstacles like the These Bricks here if

I wanted to print out now a column of

three bricks and I'll use hashtags for

these instead of anything graphical well

I think we're almost there right like I

think I can now it's almost maybe a

little easier I can go back here change

the question mark to something that

looks more like a brick like this hash

symbol and I think now I do want the new

line character because when I now do

make Mario Mario okay there's my wall of

four oh but wait I didn't want four I

wanted to be consistent just with this

particular scene here so I just want

three so I can still change it in one

place and here again is that Paradigm

even whether you're using four or three

if you get into the habit of starting

counting from zero you go on up to but

not through the value you want to count

up to so that's why I'm using less than

instead of less than or equals to there

so this would be the common Paradigm

though you could certainly count it like

we saw earlier in different ways but

what if things escalate one level

further and when you're in the

underground version of Super Mario

Brothers there's a lot of these

underground obstructions including like

grids of bricks like this and let me

conjecture that if you slice this up

it's roughly a 3X3 grid of bricks that

all interlock uh prettily to give us

just one big large brick like this so if

I want to print out a 3X3 grid now

things are getting a little more

interesting because up until now I

printed either one row horizontally or

one column vertically but we haven't

really seen any code where I'm sort of

printing or living in two different

dimensions like the game would imply but

let me propose that we could do this let

me go ahead and say all right suppose I

want to print a 3X3 grid of bricks it's

really that I want to print what three

rows of bricks like a grid is three rows

so if I take the highle idea and reduce

it to something a little simpler how do

I do that well let me get rid of the

print for a moment as I did and let me

just stipulate that this for Loop even

though it doesn't do anything useful yet

will do something how many times just by

Design all right three times right this

for Loop is good to go it will do

something three times by just using I to

do the counting all right well if I want

to print out now a row of three bricks

all on the same line that's pretty

similar to what we did earlier when I

just wanted to print out four question

marks in the sky so we've kind of seen a

solution there and I dare say we can

compose one into the other so if I want

to print out a row of bricks I could

just do this four in I gets zero I less

than three I ++ and then inside of this

inner loop if you will let me print out

a single brick like this and then I I

don't like where this is going yet but I

think I've taken two ideas and I've

combined them but what might be

problematic about lines five and seven

at the moment

what might be bad here uh yeah and

back yeah I'm using the same integer ey

which I feel like could get me into

trouble right if I'm sort of trying to

count three things here but then I'm

hijacking this variable and using it

inside of the loop like I feel like I

should avoid this this Collision of

names and so what's a good alternative

to I well a programmer if nesting Loops

in this way would pretty commonly go

with J you could certainly change this

to be like rows and columns if you want

more descriptive variables but I and J

is pretty canonical so I'm going to go

ahead and do this j++ instead of i++

everywhere and let me try compiling this

so make Mario

enter

Mario okay so a couple of things are

wrong here this is not a 3X3 grid but if

you count these things how many did I

indeed print at

least can probably just guess logically

yeah there's nine hashes there

unfortunately they're all on the same

line instead of on three different lines

so where logically can I fix this I'm

definitely printing all the bricks

they're just not on the right levels

yeah yeah so put a new line after the

first Loop this inner loop if you will

the nested Loop if you will so let me go

ahead and print out just a back slash n

here and what's this doing well I think

that's going to solve it by just moving

the cursor to the next line after you've

done one row so let me go ahead and do

make Mario enter SL Mario and now we're

in business so it's a very simplistic

version of the same graphic but I'm

leveraging two different ideas now the

same or the same idea twice rather now

I'm using one Loop to kind of control my

cursor going row by row by row but then

within that Loop I'm doing left to right

do do dot dot dot with printing out each

of these individual bricks like this now

there's a little sloppiness here still

like if I want this to always be a

square just because that's what it looks

like in the game well I could change it

to be a 4x4 uh Square by doing this or a

5x5 grid whoops by doing this why is

this perhaps not the best design to just

keep changing the numbers when I want to

change the

size where could this go AR

yeah yeah if it's always going to be a

square and height is going to be the

same as width I'm just inviting trouble

here right eventually I'm going to screw

up I'm going to change one but not the

other then it's going to come out to be

a rectangle instead of a proper Square

so I should probably solve this a little

differently so let me do that at the top

of my main function here let me go ahead

and give myself a variable called maybe

uh n for the number of bricks I want

horizontally and vertically and I'll

just initialize that to three initially

and instead of putting three here I'll

literally just use n but I'll do it in

both places so that now henceforth if I

ever want to change this and change it

to four or five or anything else like

I'm all done like it's better designed

because there's a lower probability of

mistakes but I could technically SK

still screw up somehow I could

technically accidentally write line of

code like n++ or you know I could just

change the value of that variable even

though I don't want it to ever change

and maybe it's because I'm a bad

programmer I copy pasted wrong I'm

working with someone who doesn't know

what n represents I can defend myself

and my code against human error like

that by going up here to line five and

instead of just declaring a simple

variable like we did in scratch I can

further Harden my code so to speak by

declaring it to be a constant using the

keyword const now this is just a feature

of c and some other languages to protect

you against Yourself by proactively

saying N is a constant specifically the

number five or previously the number

three you cannot accidentally write code

elsewhere that changes it the computer

will throw an error and catch that error

so it's just a way of programming a

little more defensively um some

languages have this some languages don't

but in general it's a good practice it

makes your code better designed because

it just is less vulnerable to Mistakes

by you colleagues or anyone else using

the code so let me change this back to

three just to be our default but now I'm

using n in both places and if I do make

mario. Mario we're back to where we

originally started but the code's a

little more better design and let me

note this too all this time I've been

mentioning that uh there's correctness

is important design is important there

was also this matter of style I've been

very deliberately writing pretty code if

you will not just the syntax

highlighting which is automatic but

notice that I keep indenting everything

nicely anytime I have curly braces like

on lines 4 and 14 everything is indented

one level when I have additional curly

braces on line 7 and 13 everything is

nicely indented uh as well technically

speaking the computer does not care

about that kind of white space so to

speak and you could really make a mess

of things like this because you have a

strange sense of style or just because

you're being a little sloppy but this

code is actually still correct if I

recompile it let me open up my terminal

window make uh Mario no errors Mario it

works perfectly fine but you can imagine

just how annoying this now is to read

like certainly for a TA U but certainly

for you the next day certainly for a

colleague who has to read your code this

is just bad style like it still works

and it's welld designed in that like

you're uh writing code defensively

you're using a constant but my God the

style is atrocious now you'll often find

that there's tools that can help you

format your code for you in a manner

consistent with a courses or a company's

uh style but this is the kind of muscle

memory you'll want to develop over time

to take vs code suggestions as it's

outputting lines of code for you because

it's trying to format your code in a

readable way and oh my God if and when

you do have bugs in your code and things

aren't even indented properly there's no

way you the human are going to be able

to wrap your mind around what's

happening and where like you're just

making the problem harder for yourself

so do get into this habit too of

manifesting good style as well all right

well let me propose that we don't only

want a 3X3 grid we want this to be a

little more Dynamic so suppose we moved

away from a constant to just using an

integer called n and let's ask the user

for the size of this grid as by

prompting them with get int as we've

done before and I'll store it in N here

and then I can go ahead and more

dynamically run make Mario to compile it

whoops oh I screwed up

accidentally what is it in suggesting I

do albeit

cryptically yeah I forgot to include the

cs50 header file up top and that's why

it doesn't know that get int is in fact

valid so that's an easy fix U I'm just

going to go up here and include

cs50.h now I'm going to clear my

terminal and rerun make Mario now we're

good Mario and now notice I'm prompted

for size so if I type in three it's the

same as before if I type in 10 it's even

bigger but it happens all now

automatically but there are some things

that we're not detecting for instance

suppose I type in cat well that's

handled by the get it function as I

claimed earlier that's one of the

features of using a library you don't

have to deal with erroneous input but we

only designed a function called get int

to get you an integer we don't know if

you want it to be positive negative zero

or some combination thereof and it's

kind of weird to allow the user to type

in like negative one for the size of the

Grid or you know ne3 for the size of the

grid and indeed your code does nothing

so at least it's not crashing but that's

kind of stupid right like it'd be nice

to force the user if they want a grid to

give us a positive value so how could we

do this well I could go up here and I

could say something like if n is less

than one so if it's zero or negative

which I don't want what could I do well

I could say well prompt the user again

for the size and now notice I'm not

declaring and again because once it

exists you don't have to mention the

data type again we said that earlier but

this is kind of stupid why because now

when You' given the user a second chance

okay now maybe I'll do all right if this

version of n is less than one well let's

just go and prompt the user a third time

I mean you can kind of see where this is

stupidly going like this can't be the

right solution to keep typing

recursively the same thing again and

again like where would it stop you'd

have to give them a finite number of

chances or just you know make a mess of

your code so what would be intuitively a

better solution

here yeah so some kind of loop we've

seen a while loop we've seen a four Loop

so maybe one of those so let me try this

let me delete this messiness and just go

back to the first question and let me do

this so while n is less than one so

while the number is not what we want

let's just prompt the user in a loop

this time for the size again now here

too this is better because it's only two

requests for information but clearly

line six and N are pretty much identical

other than the int and if I went in and

changed the size you know uh if I add

this if I change the wording here change

it to a different language like I have

to change it in two places that's bad

copy paste bad so what might be better

well it turns out there's another

Paradigm in C that you can use that gets

around this problem this duplication of

code it would be much nicer if I just

write this code once and I can do that

using a third type of loop called a do

while loop so it turns out in C you can

do this if you want to get the value of

a variable like n first just decree

create the variable without an initial

value so int n semicolon means it we

don't know what value it has yes but

that's okay we're going to add a value

to it eventually then I'm going to say

this do literally I'm going to open my

curly braces and what do I want to do I

want to assign to n the return value of

get in prompting the user for size well

when do you want to do that I want to do

that while n is less than one and this

code now achieves the exact same goal

but by never repeating myself why well

notice on these lines of code now I'm

literally saying on line six give me a

variable called n of type integer it

doesn't have a value initially but

that's fine you can do that line seven

says do the following what do you want

to do get int prompting the user with

the word size and just store that value

in N but because code runs top to bottom

left to right now it's reasonable on

line 11 to ask that question okay is the

current value of n which it definitely

got on line a less than one and if the

user didn't cooperate they typed in zero

or Nega 1 or3 what's going to happen

it's going to go back up here and repeat

repeat repeat everything in the do while

loop so a do while loop in C which is

not something some other languages have

like python if you know it does not have

a do while loop this is perhaps the

cleanest way to achieve this even though

it's a little weird that you have to

declare your variable create your

variable up top and then check it down

below but other wise it's similar to a

while loop it just flips the order in

which you're asking the question any

questions on this construct and Doh in

general is super useful when you want to

get input from the user and make sure it

meets certain requirements all right so

now that we have this building block

after that interlude how can I go about

cleaning up this code and then let's

conclude by taking a look at things that

our code can't do or can't do very well

or correctly let me propose that in a

final version of Mario let me just add

what are called now some comments so it

turns out in code in C you can Define

what are called comments which are just

notes to self some of you discovered

these in scratch there's little yellow

sticky notes you can use to add

citations or explanations in C there's a

couple of ways to write comments and in

general comments are notes for yourself

for your ta for your colleague as to

what your code is doing and why or how

it's a little explanatory note in

English or whatever your human language

might be so for instance what I might do

here in my implementation of this

version of Mario I might first ask

myself a question like I I might first

make a note to S like this on a new line

above this first block of code uh get

size of Grid it's just an explanatory

remark in any tur English that generally

explains the next six or so lines the

next chunk or block of code if you will

it would be a little excessive to

comment every single line at some point

the programmer should know what

individual lines of code do but it's

nice to be able to kind kind of glance

at this comment on line six that starts

with two slashes and it gets gray out

because of syntax highlighting it's not

logic it's just a note to self it

generally gives me a little cheat sheet

as to what the following lines of code

should be doing and or why and then down

here well there's a second block of code

that's a bunch of lines but together

this just what prints uh grid of bricks

and so it's another comment to myself

that just makes it a little more

understandable what these 20 some odd

lines of code are doing by adding some

English explanations thereof but now

that I have these you know wouldn't it

be nice if I could kind of abstract

these pieces of functionality away this

getting of the size and this printing of

the Grid in other words suppose that you

didn't know where to begin with this

problem and the problem at hand were

literally Implement a program that

prints a grid of bricks of some variable

size three or four or five or whatever

the human types in if you have really no

idea where to start comments are

actually a good way of getting started

because comments can be a approximation

of what we called last week pseudo code

pseudo code is tur English that kind of

gets your point across like for the uh

phone book searching like last time so

if you didn't really know where to begin

you could do something like this I could

for instance just say get size of grid

as my first step and then print grid of

bricks as my second step and that's it

for my program thus far this is now

implemented in pseudo code I have some

massive placeholders there like I still

have work to be done but at least I have

a highle solution to the problem in

comments and now I can even go this far

I could say well let's suppose that

there's just a function already that

exists called get size I could do

something like this I could do Inn

equals get size and now I just have to

assume for the moment that some

abstraction called get size exists it

doesn't this does not come with the

cs-50 library but I could invent it I

bet how else might I proceed well let's

just assume for the moment that there's

also a function called print grid that

just prints that a grid of that size n

so here too is an abstraction these

puzzle pieces don't exist these

functions don't yet exist but in C just

like in scratch I can create my own

functions how do I do that well let me

go down later in the file and by

convention you generally want to leave

main at the top of your code why because

it's the main function and it's just

where the human eye is going to look to

see what some file of code does and let

me do this I want to create a function

of my own called get size whose purpose

in life is to get the size that the user

wants I want this function to return an

integer and the Syntax for doing that is

this write similar to a variable the

data type that this function returns I

don't need this function to take any

inputs and so I'm going to use a new

keyword that we've actually been using

thus far more on it another time just

called void which just means this get

size function does not take any inputs

it does have an output it outputs an INT

and this is just a weird order in which

you write it you write the output format

the name of the function and then the

inputs if any inside of parentheses and

now I can Implement get size but I've

already implemented get size or at least

now at this point in the story I at

least know concretely what to do and I

could figure out eventually with some

trial and error perhaps all right if I

declare a variable and I do the

following n equals get in prompting the

user for size and I keep doing that

while n is less than one once that block

of code is done here is a new keyword in

C where you can return that value n so I

keep referring to these values that some

functions return as return values in C

there's literally a keyword called

return that will hand back to any

function that uses that function the

value in question so in a nutshell

between lines 15 and 21 now here is some

code identical to our solution earlier

that gets a value n from the user that

is positive it's one or two or higher

it's not zero or it's not less than one

and as soon as we've got that value we

hand it back as a return value notice

how I'm using this function on line

7even just like with get int just like

with get string I'm calling the function

nothing in the parenthesis in this case

but then I'm using the assignment

operator to copy whatever its return

value is into my variable n and so now I

have a function that didn't used to

exist called get size that gets me a

positive integer no matter what and now

for the grid how do I do this how do I

invent a function called print grid that

takes a single argument a number and

prints a grid of that size well let's go

down here I'm going to write the name of

this function print grid this function

just needs to print it has a side effect

as we keep saying so I'm just going to

say it has no return value it's just

void it doesn't have an output per se

it's just an aesthetic side effect but

it does take an an argument an argument

is an input and the Syntax for this in C

is to name the type of the input it

takes

and the name of the variable and I could

call this anything I want I'll call it

size I could call it n and it's okay to

use the same variable in different

functions but I'll call it size just to

be distinct and then in this function

I'm just going to copy from memory the

same code as before for in I get zero I

less than size instead of three I ++

inside of this four uh int J gets zero J

is less than size j++ and inside of that

print print out with print f a single

hash print out after that Loop a single

new line and that's it now I did this

fast admittedly but it's the same code

that I wrote earlier but now just like I

did with scratch let me just arbitrarily

hit enter a bunch of times to like move

the code out of sight out of mind now I

have abstractions I have puzzle pieces

that now exist called get size and print

grid Syntax for which takes some getting

used to but they now just exist except I

do need to do one thing because C is a

little naive if I try to do make Mario

now and hit enter implicit Declaration

of function get size is invalid and

we've seen that before when I hadn't

included a file right when I hadn't

included cs50 Library get in didn't work

but that's not the issue here because

this is not from a library I just

invented this C kind of takes you

literally and if you define these

functions at the bottom of your file

they don't exist on line seven or 10 so

I could do this I could all right fine

well let me just kind of highlight all

of this cut with to my clipboard and

paste it up here this would solve the

problem I could just move all of those

functions at the top of my file that's

kind of annoying because now main is

like the bottom of the file you're not

it's going to take longer to find it

it's just that's not a clean solution so

let me put it back where it was at the

bottom and let me do this this is the

only time in cs50 and really in C

programming where copy paste is

reasonable if you copy and paste the

first line of code from each function

and then end it with a semicolon you can

tease the compiler by giving it just

enough of a hint at the top of the file

that okay these functions don't exist

till down later but here's a hint that

they will exist this is how you can uh

convince the compiler to trust you so

those other functions can still be lower

in the file below main but now when I do

make Mario oh damn it oh I said print

instead of print F that's my

bad print f

so if I do make

Mario Mario now I can type in three and

we're back in business now this was a

very heavy-handed way and long way to

get to a much more complicated solution

but this solution in some sense is

better designed why because now

especially without the comments I mean

look how short my code is my main

function is literally two lines of code

why well I kind of factored out the

juicy stuff into its own functions and

now especially if I'm working with

colleagues or others you could imagine

splitting up up large programs into

smaller parts having different people

Implement different parts so long as you

all agree in advance on what those

inputs and those outputs actually are

all right so let's now consider what

computers can do well and not so well C

indeed supports a whole bunch of

operators mathematically via which we

can do addition and subtraction

multiplication division and even

calculate the remainder when you divide

one number by another in fact why don't

we go ahead and use these in a very

simple program and make our very own

calculator so let me go over here here

to vs code let me go ahead and create a

new file called calculator. C and in

this file let's go ahead and first

include a couple of now familiar header

files

cs50.h as well as standard i.h let's go

ahead then and declare main with int

main void and then inside of main let's

do something relatively simple let's

declare an INT and call it X and set it

equal to whatever the return value is of

get int prompting the user for a value

for x let's then give ourselves a second

variable we'll call it say y set that

equal to the return value of another

call to get int prompting the user this

time for that value Y and then let's

very simply go ahead at the very end and

just print out say the sum of X Plus y a

super simple calculator so I'll use

print F quote unquote percent I for

integer back sln to give me the new line

then I'm going to go ahead and do x + y

to indeed print out the sum let me go

down to my terminal window now let me do

make calculator in order to compile the

C no error messages so that's good let

me do/ calulator and let's do something

like 2 + 2 which of course should equal

4 and it does but it turns out that

sometimes there are going to be

limitations that we bump up against and

let me get a little more ambitious here

let me clear my terminal window and let

me go ahead and rerun calculator again

and this time let's type in

O2 billion for x and let's type in the

same for y and of course now the answer

of 2 billion plus 2 billion should have

of course be 4 billion and yet it's not

so curiously we see of all things a

negative number here which suggests that

somehow the plus operator doesn't quite

work as well as we might like now why

might this actually be well it turns out

that inside of your computer is of

course memory or Ram random access

memory and depending on the size of your

computer and the type of computer it

might very well look a little something

like this a little circuit board with

these black little modules on it that

actually contain all of the btes of your

computer's memory unfortunately you and

I only have a finite amount of this

memory inside of our computers which

means no matter how high we want to

count there's ultimately going to be a

limitation on high how we can count

because we only have a finite amount of

memory we don't have an infinite number

of zeros and ones to play with we have

to actually be bounded ultimately so

what's the implication of this well it

turns out that computers typically use

as many as 32 bits zeros or ones to

represent something like an integer or

in C an INT so for instance the smallest

number we could represent using 32 ins

of course using 32 bits of course would

be zero 32 zeros like this here and the

biggest number we could represent is by

changing all of those zeros to ones

which in this case will ideally give us

a number that equals roughly 4 billion

in total it's actually 4 billion 294 mil

967 295 maximally if you set all 32 of

those bits to ones and then do out the

actual math the catch though is that we

humans and Compu in general also

sometimes want to and need to be able to

represent negative numbers so if you

want to represent negative numbers as

well as positive numbers in zero you

can't really just start counting at zero

and go all the way up to roughly four

billion you got to kind of split the

difference and maybe allocate half of

those patterns of zeros and ones to

negative numbers and the other half

roughly to positive numbers so in fact

in practice when you're using even as

many as 32 bits the highest most

computers could count certainly in a

program like this in C using an INT

would be roughly 2 billion that is 2 b47

m

483,000 flow where you want to use more

bits than you have available and as a

result you sort of overflow the

available space what does this mean in

fact in real terms well let's suppose

that you only have three bits but I'm

going to gray out a fourth bit just to

convey where we'd like to uh put an

additional bit ultimately if this of

course is zero per week zero discussion

this is 1 2 3 4 5 6 7 now ideally in

binary if you want to add one more to

this value seven you're going to have to

carry the one mathematically and that

would ideally give you 1 0 0 0 but if

you don't have four bits and your

computer's only sophisticated enough to

have three bits not even 32 but three

the implication is that you're

effectively representing not one0 0 but

rather 0 z0 there's just no room to

store that fourth bit that I've gray out

here which is to say that your integer

might overflow and as soon as you get to

seven the next number once you add one

is actually going to be zero or worse as

we've seen here in my code a negative

value instead so what could we do to

perhaps address this kind of concern

well C does not have just integers or

ins it also has Longs which as the name

suggest or just longer integers which

means they have more bits available to

them so let me go back into my code here

I'll clear the terminal window and let

me go ahead and change my integers to

literally long here long here I'm going

to have to change my function in

uh cs50's library to be not get in but

get long and that's indeed another

function we provide in the library let

me change this get in to get long as

well I'll keep my variable names the

same but I do need to make one other

change it turns out that print F also

support supports other format codes so

not just percent I for integers or

percent s for Strings but also for

instance percent Li for a long integer

as well as percent f for floating Point

values with decimals so with that said

let's go ahead and change my print F

line to be not perc I but percent Li I

now let me go ahead and do make

calculator again enter no apparent

errors now do/ calulator and 2 + 2 still

equals 4 as before but now if I do

calculator again and let's do

two billion again as well as 2 billion

for y previously we overflowed the size

of an integer and got some weird

negative number because the pattern was

misinterpreted if you will as a negative

number instead but along instead of

using using 32 bits conventionally uses

64 bits which means we have more than

enough spare bits to go when we add 2

billion plus 2 billion and now in fact

we get the correct answer of four

billion which does fit inside of the

size of a long now along can count up

quite high and in fact it can count as

high as this nine quintilian and so that

will give us quite a bit more Runway but

of course it too is ultimately going to

be finite so if you have numbers that

need to go bigger than that you might

still very well have a problem now

there's another problem that we might

run into as well and we can see it in

the context of even this simple

calculator computers also suffer from

potentially what's called truncation

where especially when you're doing math

involving floating Point values that is

numbers with decimals you might

accidentally unknowingly truncate the

value that is lose everything after the

decimal point so in fact let me go back

to vs code here I'll clear my terminal

window and let's still use Longs but

let's go ahead and use division instead

of addition here so let me change this

plus to a divide operator let me go

ahead and recompile the code down here

with make calculator let me go ahead and

run/ calculator and let me go ahead and

do something like 14x and 3 for y and

we'll see that well wait a minute 1

divided 3 I learned should be 13 but in

a floating point value that should be

point0 it should be

0.33333 you know maybe with a little

line over in grade school but really an

infinite number of Threes And yet we

seem to have lost even one of those

threes after the decimal point because

the answer is coming back here as just

zero so why might that be well if I know

that two integers when divided one by

the other is supposed to give me a

fraction a floating point value with a

decimal point I can't continue to use

integers or even in this case Longs

which do not have support for decimal

points so let me go ahead and change

this format code here from percent Li I

to percent F which is again going to

represent a floating point value instead

of a long integer or even an integer and

let me go ahead further and Define maybe

a third variable Z as a float itself so

I'll give myself a variable Z equals x /

Y and now rather than print X ided Y

let's just go ahead and print Z so now

I'm operating in a world of floating

Point values because I know proactively

that an long or an INT divided by

another such value if it's meant to have

a fraction needs to be stored in a

floating point value something with a

decimal point well let me go down to my

terminal window here and rerun make of

calculator seems to work okay/ calulator

and let's do 1 / 3 again and still here

we see all zeros well so we do at least

see a decimal point so we've made some

progress thanks to the percent F in the

float but it seems that we've already

truncated the value 1 / 3 so how do we

actually get around this issue well if

you the program know that you're dealing

in a world that's going to give you

floating Point values with decimal

points you might very well need to use

what's called a feature known as typ

casting that is convert one data type to

another by explicitly telling the

compiler that you want to do so now how

do I do this well let's go back to my

code here and if the issue fundamentally

is that c is still treating X and Y as

integers or technically Longs with no

decimal point and dividing one by the

other therefore has no room so to speak

for any numbers after a decimal point

why don't I proactively do this let me

using a slightly new syntax with

parenthesis specify that I want to

convert X proactively from a long to a

float let me specify proactively that I

want to convert y from a long to a float

as well and now let me go ahead and

trust that in Z should be the result of

dividing not a long by a long or an INT

by an INT but rather a float by a float

Let Me Clear My terminal window run make

calculator again seems to work okay/

calulator and now 1 3 and hopefully now

we actually see that my code has

outputed

0.333 333 and I think if we kept showing

more numbers after the decimal point

we'd theoretically see as many of those

threes as we want but there is still one

more catch and especially when we're

manipulating numbers in this way in a

computer using a finite amount of memory

another challenge we might run up

against besides integer overflow besides

truncation is this known as floating

point in Precision just as we can't

represent as big of an integer as we

want using int or long alone because

there is going to be an upper bound

they're similarly going to be a boundary

on just how precise our numbers can be

and indeed let's go back to vs code here

I'll clear my terminal window yet again

and this time let me use some slightly

unley syntax to specify that I don't

want to see the default number of

numbers after the the decimal point

which percent F gives us automatically

let's go ahead and show me 20 decimal

point numbers after the decimal point

and the weird Syntax for this is to do

not percent F but percent period to zero

to indicate to see that I want to see 20

digits not the default after now the

decimal point let me rerun make

calculator let me do dot calculat again

and let's do one let's do three and now

this is even weirder right from grade

school you presumably learned that 1

divided 3 is of course 1/3 but that

should be

0.33333 infinitely many times or on

paper with a little line over it but the

computer is just doing some weird

approximation here it's a whole bunch of

Threes And then 43 267 44 079

590 well what's really happening under

the hood well again is this issue of

floating point in Precision if you only

have a finite number of bits and in turn

a finite amount of memory the computer

can really only be so precise

intuitively alter Converse or

equivalently the computer has decided on

some way of representing floating Point

values but the catch is per grade school

math there's an infinite number of

numbers out there and an infinite number

of floating Point values because you can

keep adding more and more digits if you

want so the computer given the way it's

implementing these floating Point values

is essentially giving us the closest

approximation that it can now how can we

go about improving the situation well

there is one alternative instead of

using float I can use something called a

double which as the name suggests uses

twice as many bits as a float so instead

of 32 typically it will use 64 and

that's just like the difference between

a long and an INT which gave us more

bits but in this case this will be used

for more Precision let's go ahead and

Cast X to a double let's cast y to a

double and now let's go ahead and using

the same format code percent 2f is still

okay for doubles let me do make

calculator let me do do slash calculator

and now let me do 1 / 3 and we still

have some of that in precision and we'd

see even more of it if we looked at more

than just 20 digits but now we have more

threes after the decimal point so it's

at least more more more precise but it's

not perfect but it it's at least more

precise so these kinds of issues then

are going to be necessary to keep in

mind anytime you do something

numerically scientifically at least with

a language like C where you're going to

bump up against these real world

limitations of hardware and intern

language now later in the semester we'll

transition to a language called Python

and that's actually going to solve at

least one of these problems for us but

just automatically giving us more bits

so to speak as we need them at least for

integers but even the issue of floating

point in Precision is going to remain

now just how real world are these issues

well back in the year 1999 we got a

taste of this when the world realized in

the Years leading up to that date that

it might not have been the best idea to

implement computers and software therein

by storing years using just two digits

like instead of storing 1999 to

represent the year 1999 a lot of

computers for reasons of space and cost

were in the habit of kind of cutting a

corner and just using two digits to keep

track of the year the problem with that

is that if systems were not updated by

the year 1999 to support the year 2000

2001 and so forth is that just like

before with integer overflow some

computers might add one to the year in

their memory 99 it should be the year

2000 but if they're only using two

digits to represent years they might

mistake the year as some systems may

very well have for the year 1900 instead

taking literally a big step backwards if

you will now you'd like to think that

kind of issue is behind us especially as

we understand all the more about the

limitations of code and Computing but

we're actually going to run up against

this very same type of issue again in

just a few years on January 19th in the

year

2038 we will have run out of bits in

most computers right now to keep track

of time it turns out years ago humans

decided to use a 32-bit integer to keep

track of how many seconds had elapsed

over time they chose a somewhat

arbitrary date in the past January 1st

1970 and they just started counting

seconds from there on out and so if a

computer stores some number of seconds

that tells the computer how many seconds

have passed since that particular date

January 1st 1970 unfortunately using a

32-bit integer as we've seen you can

only count so high at which point you

overflow the size of that variable and

so potentially if we don't get ahead of

this as humans as a society as computer

scientist on the date January 19th

2038 that bit might flip over there by

overflowing the size of those integers

bringing us back computationally to

December 13th

1901 so this is to say now with all of

this computational ability and code

comes a responsibility to actually write

correct code next week we'll peel back

some of these layers but for now this

was week one and best of luck on problem

set

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one

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all right this is cs50 and this is this

is week two wherein we're going to take

a look at a lower level at how things

work and indeed among the goals of the

course isn't this bottom up

understanding so that in a couple of

weeks time even a few years time when

you encounter some new technology you'll

be able to think back hopefully on some

of this week's and this courses basic

building blocks and Primitives and

really just deduce how tomorrow's

Technologies work but along the way it's

going to seem it's going to be a little

hard perhaps to see the forest for the

tree so to speak and so the goal at the

end of the day still is going to be

problem solving and so we thought we'd

begin today with a look at some of the

problems we'll talk about or solve this

coming week uh and for that we have some

Brave volunteers who have already come

up if we could turn on some dramatic

lighting and meet today's

volunteers so on my left here we have hi

my name is Alex I'm a first year at the

college and I'm from Chapo North

Carolina Welcome to Alex and to Alex's

right um I'm Sarah I'm from Toronto

Canada and I'm also a first year student

at the college wonderful well welcome to

both Al and Sarah so one of the problems

you'll perhaps solve this week for

problem set two is to analyze the

reading level of a body of text whether

someone reads at a first grade level

second grade level third grade level all

the way up to 12 or 13 or Beyond but

you've perhaps never quite thought about

certainly in terms of code like how you

would analyze some text some book and

figure out what reading level is it at

and yet surely our teachers growing up

kind of knew or had an intuitive sense

of this so let's consider some sample

text for instance Alex what have you

been reading lately um

One Fish Two Fish Red F Fish Blue Fish

wonderful so given that what grade level

would you say Alex is currently reading

at feel free to just shout it

out first first so indeed you'll see

this week if you run your code on Alex's

text it actually turns out he reads

below a first grade reading level but

but why might that be what might your

intuition be for why we've

uh why we've accused Alex of reading at

this level feel free to shout

out yeah so very few syllables short

words short sentences and so there's

some puristic perhaps we can infer from

that short text that that probably means

that it's best for younger children now

Sarah by contrast what have you been

reading Mr and Miss dersley of number

four privet Drive were proud to say that

they were perfectly normal thank you

very much they were the last people

you'd expect to be involved in anything

strange or mysterious ious because they

just didn't hold with much nonsense all

right now irrespective of what grade you

were in when you might have read that

text what grade level does Sarah seem to

be reading

at so eighth grade second grade okay so

hearing a bit of everything so that at

least according to code would actually

be seventh grade and what might the

intuition there be why is that a higher

grade level even though we might

disagree exactly which grade it is comp

yeah so complicated sentences longer

sentences so indeed a lot more more

words were being spoken by Sarah because

there was so much more there on the page

so we'll translate these ideas this

coming week and problem set two if you

tackle this one to code so that you can

ultimately infer things of these

quantitatively but to do so we're going

to have to understand text so let's

first thank our volunteers and then

we'll dive in to that lower

level stress

balls sure you can keep those yeah all

right so besides that let's consider one

another body of text perhaps that you

might see this week which is namely a

little something like this what I have

here on the screen is what we'll start

calling today Cipher text it's the

result of encrypting some piece of

information and encryption or more

generally the Art and Science of

cryptography is all around us it's what

you're using on the web on your phones

with your Banks and anything that tries

to keep data secure is using encryption

but there's going to be different levels

of encryption strong encryption weak

encryption and what you see here on the

screen isn't all that strong but we'll

see later today how we might decrypt

this and actually reveal what the plain

text is that corresponds to that Cipher

text but in order to do so we have to

start taking off some training wheels so

to speak and believe it or not even

though your time with C this past week

for the first time probably might have

been rather in the weeds and much more

complicated seemingly than C it turns

out that along the way we have been

providing and will continue to provide

certain training wheels for instance the

cs50 library is one of them and even

some of the explanations give up topics

for now in these early weeks will be

somewhat simplified abstracted away if

you will but the goal ultimately is for

you to understand each and every one of

those details so that after cs50 you

really can stand on your own and

understand and wrap your mind around any

future Technologies as well so let's

consider first the very first program

with which we began last week which was

this one so hello world and C at the end

of the day it was really the print a

function that was doing the interesting

part of the work but there was a lot of

technical stuff above and below it the

the curly braces the parentheses words

like void and include and then of course

the angled brackets and more but at the

end of the day we needed to convert that

source code in C to machine code the

zeros and ones in binary that the

computer understood and to do that of

course we ran we compiled the code we

ran make and then we were able to

actually run that code there so let me

actually go over here to VSS code and

really quickly recreate that hello.c

pretty much by transcribing the same so

I have here uh include standard

i.h uh int main void and then in here I

had quite simply hello comma world with

my back slend quotes and more now last

time to compile this I indeed ran make

hello followed by enter hopefully you

see no errors and that's a good thing

and if you do do/ hello you see in fact

the results of that program but it turns

out that make is not actually a compiler

as I alluded to last week it's a program

that clearly makes your program but it

itself just automates the process of

using an actual compiler and there's

lots of different compilers out there

and the one that it's actually using

underneath the hood is a little

something called clang for C language

and clang is a pretty popular compiler

nowadays there's another one that's been

around for ages called GCC but these are

just specific names for types of

compilers that different people

different companies different groups

have actually created but if you use in

week one a compiler yourself manually

you have to know you have to understand

a little more about what's going on

because it's even more cryptic than with

just make a loan so in fact let me go

back to my terminal window here let me

go ahead and clear the screen a little

bit and just run really the raw compiler

command so what make is automating for

me let me actually do this manually for

just a moment so if I want to compile uh

hello.c into an executable program I can

run I can do this uh clang space hello

C and then enter and now there's no

output which is a good thing in this

case no errors but notice this if I go

ahead and type LS it turns out there's a

uh a file that's been created suddenly

in my current folder weirdly called

a.out that stands for assembler output

and long story short that's actually the

default name of a program that's created

when you just run C by itself now that's

a pretty uh bad name for a program

because it doesn't describe what it said

does so better would be here to perhaps

do well instead of a.out which yes still

prints hello. world but isn't really a a

clearly named program it'd be nice to

name this hello so what could I do I

could do like we learned last week well

I could rename a.out to Hello by using

linux's MV command so I'm going to move

a. out to become hello but that too

seems kind of tedious now I have three

steps like write my code compile my code

and then rename it before I can even run

it like we can do better than that and

so it turns out that certain commands

like clang support what we're going to

start today calling commandline

arguments a commandline argument unlike

an argument to a function is just an

additional word or key phrase that you

type after a command at your prompt in

your terminal window that just modifies

the behavior of that command it

configures it a little more specifically

so what you're seeing here on the screen

is a summary of a better command with

which to run clang so that now I can

specify the output of this command this-

o so what do I mean by that well let me

go ahead and clear my terminal window

again and more explicitly type clang d o

hello hello. C and then enter nothing

again appears to happen but that's a

good thing when you see no errors and

now the program I just created is indeed

called hello so it achieves really the

same exact effect as make did but what I

don't have to do with make is type and

remember something as long as this

command and this too is a bit of a white

lie it turns out we have preconfigured

vs code in the cloud for you to also use

some other features of clang that would

be even more tedious for you to write

yourselves and so really this is why we

distill this as ultimately just running

make so let me pause here to see first

if there's any questions on what I've

done by taking my very first program in

C and just now compiling it first with

make but then starting over and now

manually compiling it with clang with

what we'll call command line arguments -

o space hello and then the name of the

file

yeah yeah so a.out is a historical name

it refers to assembler output more on

that soon and it's just the default file

name that you get automatically if you

just run the compiler on any file so

that you have just a standard name for

it but it's not a very well-named

program instead of running Microsoft

Word on your Mac or PC it would like be

like double clicking on a.out so instead

with these command line arguments you

can customize the output of clang and

call it hello or anything you want other

questions on what I've done here with

clang itself the

compiler

yeah so- o and you would only know this

from reading the manual taking a class

means output so- o means change clangs

output to be a file called hello instead

of the default which is a.out and this

too is again a detail you would have to

uh look it up on a web page read the

manual hear someone like me tell you

about it and in fact there's even more

than these options but we'll just

scratch the surface here all right so if

we now know this what more is actually

happening underneath the hood well let's

take a a closer look at not just this

version of my code but my slightly more

complicated version last week which

looked a little something like this

wherein I added in some Dynamic input

from the user so I could say not Hello

World to everyone but hello David or

hello to whoever actually runs this

program so in fact let me go ahead and

change my code here in vs code just to

match that same code from last week so

no new code yet I'm just going to in a

moment compile it in a slightly

different way so I did last week string

uh I think answer equals get string

quote unquote what's your name just like

in scratch and then down here instead of

doing world I initially wrote answer but

that didn't go well what did I

ultimately do instead to print out hello

David or hello so and so yeah sorry a

little louder yeah so percent s the

so-called format code that printf just

knows how to deal with and I had to add

one other thing someone else besides

percent F

yeah the name of the variable that I

want to plug into that placeholder

percent s and in this case it's answer

now let me make one refinement only

because now we're in week two and we're

going to start writing more lines of

code even though scratch called the

return value of the ask puzzle piece

answer always and see we have full

control over what our variables are

called and now it's probably good not to

just generically always call my variable

answer if I'm using get string let's

call it what it is so this is now just a

matter of style if you will let me

change the variable to be name just so

that it's a little clear to me to you to

a TF or ta exactly what that variable

represents instead of more generically

answer all right so that said let me go

down to my terminal window and last week

again I ran make to compile this exact

same program now though let me go ahead

and just use clang so clang d o I'll

still call this version hello space

hello.c so exact same command as before

the only thing that's different is I've

added a couple of more lines of code to

get the user's input let me hit enter

and now darn it our first error so

output from clang and make is not a good

thing and here we're seeing something

particularly cryptic uh so something in

function main undefined reference to get

string and then Linker command failed

with exit code one so there's actually a

lot of jargon in there that will tease

apart today but my hint is that clearly

my problem's in Maine although that's

not surprising because there's nothing

else going on here get string is an

issue and the uh issue is that it's an

undefined reference and yet notice I was

pretty good I added the cs50 header file

and I said last week that that's enough

to teach the compiler that functions

exist but the problem is that even

though this does in fact teach clang

that get string exists it is not

sufficient information for clang to go

find on the hard drive of the computer

the zeros and ones that actually

Implement get string itself so in other

words this include line per last week is

a little bit of a hint it's a teaser to

CLA that you're about to see and use

this function somewhere but if you

actually want to use the zeros and ones

that cs50 wrote some time ago and bake

those into your program so your program

actually knows how to get input from the

user well then I'm going to have to go

ahead and run a slightly different

command so let me do this let me clear

my terminal window just to get rid of

that distraction and let me propose now

that we run this command instead almost

the same as before clang - o space hello

then hello. C but with one additional

command line argument at the end and

this is a-h L not a number one so- L

cs50 with no space in between those two

now the L is going to result in all of

those zeros and ones that actually were

written by cs50 being linked into your

code your few lines of code or mine here

but that's the second step that the

compiler requires in order to know how

to actually execute and rather compile

your code and cs50's and cs50 is not the

only one that does this if you use any

third-party library in C that doesn't

come with the language you would do- L

such and such where whoever however

they've named their own library but you

don't have to do it for built-in things

like uh like we've been using thus far

all right so let me go ahead and try

this I'll go back to vs code here and

let me go ahead now and run clang - o

hello then hello. C and now instead of

just hitting enter - L cs50 with no

space between the L and the cs50 enter

now nothing bad happens and now I can

do/ hello what's your name I'll type in

David enter and now we see hello David

now honestly this is where we're really

getting into the weeds and now this is

taking this is really just adding new to

the process of compiling and running

your code and so the reality is even

though this is indeed what is happening

this is why we used last week and we're

going to continue using this week onward

make because it just automates that

whole process for you but it's ideal to

understand what's going wrong because

any of the error messages you saw for

problem set one any of the error

messages you see for the next few weeks

probably aren't coming from make they're

coming from clang underneath the hood

because make is just automating the

process but with make you literally just

write make and then the name of the

program you don't have to worry about

any of those command line arguments

questions then on compiling with- l cs50

or anything else

yeah sorry what is the benefit

of what is the benefit of using clang

manually none really in fact all main is

doing is just sa make is doing is saving

us some keystrokes um if you prefer

though and you just like to be more in

control you can totally run clang

manually if you remember the various

command line arguments yeah

exp exactly why did I have to explain

that is provide a hint to cs50 with the

cs50.h henter file but I didn't have to

do that with standard i.h just because

standard i.h comes with C just like a

few other libraries come with C that

we'll start seeing today um cs50 though

is not built into C everywhere and so

you do have to explicitly add that one

there

yeah a command line argument is a a word

or phrase that you type at the command

line AKA your terminal in order to

influence the behavior of a

program for whatever you're doing yeah

it changes the defaults right in our

guey World graphical user interface you

and I would probably click some boxes we

would select some menu options to

configure a program to behave in the

same way at a command line interface you

have to just say everything all at once

and that's why we have command line

arguments

yeah no make is not just for cs50 it's

used globally in any project really

nowadays using C C++ even other

languages as well in fact most every

command you see in this class unless it

has 5 zero at the end of it is globally

used only those suffix with 50 are

indeed course specific and even those

will gradually take training wheels off

of so that you know exactly what those

commands are doing as well all right so

what is it that we've just done

everything we've just done of course I

keep calling compiling but let's just go

down one Rabbit Hole so that you

understand that when you compile code

there's actually a whole bunch of steps

happening and this is going to enable uh

a lot of features like companies can

write code and then convert it to run it

on Macs and PCs alike or phones or the

like so it's not just a matter of

converting source code to machine code

there's actually four steps involved in

what you and I as of last week know as

compiling and these aren't terms that

you'll have to keep in mind constantly

because again we're going to abstract a

lot of this away but just so we've gone

down the rabbit hole once let's consider

each of these four steps that have been

happening for you for a week

automatically uh the first of which is

called pre-processing so what is this

mean well let's consider that same

program as before notice that a two of

the lines of code start with a hash mark

that is a special symbol in C and it's a

so-called pre-processor directive you

don't need to memorize terms like that

but it just means that it's a little

different from every other line and

anything with a hash symbol here should

be pre-processed that is analyzed

initially before anything else happens

so let's consider these two lines up top

what exactly is happening well it turns

out with these two lines you have two

header files of course cs50.h and

standard. i.h where are those files

because they've never been in VSS code

for you seemingly if you type LS if you

open up the file explorer in the GUI you

have never seen probably cs50.h or

standard i.h they just work but that's

because there's a folder somewhere on

the uh the hard drive that you're using

on your Mac or PC or somewhere in the

cloud as in our case and inside of this

folder traditionally called sluser SL

include and user is deliberately

misspelled it's just slightly more

succinct although it's a little weird

why we drop that one letter but user SL

include is just a folder on the server

that contains cs50.h standard i.h and a

bunch of other things as well so in fact

if you type in uh VSS code in your

terminal window uh when you're using

Code spaces in the cloud and type LS

space SL user include you can can see

all of the files in that folder but

we've pre-installed all of that stuff

for you so let's consider what's

actually in those files here where if I

highlight these two lines up top that

start with hash include well I kind of

hinted last week that what's in that

first file is a hint as to what

functions cs50 wrote for you so you can

kind of think of these include lines as

being temporary placeholders for what's

going to become like a global find and

replace that is the first thing clang is

going to do it's pre-process this file

it's going to look for any line that

starts with hash include and if it sees

that it's going to essentially go into

that file like cs50.h and then just copy

and paste the contents of that file

magically there for you you don't see it

visually on the screen but it's

happening behind the scenes and so

really what's happening with this first

line is that somewhere in

cs50.h is the Declaration of get string

like we talked last week and it probably

looks a little something like this and

we didn't spend much time on this yet

this past week but we will in time more

notice that this is how the a function

is declared that is it is decreed to

exist the name of the function of course

is get string inside of the parenthesis

are its arguments in this case there's

one argument to get string I claim today

but you've known this implicitly and

it's a prompt it's the prompt that the

human sees when you use get string what

is that prompt well it's a string of

text like quote unquote what's your name

or anything else that I asked last week

mean meanwhile get string as we know

from last week has a return value it

returns something to you and that too is

a string so again this is also called a

functions prototype it's the thing

toward the end of last week that I just

copied and pasted from the bottom of my

file to the top just so that it was like

this teaser for clang as to what would

exist later so you can think then of

these include lines as just kind of uh

combining all of those function

declarations in some separate file

called cs50.h so that you yourself don't

have to type them every time you use the

library or worse so that you yourself

don't have to copy and paste those lines

this is what clang is doing for you in

its first step of pre-processing second

and last in this example what happens

when clang pre-processes this second

include line well the only other

function we care about in this story is

printf of course which comes with C so

essentially you can think of printf's

prototype or Declaration as just being

this print f is the name of the function

it takes a string that you want to

format like hello comma world or hello

comma percent s and then with dot dot

dot this actually has technical meaning

it means of course that you can plug in

zero variables one variable two or 10 so

dot dot dot means some number of

variables now we haven't talked about

this yet and we won't really in general

print F actually returns a value a

number that is an integer but more on

that perhaps another time it's generally

not something the programmer tends to

look at but that's all we mean by

pre-processing so that the the end of

this process even though there's more

lines of code in cs50.h and standard i.h

what's really just happening is that

clang in pre-processing the file copies

and pastes the contents of those files

into your code so that now your code

knows about everything get string printf

and anything else any questions then on

that first step

pre-processing

yes

good question when you include a file

does it only include what you need or

Does it include everything think of it

as including everything so if it's a big

file that's a lot of code at the very

top and that's why if you think back to

all of the zeros and ones I showed a

little bit ago as well as last week

there's a lot of zeros and ones that end

up on the screen as a result of just

writing hello world a lot of those zeros

and ones are perhaps coming from code

that you didn't actually necessarily

need but some of it is perhaps there but

there are ways to optimize that as well

all right so step two of compiling is

confusingly called compiling it's just

this is the term that most everyone uses

to describe the whole process instead of

just this one step but once a program

has been pre-processed uh behind the

scenes by the compiler for you it looks

now a little something like this and

I've put dot dot dots just to imply that

yes to your question there's more stuff

above it there's more stuff below it

it's just not interesting right now for

us so now we have just C code there's no

more pre-processor directives at this

point all of the hash symbols and those

lines of code have been pre-processed

and convert it to something else and so

now and this is where things get a

little spooky looking uh here now is

what happens when clang or any compiler

literally compiles code like this it

converts it from this in C to this in

assembly code so this is among the

scarier languages I myself don't really

have fond memories this is not language

that many people program in if you take

a subsequent class in computer science

in systems uh a higher level class you

might actually learn this or some

variant there of but there's at least a

few people out there that need to know

this stuff because this is closer to

what the computers themselves nowadays

understand like the Intel CPUs or the

AMD CPUs the brains of today's computers

and phones understand stuff that looks

more like this and less like C now it's

completely uh esoteric but let me just

highlight a few phrases there's some

stuff that's a little familiar there is

mention of Maine at the top there in

yellow there is mention of get string

toward the bottom there is mention of

prf down below so this is just another

programming language called Assembly

Language that decades ago humans myself

included in school did write code in and

absolutely some people still write this

code especially since you can write very

very efficient code but it's a lot more

uh Arcane it's a lot more uh lot less

user friendly so you'll see in Yello now

the these are the so-called instructions

that a computer's brain or CPU

understands pushing values around moving

them subtracting values uh calling

functions and move move move so really

the low-level operations that computers

understand tend to be arithmetic

operations subtraction addition and the

like moving things in and out of memory

it's just a lot more tedious for folks

like us to write code like this this is

why you and I tend to write stuff like

this and ideally still people like you

and I tend to drag and drop puzzle

pieces that sort of abstract all of that

away further but for now this is again

called Assembly Language it is what

happens when the compiler literally

compiles your code but of course this

still not zeros and ones so we got two

steps to go so when a compiler proceeds

to step three this is where things get

converted to machine code and when a

compiler assembles your code for you it

converts what we just saw on the screen

here to actual zeros and ones the

so-called machine code that your phone

or your computer understands but it's

worth noting that these are not

necessarily all of the zeros and ones of

your program yes they re uh they are the

zeros and ones that correspond to your

hello program or printf and get string

and the like but notice that here we

need one final step in those zeros and

ones are only your lines of code but

what about cs50's lines of code that we

wrote to Implement get string what about

the lines of code that humans wrote

decades ago to implement printf those

are somewhere on this hard drive like on

my Mac my PC or somewhere in the cloud

but we need to combine all of those

zeros and ones together and Link My code

with cs50's code with uh standard io's

code all together and so what happens in

the last step ultimately is that if we

have my code here in yellow and then the

code that cs50 wrote and the code that

the authors of C itself wrote what

really is happening is that somewhere we

have not only hello.c which obviously I

wrote and wrote with us live here

there's also let's assume somewhere on

the computer a cs50.c file that

coincidentally I and cs50 staff wrote

years ago and also somewhere on the

computer there's another file Let Me

oversimplify by just calling it standard

io. C in practice it's probably

specifically called print F.C but

there's somewhere these two other files

and so this last step called linking

takes my zeros and ones from the code I

just wrote namely this code on the

screen here it then grabs the zeros and

ones that cs50 wrote and it grabs the

zeros and ones that the authors of C

wrote In order to implement the standard

IO library and lastly voila links them

all together and this is the same blob

of zeros and ones that we saw earlier

it's just now the result of

pre-processing your code compiling your

code assembling your code linking your

code and my God it's at this point like

if there were any fun in programming for

you yet we've just taken it all away we

just call this whole process compiling

why because now that we know those steps

exist and smart people solve that

problem for us you and I can kind of

operate at this level of abstraction and

just assume that compiling converts

source code to machine code questions

though on any of these

intermediate steps

yeah

a good question so where are all of

these zeros in one store because you and

I we've been using a browser at code.

cs50. of course is this web-based user

interface but again recall from last

week even though you're using a web

browser to access VSS code that

web-based version of vs code is

connected to an actual server somewhere

in the cloud and on that server you have

your own account and your own file and

really your own hard drive virtually in

the cloud think of it a little like

Dropbox or box or Google drive or one

drive or something like that so you have

a hard drive somewhere out there that

we've provisioned for you and it's on

that hard drive that we have uh your

code that you just wrote or I just wrote

cs50.c standard I.C and all of the other

code that implements the math functions

and everything else that c

supports good question

yeah

c

good question that uh hash includes

cs50.h line at the top of my code if I

just replace that with the contents of

cs50.c would that work short answer yes

that would work you could copy all of

the code there however there's some

order of operations that might come into

play and so it's probably not quite as

simple as copy paste but conceptually

yes that's what what's happening now

with that said in

cs50.h are only the prototypes of the

functions the hints as to how the

functions look what their return type is

what their name is and what their

arguments are it's in the C file that

actual code tends to be written and this

is a little confusing now because you

and I have only written code in C files

but in the next few weeks you'll

actually start writing some of your own

files as well just like cs50 just like

standard iio but in essence that line of

code just makes it easier to use and

reuse code that's already been written

and that's the whole point of a

library I say that little

louder yes does linking happen when you

compile your code yes when you run make

as we have been doing the past week now

all four of these steps are happening

pre-processing converts the hash include

lines to something else compiling

technically converts it to assembly code

which the Mac the PC the server more

closely understands assembly converts

that language to Binary machine code

that this computer actually understands

and then linking combines everything

together and in fact if you think back a

few minutes ago to when I did this- L

cs50 the reason I had to add that and

the reason my code did not compile at

first was because I forgot to tell clang

to link in cs50's zeros and ones per

that last step I don't need to do- L

standard IO because it comes with c so

that would just be tedious for everyone

in the world but cs50 does not come with

C so we link that in and to be clear too

we won't always use cs50's Library

that'll be yet another pair of training

wheels we take off in the coming weeks

but for now it makes a few things

simpler

yeah short answer yes so what do the

zeros and ones the machine code

translate to yes there is a one toone

relationship between the machine code

and the assembly code assembly code it's

not really English but at least it's

symbols I recognize it's not zeros and

ones machine code of course is just

zeros and ones so back in the day before

c existed people were programming only

in assembly code before assembly code

existed people were coding in zeros and

ones and you can imagine just how

painful that was and so each of these

languages makes life for us sort of

easier and easier in a few weeks we'll

transition to python which will in turn

make C even uh simpler or coding in

general simpler to do to all right so

with that said what now can we uh what

could go wrong with this well it turns

out that besides compiling technically

speaking there's decompiling and we've

not done this and we won't do this but

it's worth considering for just a moment

uh if you were to not compile your code

but decompile it as the word suggests

this just means reversing the process

converting it ideally from machine code

zeros and ones maybe back to C now this

would be cool perhaps if all you have is

a program you can convert it and see the

actual source code what might a downside

be if if anyone on the Internet is able

to decompile code on their machine

yeah okay so it's easier to find bugs in

the code that oh to exploit so it might

be easier to uh hack into the software

by finding mistakes you and I made

because literally they're staring at you

in code worries the zeros and ones make

it way less obvious other downsides of

what I call decompiling

yeah

yeah yeah if your code your work is your

intellectual property copyrighted or

otherwise you know that's kind of

obnoxious that someone can just like run

a command and boom they can see the

original code that you wrote now it

turns out it's not quite as simple as

that and so even though yes you could

take a program like hello or even

Microsoft Word and convert it from zeros

and ones back to some form of source

code be it in C or Java or python or

something else whatever it was

originally written in odds are it's

going to be an utter mess to look look

at why because things like variable

names are not retained in the zeros and

ones typically function names might not

be retained in the zeros and ones the

code is the logic is but the computer

doesn't care what pretty variables you

chose and how nicely named your uh

functions were it just needs to know

them as Z in ones moreover if you think

about last week we introduced things

like loops and c and besides four Loops

there's what other kind of loop for

instance so a while loop and even though

they look different and you have to

write different code they achieve

exactly the same functionality which is

to say when you compile a for Loop or

you compile a while loop if they

logically do the same thing they might

end up looking identical as zeros and

ones and so therefore it's not

necessarily predictable that you'll get

back the original code why cuz because

the zeros and ones might not know so to

speak whether it was a for Loop or a

while loop so maybe decompiling will

show you one or the other and honestly

decompiling while possible and it's one

way of reverse engineering someone's

product odds are if you're good enough

to start reading code that's been

decompiled and reading through the

messiness of it odds are you have the

talent probably to just write that same

program from scratch yourself now that's

an overstatement perhaps but it's not

quite as easy or threatening um as you

might first think so in general once

codee is piled it's pretty challenging

timec consuming costly to reverse

engineer it um much like it would be in

the real world right like all of us have

some kind of phone probably nowadays in

our pocket there's nothing stopping you

from opening it up somehow poking around

recreating what's there that's a huge

amount of effort most likely and at that

point maybe you should just invent the

phone instead of trying to reverse

engineer it so same kind of idea in the

physical world any questions then on

compiling or even decompiling in these

forms

all right so odds are at this point not

only I but you have made mistakes and

you've written buggy code a bug in a

code is just a mistake a logical error

or otherwise where the code just does

not behave correctly as you intend and

up until now odds are your debugging

techniques have been to maybe look back

at what I did in class maybe ask a

question online or in person but

ultimately it'd be nice if you had some

tools of your own with which to debug

code and this honestly is a lifelong

skill you're going to emerge from cs50

and even 20 years from now you're not

going to be writing if you're writing

code at all correct code all of the time

like all of us on the staff continue to

write bugs hopefully they get a little

more sophisticated and not sort of like

oops I missed a semicolon but even those

kinds of mistakes we make too but

there's tools out there and techniques

that can make your life easier when it

comes to solving those problems now the

term bug has actually been around for

decades but a fun story to tell is that

the first documented actual bug was

actually somehow connected to Harvard in

fact this is the log book relating to

the Harvard Mark 2 computer from 1947

whereby if you read the notes here and

if I zoom in this was an actual moth

discovered inside of this big Mainframe

computer that was causing some kind of

problems and the engineers at the time

actually thought it was funny that wow

physical bug actually explains the issue

and it's been forever uh taped to the

sheet of paper which I believe now is on

display in the Smithsonian uh with that

said this is just represented two of a

logical bug and that story is actually

uh that story was often retold by a

famous mathematician then computer

scientist really uh Dr Grace Hopper who

actually worked not only on the Harvard

Mark 2 computer but its predecessor the

Harvard Mark 1 and if you ever spent

time yet in the engineering building

across the river here you can actually

see much of this computer which is along

the wall when you first walk into the

science and engineering complex and

indeed as you've probably heard growing

up this is a Mainframe computer like

this is what Macs and PCs so to speak

looked like back in the day with very

physical things that essentially

implemented the zeros and ones that you

and I take for granted now being

miniaturized in our laptops and phones

so there's a piece of history there if

you visit campus that side of Campus

sometime do take a look but let's

consider then how we solve not of course

physical bugs but logical bugs and let's

consider something like this from last

week whereby we were trying very simply

to print like this uh column of three

bricks using hashtags of sorts so let me

go over here in just a moment to VSS

code and I'm going to go ahead and open

a program I wrote in advance and I'm

bringing it to class because there's a

bug in it and I'd like to figure out how

to solve this bug so let me open up uh

buggy z.c which is version zero of my

code and let's just take a quick peek at

what's here it's pretty short it

includes only standard i.h it uses

printf it uses a for Loop and the goal

quite simply is to print out that column

of three bricks now it's short enough

that some of you if you're getting comfy

already with see you might already see

The Logical bug it's not a syntax error

like it will compile and run but there's

a bug there and suppose that I'm very

new to see I'm very uncomfortable with C

it's 2 a.m. and I just can't see the bug

what are my recourses here for actually

finding a mistake like this well first

let's look at the symptom let me go down

to my terminal window I'm going to use

make buggy zero because again the file

is called buggy zero. C I'm not going to

use clang in fact I'm never really going

to use clang manually here and out I'm

just going to use make because it makes

our lives easier it does compile no

errors so it's not syntax it's not

something silly like a missing semicolon

but when I runbuggy Z I of course see 1

2 3 4 and this of course does not match

the zero the one two three bricks that I

actually intended for that column and

yet I'm starting counting at zero as I

usually do I've got three I'm going up

to three so where is my logical error if

it hasn't obviously jumped out at you

already well how can I solve this well

first and foremost perhaps the best

technique for solving bugs at least

early on is just use printf like thus

far we've used printf to say hello and

other things on the screen but print def

is just a function for printing anything

and there's no reason you can't

temporarily use print def to like print

out the contents of variables what's

going on inside of your program just to

figure out where your mistake is and

then you can delete that line of code

later it doesn't have to stay there

forever so let me do this instead of

just printing out in vs code the hash

symbol let me do a little safety check

here and print out the value of I so let

me go ahead and say something like I is

now I want to say I is this but of

course this is not how I print out the

value of I if I want to print out the

value of I what should I put

here so percent I for integer instead of

percent s for string so they're still

placeholders but we use percent s for

integers and now if I want to print out

I I just need the comma as the second

argument and then I all right let me go

ahead and back to vs uh to my terminal

window let me recompile the program

because I've changed it that still works

fine do/ bugy Z and now let me increase

the size of my terminal window here you

just see some diagnostic information if

you will like this is not the goal this

is not what you should be submitting for

this homework problem we're at one uh

but it is helping us diagnostically know

that okay when I is zero here's a hash

when I is one here's a hash when I is

two here's a hash when I is three here's

a hash well wait a minute that's one two

three four so clearly I'm printing it

one too many times so let me look back

at the code here by shrinking my

terminal window and let me just ask the

group where is in fact the mistake or

what equivalently would be the solution

yeah in the

middle yeah instead of less than or

equal to use just less than so you got

to kind of pick a lane here like if

you're going to start counting from zero

you generally use less than and go up to

but not through the value or if you

prefer like in the human world counting

from one on up you can use great less

than or equal to but you have to be

consistent and in general as a

programmer just always start counting

from zero if you're doing something

canonical like this but the solution is

indeed just to change this by changing

the greater less than or equal to to

less than if I re compile this program

with make buggy zero and then do buggy

zero again and let me increase the size

of my terminal window now you see okay

almost the same output but indeed I

starts at zero goes up two but not

through three all right so printf in

short should be can be your first uh

diagnostic tool instead of just staring

at the screen or raising your hand I

mean use printf to see literally what's

going on inside of your program by just

printing out things of interest and then

once you've solved the problem you can

go back into your code AS I'll do here

by shrinking my terminal window I'll

delete the print F line and now I'm

ready to share this program with the

world or submit it as homework or the

like it's just meant there to be

temporary any questions on printf as a

debugging

tool no all right well that only gets us

so far and honestly as your programs

grow and grow and grow it's going to

actually get really annoying to start

going in and adding printfs then

removing them and figuring out if you've

got multiple printfs well which one

printed what like it just gets messy

eventually to rely on printf alone so

being a computer scientist computer

scientists have written software to make

it easier to debug code that software is

what we would generally call a debugger

which would be the second tool of the

trade that you can use to actually solve

problems in your code now in the world

of VSS code there's actually a debugger

built in so the graphical user interface

you're about to see in VSS code isn't

specific to cs50 it actually comes with

vs code and it supports C and C++ and

Java and Python and lots of other

languages too but it's admittedly a

little complicated to just start using

the debugger you have to like create a

configuration file and do like some

annoying steps that just get in the way

of solving real problems so we have

automated the process for you of just

starting the debugger and thereafter

it's sort of Industry standard how you

use it but we save you the headache of

having to create those configuration

files so suppose I want to do this

suppose I want to try to debug this

program step by step using special

software well how can I do that well let

me propose that if I revert this back to

the original version where I was less

than or equal to three I'm pretty sure

that I was printing to hashes so I'm

going to do this and you might have done

this accidentally or never at all but

notice if you hover over the gutter so

to speak in vs code the part of it all

the way to the left of the editor you

see this sort of grayed out uh Red Dot

if you click there it becomes a brighter

Red Dot and this represents what we're

going to call a break point and this is

just a visual indicator that you've put

like a stop sign equivalent there and

you're telling the debugger in a moment

stop running my code there why because I

prefer to step through my code at sort

of a human speed and not as computer

speed where it runs all at once so I've

set my breakpoint which is step one and

then step two is quite simply this

instead of running the program itself

run a command called debug 50 and then

do/ bugy Z and now this will start your

program but inside of the debugger which

is a special program that smart people

wrote that will Empower you to now step

through your code line by line at again

at your own Comfort Pace I'm going to

hit enter some stuff's going to happen

on the screen whoops uh notice this is a

common mistake that I made accidentally

here looks like I've changed my code I

did because I went in and changed the

less than or equal to sign so let me go

ahead and rerun make buggy zero enter

good now let me rerun debug 50 enter and

now some stuff just happened on the

screen and it takes a moment to get

started but once it's started you'll see

this you'll still see your code but

you'll see this yellow highlight which

you've probably not seen before and

notice that it's specifically

highlighted in the same line that I set

a breakpoint on why that just means the

program has EXE the debugger has

executed all of these lines except for

line seven it has broken at not in a bad

way but it has paused execution on line

seven so it hasn't yet printed any

hashes and you can see that no hashes in

the terminal window yet it's paused

execution but what's interesting with

the debugger is the stuff over here on

the left hand side in the debugger here

you'll see under variables all of your

so-called local variables and we haven't

really made a distinction between local

and something called Global but for now

local variables just means all of the

variables that exist in your function so

I currently has a value of zero okay and

that makes sense so now how do I step

through my code and see what it's doing

well at the top of the screen here

you'll see some playback icons kind of

like a video player but they have

special meaning this first one will just

play the rest of your program all the

way to the end so you only click that if

you sort of solved a problem and you

just want to run run it to completion

like before but the next three or next

two really are really the juiciest the

second one here if you hover over it

eventually you'll see that it's called

step over step over means that the

debugger will run this currently

highlighted line of code but it's not

going to dive into it so if it's a

function like print F it's not going to

start stepping through print F line by

line why because I can pretty much

assume print F written decades ago is

correct problems probably with me but

this next line if I did really want to

step in into the printf code to figure

out how it works or find some problem in

it all these years later you can step

into printf and then the screen would

change and you'd see each of the lines

for print F line by line at least if you

have the source code for print F

installed all right I'm going to use the

first one step over and watch as the

yellow highlight moves and watch as in

the terminal window there's a hash

symbol here we go there's one hash now

notice line five is highlighted that

means it has paused on line five line

five has not yet been exec executed so

what does that mean the value of I per

the top left hand corner is still zero

but as soon as I click step over again

Watch What Happens at the top left where

I is a variable on the

screen now I and it flashed briefly has

a value of one and now if I step over

again watch the terminal window there's

my second hash now let me click step

over on the for Loop watch the variable

at top left now one goes to two now let

me click it again third hash and Here's

Where The Logical error is perhaps

revealed let me go ahead and step over

the loop now I is three wait a minute

I'm still going to print out a hash

there it is there's the fourth hash and

at this point hopefully the light bulb

proverbially has gone off I realize oh I

screwed up I can either stop the program

Al together with the red square or I can

just let it run all the way to the end

which just terminates everything at this

point I just want to get back into my

code and start fixing things and you can

close for instance as I will here the

file explorer just to hide the panel

that open so that's debug 50 but it's

not a cs50 thing that just starts the

debugger for you which is something

you'd find in most any programming

environment nowadays questions on

debugging questions

yeah good question where does it tell

you where it went wrong so sadly it does

not tell you any of that the onus is

still on you the human to use this tool

productively to walk through your code

at a a saner pace but your brain is is

the one that still needs to solve it and

I don't doubt down the line with

artificial intelligence and more

programs like this will get all the more

helpful and start answering questions

like that for us and there are other

tools we'll introduce you this semester

that are even more powerful than this

but for now it's just a tool really to

slow things down and not have to change

your code the fact that I had that panel

on the left that just showed me eyes

changing value is just an alternative to

print F and I can step through it a

little more slowly other questions on

debugging now let me show you one final

example with this debugger here and this

one too I wrote in advance let me close

buggy z.c and let me open up buggy1.c my

second version thereof let me close my

terminal window for a second and give

you a quick tour of this program which

similarly has a mistake now at the top

of this program some familiar includes

cs50.h and standard i.h this is not

something we've seen before it's

specific to this example a function

called get Negative int takes no

arguments and it returns an integer what

does it do it literally gets a negative

integer ideally from the user fun fact

though it doesn't correctly that's the

bug get Negative int is broken at the

moment so what does Maine do well main

just calls this function passing in

nothing in parenthesis no inputs and it

stores the return value in I and then it

just prints out I on the screen so

honestly just by eyeballing this you

know I feel comfortable enough with

programming and see I think main is

correct let me just stipulate main is

correct but there is going to be a bug

down here now what's the bug down here

here well let me look at get Negative in

implementation notice this first line 12

is identical to the Prototype up here

the Prototype is sort of stupidly

required up here because C reads things

top to bottom left to right the compiler

technically does so if you reference get

Negative in here but you don't implement

it until down here and you haven't told

C in advance that it will exist again

you get the error we saw last week all

right so how does get Negative int work

we declare a variable called n

we've got a do while loop that does what

it uses get int which comes with the

cs50 library per last week it prompts

the user for negative integer quote

unquote and stores the value in n i then

do all of this while n is less than zero

right remember we used a do while loop

last week to make sure the human

cooperates and doesn't give us the wrong

type of value be it positive or negative

or something else and then we return n

and there's some subtleties anyone

recall or have an intuition for why I've

declared clared in on line 14 instead of

on line 17 this is a c specific

thing exactly there's this notion of

scope in see and we'll continue to see

this over time whereby a variable only

exists inside of the most recent curly

braces that you've opened so if I've

declared and here on line 14 I can use

it anywhere between lines 13 and 21

because those are the nearest curly

Braes if by contrast as you note if I

instead said this int n equals get int

and so forth and didn't have the current

line 14 well n would exist inside of

these curly braces but not here which is

too late and definitely not here so you

just have to declare it first and then

use and reuse it as such now let me just

show you how I can debug this but let me

show you the symptoms first let me open

my terminal window let me run make buggy

one compiles okay so it's not something

silly like a semicolon dots SL buggy one

and I'm asked for a negative integer all

right let me give it negative one

enter well the main function supposed to

print out what I typed but it clearly

didn't it's prompting me again all right

so maybe it'll like -2 no maybe

-3 50 okay so it's definitely broken

right it kind of seems logically to be

doing the opposite now you can perhaps

see why this is happening already these

are deliberately simple programs by uh

for demonstration sake but let's do this

let me go ahead and set a break point in

main even though I'm pretty sure main is

correct but it just helps me start my

thought process start with Main and then

take it from there let me run now uh

debug

50 bugy one enter and let's see with

that breakpoint now the the goey is

going to reconfigure itself it's going

to pause on line eight because that's

the first interesting line inside of

Maine so I could have just put the

breakpoint on line eight too it's smart

enough to know that if I set it on six

eh you really mean line eight because

that's the first actual line of code and

watch when now what happens if I step

over this line notice that I which at

the moment seems to have a default value

of zero more on that another time but if

I click step over like before I'm

prompted for a negative integer let me

type negative 1 enter and now

notice there's no additional yellow

highlight why where am I currently stuck

logically yeah just logically I must be

in that D while Loop and even if you

don't understand it like that's the only

explanation if you keep getting prompted

surely there's a loop going on there's

only one Loop in my code so there's

probably a problem there so okay I can't

just set a breakpoint in Main and then

wait for this to work so let me just uh

let me stop this with the red square and

let me think all right instead of I can

still set my break point in main but let

me rerun the debugger instead and this

time not step over that line of code let

me step into that line of code so Watch

What Happens now instead of clicking the

second icon here let me click the third

whose name is indeed step into and watch

as the yellow highlight does not move to

line n it dives into line8 the function

on line eight thereby bringing me whoosh

down to line 17 it's kind of going down

into that next function now it didn't

bother pausing on line 12 or 13 or 14

because there's nothing intellectually

interesting there happening yet the

juicy part really starts it would seem

in line 17 so now notice n is my

variable at the top left if I click I

don't want to click step into now

though what what would go wrong if I

click on step into or what would it do

that I don't think I want to

do

yeah yeah it would step into get int but

I'd like to think that the staff's

version of get in is correct and that's

not our problem today so I want to step

over it and watch now at top left that

nothing happens yet to the value of n

until I go to the terminal window now

and type in something like Nega 1 now

notice it jumps to line 19 which is the

next interesting line top left n indeed

is1 and here's where I can now pause as

a human and think all right so while n

is less than zero all right n per the

top left corner is negative 1 so all

right while negative 1 is less than zero

well obviously that's true

mathematically so what's going to happen

it's a do while loop so when I click on

step over again it's going to go to this

line cuz it's at the end of the inside

of that Loop and now here it's looping

through again and again all right let me

do this once more I'm going to step over

all right I'm going to type in -2 and

it's the exact same thing now is my

chance on the yellow line okay wait a

minute -2 is obviously less than zero

let me try this one more time click it

once here and now all right let me give

it 50 and now okay while 50 is less than

zero that's not true so the loop is over

because it's not going to do it while 50

is less than zero that's not true so now

watch when I click step over once more

it then finishes the loop even though

there's nothing more to do it's now

about to return n it jumps back up to

main where I left off on line nine it

now prints in my terminal window the

number 50 and hopefully at this point to

your question earlier my human brain has

realized oh I'm an idiot like I flipped

my my sign there so I probably let me

stop this I probably want to do

something like this if the goal is to

get a negative integer I probably want

to say well n is for instance greater

than or equal to zero would work so

while n is greater than or equal to zero

keep doing this and that's the logic I

wanted to express so the debugger just

saves me from staring at the screen

raising a hand sort of asking someone

else at least in this case it allows me

to go through it at a healthier Pace

questions now on debug 50 which should

be your new friend even if it's not your

first instinct after

printf any questions on debug 50 no all

right all right well there's one last

Technique we can equip you with here um

and that is in addition to printf and a

debugger no joke um a rubber duck is

actually a reasonably recommended uh

solution to finding bugs in your code to

your question earlier the duck two is

not going to solve the problem for you

but if you've wondered why this little

guy has been here for so long um there's

this technique has its own Wikipedia

article of called rubber duck debugging

the idea of which is that if you're home

in your dorm room wrestling with some

bug in your code printf didn't quite

reveal the source to you debugger isn't

really helping honestly maybe it would

help to just sound out what problem

you're having similar to going to office

hours talking to a a TA or a professor

just walking through your problems

because in sort of talking to the duck

about you know the fact that you're

doing this while uh n is less than zero

and then if it is I wait a minute I'm an

idiot not just for talking to the rubber

duck you realize hopefully in expressing

yourself literally verbally you probably

will hear with non-zero probability like

some illogic in your statement and just

by sounding things out you'll realize

like oh that's my problem and so frankly

if you have roommates you can also use a

roommate for this but the rubber duck is

just sort of a go-to when your roommates

have no interest in your you know C

problem set talking something through

that um as follow as such and this is an

invaluable technique I admittedly tend

not to do it so much with a rubber duck

but ideally with colleagues human

colleagues but just talking through

things often will help you just realize

oh I said something logical now I can go

back to the code so don't solve problems

by staring at your screen endlessly for

minutes for hours at that point it's

time for a break time to walk away time

to talk to the duck if you've already

exhausted some of those other tools um

as an aside on your way out today at the

end of class we have H clearly plenty of

rubber ducks uh for you um and uh it's

become a thing over the years at least

among some uh to bring the duck with

them when they travel and send us photos

here for instance is uh cs50's rubber

duck debugger AKA ddb for duck debugger

which is a pun on a geekier program

called GDB the ganu debugger which is an

actual piece of software for debugging

this is cs50's debugger uh in the hills

of Puerto Rico uh also here on the sea

uh he made its way to San Francisco here

uh also down by Fisherman's dwarf by the

sea lions if familiar uh here at

Stanford where there's a William Gates

computer science building for computer

science uh down the road in SF at Google

uh and this is the Tre fountain in Rome

and lastly uh the Coliseum so we'll be

curious to see in the coming years where

your duck two travels so that then was

quite a bit why don't we go ahead here

and take a short five minute break no

snacks yet you're welcome to get up or

sit down we'll return in about five all

right so we are back and if the goal

ultimately today is to have a better

understanding of things like strings so

that we can solve problems with text

let's consider some simpler types of

data first how we might represent those

and then see if that doesn't lead us to

a discovery as to like how strings in

just today's modern software is using

things like that so when we talked on

week zero about representation of data

we had different ways of doing it in

terms of binary and decimal and uh unary

even when we started talking about the

same last week in code we started

talking about uh data types instead and

these data types were a way of telling

the computer like do you want an integer

do you want a character do you want a

floating point value like a real number

or even a string as we've seen but it

turns out that computers of course only

have finite amounts of resources your

computer only has a fixed amount of

memory or RAM and that actually has very

real world implications so for instance

here are some of the data types we've

seen thus far and it turns out that each

of these in C has a specific number of

bits allocated to it now admittedly this

can vary by System it's not so much the

case nowaday days but for many years for

decades computers were getting better

and better the earliest computers might

have used fewer bits for some of these

data types more modern computers might

use more bits so the numbers you're

about to see are pretty much where we

are present day so when it comes to

these data types A bu which is true or

false somewhat curiously uses a whole

bite even though that's way Overkill

because for a bull true or false you of

course only need one bit but it turns

out even though it's wasteful to use

eight bits or one bite just to represent

true or false it's just easier for

computers so a bull tends to be one bite

an INT which we've been using a lot uses

four bytes typically or 32 bits and if I

do some quick math from week zero with

32 bits you have four billion possible

values roughly but if you want to

represent positive and negative that

means you can represent roughly -2

billion all the way up to positive2

billion so that's the range typically

within if that's to few numbers for you

turns out there's things called Longs

and longs use 64 bits which allow you to

have like a quintilian number of

possibilities which is a lot certainly a

lot more than 4 billion so sometimes you

might use a long but even that's finite

and so uh as we discussed at the end of

last week bad things can happen if you

make certain assumptions as the data

because of things like Inger overflow or

the like where things wrap around then

there's a float which is a real number

something with a decimal point by

convention it's four bytes or 32 bits

which gives you in short only a specific

amount of precision it doesn't

necessarily dictate how many numbers to

the left or to the right in the

aggregate ultimately you have though uh

three uh 4 billion possible permutations

still if you need more Precision for

scientific for medical for financial

applications you might use eight bytes

AKA a double which just gives you more

digits of precision they eventually get

imprecise per the example we looked at

last week but it at least gets you

further down the line as an aside in

really really important applications in

finance and medicine and Military

operations and the like where you really

can't have rounding errors long story

short humans have developed libraries in

C and other languages that use more even

than 8 bytes so there are solutions to

these problems but they're always finite

you have to pick uh an upper bound then

there's Char which we saw briefly last

week when I asked the user for y or n

for yes or no and then there's string

which I'm going to propose as a question

mark because a string totally depends

like high h i exclamation point would

seem to be three bytes d a ID would seem

to be five so strings clearly are

variable based on what you or the human

type in so we'll see what this means

though in just a bit this though is the

thing inside of your Mac your PC your

phone might not look exactly like this

but this is a a memory module for a

modern computer and let's go ahead and

use this really as just representative

of the finite amount of memory that any

computer indeed has let's zoom in on one

of these little black chips on the uh uh

circuit board here zoom in and let me

propose that this rectangle really

represents some number of bytes like

tucked inside of this little black uh

circuit on the board is maybe I don't

know a gigabyte a billion bytes maybe

it's a 100 btes some number of bytes it

totally depends on the computer and how

much you paid for the Stick of memory

but if there's a finite number of bytes

physically implemented somehow digitally

inside of this Hardware well then it

stands to reason that we could number

those bytes we can just arbitrarily

decide that the top left corner is bite

number one or really bite number zero

the one next to it is number one then

number two number three dot dot dot

number two billion or whatever it is

however big this this memory is so if

you are use a variable in a c program

that's only one bite like a Char it

might literally be stored in that top

left hand corner of the memory like in

practice you don't care where physically

it is but really the artist rendition

would be this a Char might use one of

those single btes somewhere in the

computer's memory if you use an INT

which is four bytes it would give you

four bytes contiguous that is left to

right top to bottom but all 32 bits

would be next to each other so the

computer knows that those indeed all

belong to the same int if you need a

long or a double for that matter then

you might use a full eight bytes in this

case and you just keep using and using

this memory kind of like a a canvas you

know almost in Photoshop or a

spreadsheet where you can just move uh

pixels or you can move data around

that's really what your computer's

memory is a canvas for storing uh

information in units of bytes or 8 Bits

now we don't need to keep looking at

these circuit boards can abstract it

away as we often do and let's go ahead

and zoom in on this grid just to

consider some very specific variables so

let me zoom in and now I see fewer but

larger uh boxes on the screen Each of

which again represents a bite and now

let me propose that we play with some

actual code so here in C albeit without

a full program or three ins score one

score two score three I have

coincidentally given myself two uh uh

two scores at around 72 and 73 and then

a pretty low score at 33 of course

course last week or two weeks ago this

would have been high but now we're

dealing with actual integers so these

are three so so scores on my quizzes or

tests or the like so let me go to vs

code here and let's make a program

called scores. C so I'm going to write

code scores. C that's going to give me

my new file and let me go ahead and

Implement something like this include

standard i.h int main void and then

inside of here let me do int score one

will be 72 uh int score two be 73 and

int score three will be 33 and then let

me just do something like write a

program to average my three test scores

together something like that so let me

do print F quote unquote my average is

and I'm going to go ahead and do say

percent I back sln and now let me plug

in the results and this is kind of grade

school math now how do I compute the

average of three values well just like

in on paper I can do score one plus

score two plus score three in

parentheses because of order of

operations divided by three since

there's three total scores all right so

I think this checks out and indeed you

can use parentheses and operators like

plus and your code like this in C let me

go ahead now and do make scores no

syntax error so that's good nothing

missing there and now let me do dot

scores and see what my test average is

all right you know it's not great but I

I I think I still passed and indeed my

average here is is 59 is it precisely 59

though well let's see let's let's

actually instead of using an INT how

about we go ahead and use something like

a floating point value here and let me

go ahead and do this so let me recompile

my code make scores huh all right I've

got an issue let me zoom in on my

terminal window we've not seen this one

necessarily before but error on line

nine format specifies type double which

is a lot of precision but the argument

has Type in so what does this mean well

it's showing me with these green

squigglies that something's bad between

the percent F and this thing over here

well on the left I'm implying a float or

a double for that matter on the right

though what data type are score one

score two score three all right so

they're ins so Cent does not like this

the compiler just doesn't like that I'm

using ins on the right but I want floats

on the left so there's going to be

different ways of solving this one way

would be to just ignore the problem like

I originally did and just go back to

percent I or as an aside percent D is

often an alternative to percent I for a

decimal number but we use perc I cuz it

sounds like int so percent I is fine

here too but I don't want to just avoid

the problem I want to actually display a

floating point value so how can I fix

this well it turns out I can solve this

in a few different ways the simplest is

just to make sure that at least one

number on the right is a floating point

value like

3.0 instead of just three now I think

clang will be happier let me do make

scores enter and indeed it's okay why as

soon as you have at least one more

precise data data type on the right it

just treats everything at that point as

floating point value so that the math

works out so/ scores enter and now there

we go right you know uh some of us might

really want that third of a point our

average was not 59 it's 59 and a third

as in this case here all right so we've

solved that there as an aside though

there's one other technique to uh to

show here if you didn't want to change

it to 3.0 because that's a little weird

because I there were literally three

scores it's not like that needs to have

a decimal point you could also

explicitly convert the three to a float

by saying in parentheses float this is

what's called type casting and this will

just convert the thing right after it to

that data type if it's possible so if I

do this again make scores no errors now

do/ scores and I get in fact the same

result there's a bit of a rounding issue

here but we know the rounding relates to

the imprecision from last week for now

let me just be happy with my 59.3

something I'll take that for now but

this is you know is close to um a good

enough correct answer for me now but how

do I think about now what's going on

inside of the computer's memory well

let's consider here's that same grid of

memory each box represents a bite where

are score one score two and score three

in my memory well score one let me just

propose is at the top left but it's

taking up four boxes for four bytes

score two probably ends up right next to

it in memory though this isn't always

going to be the case but I've chosen

simple examples 73 is next to it also

taking up four bytes and then lastly 33

is in score three uh down there

underneath now if we don't if we really

look at the computer's memory look at it

with some kind of microscope or the like

there's actually 32 bits 32 bits 32 bits

in each of those four uh groups of four

bytes representing those values but

again for today's purposes onwards we

don't really need to think again and

again in binary it's just indeed these

decimal numbers being stored there but I

claim now this isn't the best design

even if if you have never programmed

before cs50 what you're looking at here

on the screen as an excerpt in what

sense is this perhaps bad design even

though it's a correct way of storing

three test

scores what's kind of bad here

yeah yeah always do exactly what you did

extrapolate to four scores five scores

50 scores this can't be that welld

designed because now you're going to

have four lines of code five lines of

code 50 lines of code that are almost

identical except for this like arbitrary

number that we're updating at the end of

the variable so indeed there's probably

going to be a better way even though at

least in C we haven't yet seen that

technique but the solution today onward

is going to be something called an array

an array is a uh way of storing your

data back to back to back in the

computer's memory in such a way that you

can access each individual member easily

put another way with an array you can

instead do something like this instead

of saying int score one int score 2 in

score three giving each a value you can

first tell the computer please give me a

variable called scores plural that you

can call it anything you want of size

three Each of which will be an integer

that is to say this is how you declare

an array in C that will have enough room

to store three integers put another way

this is the technical way of telling the

computer please give me uh 12 bytes in

total

3 * 4 each for an inch so give me 12

bytes in total and what the computer

will do is guarantee that they're back

to back to back in the computer's memory

and that'll be useful in just a moment

so let me go ahead and do something

useful with this let me store three

actual scores here's how I could now

store those same numeric scores in this

array syntax is a little different but

there's one variable called scores but

if you want to go to its first location

starting today you use square brackets

and go to location zero first which

because things in C are zero indexed uh

so to speak you start counting at zero

the first int is at bracket zero second

int is at bracket 1 third int is at

bracket two so it's not 1 two three it's

literally 0 one2 and this is not

something you have control over you must

start at zero so these lines now create

an array of size three and then insert

one two three values into that array but

the upside now is that you only have one

name of the variable to remember it's

just called scores yes you need to go

into the array to get individual values

you need to index into it using those

square brackets but at least you don't

have this hackish approach of declaring

a separate variable for each and every

one of these values so let me go back to

scores do c here and let me propose that

I do this let me just kind of use that

same idea to do the following let me get

rid of these three separate integers let

me give myself an INT scores array of

size three and then scores bracket Z

will as before be 72 scores bracket 1

will be 73 and scores bracket 2 will be

33 and let me get rid of the Little Dot

there all right so now if I go ahead and

run this again with make uh scores

enter huh what did I do wrong here I

think I got a little too ahead of myself

let me increase my terminal

window let's focus on line 10 here first

error use of Undeclared identifier score

one what did I do here that was dumb

yeah right so I didn't declare score one

I've got old code right so I just kind

of honestly got ahead of myself here not

even intentionally so let me go ahead

and Shrink my terminal window again I

need to finish my thought here so let me

clear my terminal and let me change this

now to be scores brackets zero plus

scores bracket 1 plus scores bracket two

so it's a little more verbose because

I've got these these um square brackets

so to speak but I think now my code is

consistent so let me make scores now now

compiles do/ scores gives me indeed the

same rough average with those same

values all right so let me go ahead and

maybe um tidy enhance this a little bit

it's a little silly to have to write a

special program just to check your

average of three test scores like 72 73

33 why don't I actually make the program

Dynamic and ask the human for that

average uh for those scores so instead

let me do do this how about we get rid

of the 72 and change this to get int and

I'll just prompt the user for a score

let me get rid of the 33 and change 73

and get this to be get int score quote

unquote and then lastly get rid of the

33 and replace it with get int quote

unquote score get int is a cs50 thing

for now so I need to include cs50.h as

always but I think now it's sort of a

better program because now I can compile

it once I can even share it with my

friends and now any of us can average

three scores on some classes test they

don't need to know the code or rewrite

the code just to T type in their scores

so make scores worked scores now I can

type anything I want maybe it's a 72 73

33 still get the same answer or maybe

I'm having a better semester 100 100

maybe 99 and now we get still a pretty

high score there but now it's Dynamic

now you don't need the source code you

don't need to recompile the program it's

just going to work work again and again

but this too let me propose that this

code is correct if I want to get three

scores from the user but these

highlighted lines now 6 through n are

they welld designed would you

say

yeah yeah right this is we can use a

loop is the spoiler here why I mean my

God it's like the same code again and

again and again the only thing that's

changing is the number and you it should

have kind of had some code smell again

because if I keep typing the same thing

again again and again like that's

clearly an opportunity to better design

something so let me do this let me go

ahead and still create my ver my array

of size three and but let me use our old

friend the for Loop for in I equals z i

Less Than 3 I ++ and then in here let me

do scores bracket we haven't seen this

before but any

intuition scores bracket I because that

will use whatever I is be it Z or one or

two in iteration and then I can get an

INT asking the user for score without

having to repeat myself again and again

so hopefully if I didn't make any typos

make scores all good do/ score 72 73 33

and we're back in business but the code

is arguably now better designed because

now I haven't actually uh I haven't

actually hardcoded the scores and I

haven't actually copied and pasted any

of that code well if we consider now

what's going on inside of the computer's

memory it's pretty much the same in

terms of the values but instead of the

variables being literally score one

score two score three there's just one

variable it's an array called scores but

you can index into its three locations

by using scores bracket Z to get the

first scores bracket one to get the

second scores bracket two to get the

third but this is key the memory is

contiguous it's only the screen is only

so large so it wraps around but

physically digitally the memory is

contiguous top to bottom left to right

and that's important why because the

brackets indicate 0 1 2 that each of

these integers is just one integer away

from the next it can't be randomly down

here all of a sudden it's got to be back

to back to back all right now equipped

with that Paradigm what more could we

actually do here well it turns out it's

worth knowing that it's possible in code

to even pass arrays around as arguments

and let me just whip this program up

somewhat quickly just so you've seen it

before long but let me go ahead and do

this let me propose that I create a

function that does this averaging for me

so I'm going to create a function called

average that returns a float uh and the

arguments this thing is going to take uh

let's see it's going to be the array so

it turns out if you want to take in an

array of numbers you can call it

anything you want this is how you tell C

that a function takes not an integer but

in Array of integers and you don't have

to call it array I'm doing that just for

the sake of discussion it can be called

X it can be numbers it can be anything

else I'm just calling an array to be

super explicit as to what it is there

now how do I change my code down here

what I think I'm going to do for the

moment is just this I'm going to get rid

of this code here where I manually

computed the average and let me just

call the average function here by

passing in the whole array of scores so

this is just an example of abstraction

like now I have a fun function called

average I don't care I don't have to

remember how it works once I implement

it it just kind of tightens up my main

code a little bit but I do still have to

implement this so later in my file let

me repeat myself before the only time

it's okay and see to repeat yourself

again and again by typing out again

average and then int array Open Bracket

but now not a semicolon now I have to

implement this thing and I can implement

this in a bunch of different ways uh but

I don't know huh in advance I can't just

do this I can't just do array bracket 0

plus array bracket 1 plus array bracket

2 unless unless this program is only

ever going to work on three numbers so

huh let me let me go ahead and do this

let me first propose that there's a poor

design here in my main function what

value have I repeated

twice among the highlighted lines what

jumps out at use twice L of the array

yeah the length of the array is just

three now it's not a huge deal that I

type the number three on line eight on

line nine but this is exactly the kind

of like shortcut that's going to get you

in trouble eventually why because

eventually you or someone else is going

to go in make the array bigger or

smaller and you're not going to realize

that magically that same number is in

two places and indeed this is what a

programmer would often call a magic

number a magic number is one that just

kind of appears magically and you're on

the honor System to change it here if

you change it here and then you change

it over here like that's not going to

end well if it the onus is on the

programmer to remember where they

hardcoded that is rote out three

explicitly so anytime you reuse a value

like this you know what we should

probably do what we did last week which

was to declare a variable uh perhaps at

the very top of my program so it's super

obvious what it is called maybe n and

set that equal to three better yet what

did I do last week to make sure that I

Can't Screw Up and accidentally change

that value yeah constant and the keyword

there was just const for short and now I

have a global variable Global in the

sense that I can access it anywhere that

is called n it's an INT and it's always

going to be three and now I can improve

my main function a little bit by just

changing the threes to n so now if I if

a colleague realize oh wait a minute

there's four tests this year you change

n to four recompile the code and it just

works everywhere else except in my

average function let me change it back

to three just for consistency this is

not going to fly now to just uh sum up

things like this for instance and then

return this divided three why will this

not work

now as I've defined

it

yeah okay I might be in returning an

integer value when I intend to return a

float per this but I think I'm okay CU I

Ed that little trick where I made sure

that at least one of the numbers in my

my arithmetic expression is in fact a

floating point value and just by adding

the point zero make sure that everything

gets treated as a float so I think

that's

okay sorry a little

ladder exactly so left hand's not

talking to the right hand here and that

my current implementation of average is

still assuming that there's only going

to be three tests or whatever but wait a

minute I just went through the trouble

of modifying this to be n generically

and if I change this to four I'm not

going to be happy perhaps with my

average because now I'm going to ignore

one of my test scores Al together so let

me change this back to three and

unfortunately if it's a variable now n

and therefore I have literally a

variable number of scores how do I take

the average of a variable number of

things I mean what's my building block

there

yeah

yeah why don't I use a loop that goes

through the array and adds things up as

you go I mean kind of like grade school

as you take the average on your

calculator or paper pencil you just keep

adding the numbers together and then you

divide at the End by the total number of

things so how can I do this well let me

change my implementation of average to

First declare a a variable called sum or

whatever set it equal to zero so this is

like me on my piece of paper getting

ready to count or my calculator of

course when you turn it on typically

defaults to zero and now let me do four

in I equals z i is less than a VAR I no

I didn't do that n i is less than n

i++ and now in here let me go ahead and

add to the current sum uh whatever is in

the arrays location I and then down here

I think I can just return sum divided by

3 nope not 3.0 n perhaps here and

actually I think I'm going to get let's

make sure it's a float let's use the the

type casting trick just to make sure I

don't accidentally Short change someone

and uh throw away everything after the

decimal point so it just escalated

quickly right like average just got a

lot more involved it's just not just a

single one line of code but now it's

Dynamic I initialize a variable called

sum to zero in this loop I go through

and just keep adding to sum which is

initially zero whatever is in Array

bracket I or specifically array bracket

zero array bracket one array bracket two

that gives me a total sum that I return

divided by the total number of things

now this I can tighten slightly recall

that this is syntactic sugar for just

adding things I can't use plus plus

because that only literally adds one but

I can use here plus

equals questions on this implementation

here really the only takeaway or the

most important takeaway is that this is

the Syntax for how you tell a function

that to it expects a whole array not a

single variable like an INT or the like

you literally use square brackets but

you don't specify the length inside

there

yeah what about the variable at the

top good question what do I have it

defined as at the top this variable n it

must be an integer if you're going to uh

use it inside of in arrays square

brackets here so this line 10 notice no

longer says three it says n and so

whatever n is three or four or something

else that's how many integers I will get

in that array and it must be by

definition of an array an integer that

goes in those square brackets and here's

a common source of confusion when you

create the array that is declare it you

use square brackets like this where you

put the total number of elements you

want when you subsequently use the array

like I'm doing here you don't mention

int again just like you don't mention in

again and again once a variable exists

you use the square bracket still but you

don't use n you use zero or one or two

or generically here I so when C was

designed they sometimes use the same

Syntax for two different ideas or

contexts

yeah good question do I have to include

line six short answer yes because of the

reason we ran into last week C or clang

really reads your code top to bottom

left to right and so if the compiler

sees some mention of this function

average on line 16 but you haven't told

the compiler that average exists you're

going to get an error on the screen so

the conventional way to do that is you

just copy paste the one first line of

code from the function it's so-called

prototype or Declaration

yeah really good question uh in a

perfect segue is there a library you can

use if you don't know the size of the

array no and so if any of you have

programmed in uh in uh Java or python or

other Lang languages you can actually

just ask the array like how big is it in

C you and I the programmers have to

remember it and so short answer no

there's no function that will just

automatically do this for us and in fact

let me make a more subtle claim that

it's fine to use Global variables like

this if they're really for configuration

options why it's just convenient to put

them at the very top of the file because

everyone you your colleagues your Tas

are going to see them at the top of the

code but you really shouldn't be using

them everywhere throughout your code

it'd be better if the average function

itself we're independent of that special

variable so by that I mean this you know

what I should really do if I really want

to be welld designed I should pass in

the length of the array to the average

function I should give the average

function a second argument I'll call it

length for instance but I could call it

anything I want and so rather than

putting n all the way down here at the

bottom of my file let me just

dynamically say length instead and this

is a subtlety and no need need to get

too tripped up over this but this now is

just an example of how the same function

can take not one but two arguments but

indeed in C you must remember yourself

what the length of an array is you can't

just ask the array via some syntax like

you can those of you who've programmed

before in Java or python

yeah good question would it be better to

write a function that computes the size

short answer can't do that in C as soon

as you pass an array into a function in

C you cannot figure out its size if it's

a generic array like that of integers

there are special cases that uh you can

do that but in general no it's just not

possible in see and if that's some

frustration honestly this is why more

modern languages add that feature why

because it was really annoying um as I'm

alluding here to not having that

information now just to make sure I

didn't screw up anywhere let me compile

this final version of

scores

suspense all good/ scores 72 73 33 and

we're still back in business so this

version is more complicated and as

always we'll have this version on the

course's website for reference but the

point really is that arrays not only can

be used as containers to store multiple

values three or more in this case um you

can also even pass them around as

arguments as such all right now besides

that let's let's simplify for just a

moment and consider now the world of

chars if we've just got single bites

where uh where does this lead us and how

does this get us ultimately to strings

to solve problems like readability and

cryptography and the like well here for

instance are three lines of code out of

context that simply store three chars

and you can already see where this is

going having three variables called C1

C2 C3 is clearly going to end up being

bad design because of all the silly

redundancy here but notice I'm using

single quotes like last week because

these are single chars what does this

look like in the computer's memory well

it looks a little something like this if

we clear out the old memory C1 C2 C3

probably will end up here maybe not

literally in the top leftand Corner this

is just an artist rendition but C1 C2 C3

will probably end up like that now

what's really there it's really those

same three numbers 72 73 33 but how many

bits does a bite

have just eight so if we were to look at

the binary representation of these

characters it would only be eight bits

each that's enough to store small

numbers like 72 73 33 we're not dealing

with Unicode and emoji and the like but

the point is the same you don't have to

use four bytes to store these numbers

you can use a different data type like

chars and underneath the hood it's

indeed going to use just single bytes

for each but this is sort of like a this

isn't really how we Implement strings

right when you wanted to say hi last

week or this we use double quotes and we

wrote all of the things together and

used one variable not three right when I

typed in David I didn't have a variable

for d a v i d i had one variable called

name that stored the whole thing so in C

we keep talking about these things

called strings we'll see eventually that

strings are not necessarily what they

seem to be but for now the key thing

about strings is that they're variable

length uh so to speak right they might

be three characters high or five

characters David or anything uh smaller

or larger so how do we go about

implementing strings if all we have at

the end of the day is my memory well

here is an example of just creating uh

declaring and defining a string called s

s because it's just a simple string and

quote unquote high in double quotes what

does this look like in the computer's

memory well let's clear it again and

here now because it's technically stored

in one variable s here is how I might

draw it as an artist it's three bytes in

total H exclamation point but there's no

C1 C2 C3 it's just the whole thing is uh

s but it turns out that a string fun

fact is really just what underneath the

hood kind of leading up to this what is

a string if this is how it's laid out in

memory literally it's just an array of

characters and we didn't have to know

about arrays last week to use strings

this is where again the training wheels

are starting to come off but a string is

just an array of characters Hi

exclamation point for instance so

technically an array a string called s

is really a variable called s that

allows you to get at the first character

with s bracket zero if you want s

bracket 1 s bracket2 you can literally

get individual characters just by

treating S as though it's an array which

it really is underneath the hood in this

case but there's a catch how do you know

where strings end in the past when I

drew some integers on the screen I know

I claim they always take up four bytes

if I had drawn a long it always takes up

eight bytes if I drawn a a character it

always takes up one bite but how many

bites does a string take up yeah I mean

that's kind of the right answer in this

case three it would seem but if it's

David that's a good five characters but

where do we put the number three where

do you put the number five right this is

literally all that's into your computer

this is all our building blocks in front

of us so how can we where does the three

go where does the five go well it turns

out you can solve this in a couple of

different ways but the way humans

decided to implement strings years ago

is indeed an array but they added one

extra bite at the end of every such

string array just to make clear with a

so-called Sentinel value that the string

ends here why so that if you have two

strings in the computer's memory like hi

and by you know where the barrier is

between like the exclamation point of

one and the letter B in the next right

you need some kind of delimiter and so

what really is underneath the hood is

this when you store a string in memory

when you type in a string as the user if

you type in three characters it's it's

going to use 3 + 1 = 4 bytes in total if

you type in David it's going to use 5 +

1al 6 bytes in total why because C

automatically adds this special zero at

the end of the string I've drawn it with

back SL Zer because this is how you

represent zero as a Char as a character

but this is literally just zero as we'll

soon see so anytime there's a string in

memory it always takes up one more bite

than you yourself as the programmer or

human typed in in fact if we convert

this again just for discussion sake to

those integers what's literally stored

in the computer's memory is going to be

72 73 33 and now a zero and the computer

because of c and how it was invented

it's just smart enough to know that when

you print out a string it prints out

every character until it sees a zero and

then it just stops printing in

particular printf knows how this works

and this is why print F knows when to

stop printing decimal numbers are not

that enlightening we'll generally write

the characters like this and again back

back sl0 is just special symbology like

it's what the programmer types to make

clear that you're not saying hi zero

you're saying hi and then it's a special

zero specifically it is eight zero bits

that indicate that it's the end of the

string technically that back sl0 if you

want to be fancy it's called null

NL and it turns out you've seen this

before that we didn't call it out here's

that same asky chart from the past

couple of weeks if I highlight this what

is decimal number zero mapping to n

which is just programmer speak for the

special null character all zero bits

that means the string ends here this all

happens automatically for you you do not

need to create these null characters or

these

zeros any questions

then on this implementation thus

far any questions here no well let me do

this let me go back to vs code in a

second and let's actually corroborate

this with some code let me go ahead and

create a small program called high. C

and how about we do this let me include

standard i.h let me include uh let me

type out int main void as always and now

let me do something simple and kind of

bad but Char C1 equals quote unquote H

in single quotes Char C2 equals quote

unquote I in single quotes and lastly

Char C3 equals exclamation point in

single quotes and now let me just print

this out I can't use percent s cuz that

is not a string that's literally three

chars cuz that's the design decision I

made but I could do this percent C

percent C percent C which we haven't

seen before but percent s is string

percent I is int percent C is indeed uh

Char so let me put a back sln at the end

for cleanliness and now do C1 C2 C3 so

this is like a Char based version of

printing string so let me make high and

then let me do/ high and it looks like I

use print f with percent s but I did

things very manually by printing out

each individual character what's cool

now though is that once you know that

characters are just numbers and strings

are just characters you can kind of poke

around let me change all three

placeholders to percent i instead and

this is totally fine too let me rerun

this make

High um actually let me make one change

just so we can see this let me add

spaces just for Aesthetics sake let me

do make high do SL High enter and voila

like now you can actually see the

numbers that I claimed back in week zero

were in fact happening underneath the

hood well this is not how you would make

strings it'd be incredibly tedious to

have three variables for threel Words

five variables for five letter words

we've been using of course strings since

last week so let's do that instead uh

string uh s equals quote unquote double

quotes high for this no because of these

training wheels I need to include the

cs50 library but we'll come back to that

in the coming week but for now I'm going

to go ahead and create a string s called

quote unquote high and now I'm going to

change this to be my familiar percent s

and now just print out s itself this of

course is the same thing as last week

high gives me the exact same thing but

now we're dealing of course with strings

but how can we see a little beyond that

well how about this let's poke around

further with today's Primitives even

though s is a string I could technically

print out its first character with

percent C by doing s bracket z i could

technically print out its second

character with percent C by doing s

bracket one I could print out its third

character with percent C and printing

out s bracket 2 so again this just

derives logically from my understanding

now of that strings or arrays as you

note let me do make let me do make high

SL high and no visual change but I'm

just kind of now tinkering around and in

fact if you're really curious let me do

this let me change these back to I back

to I I oops back to I and let me add a

fourth one because if I'm really curious

now let's see it what's S braet 3 this

is the fourth bite and even though the

string itself is hi I think we can

corroborate this whole null thing make

high SL High enter and there it is you

could have done this last week if you

really wanted to geek out on strings but

like for now it's just revealing what's

going on underneath the hood questions

then on what these strings are

yeah why do we need the

bracket uh uh why do you not need

brackets good question why do I not need

brackets on line six uh to because uh s

is a string we'll see in a couple of

weeks that s is essentially implemented

underneath the hood indeed as an array

but that happens automatically for you

you can treat S as just a variable name

without square brackets you will use

square brackets when you have arrays of

ins or you manually create arrays of

chars or doubles or Floats or anything

else but strings are special why I mean

every program you write seems to use

strings text in some form we're humans

we like text not just numbers and and

such so this is just treated a little

specially in C and many other languages

as

well other questions on this here no

let's add then one other string to the

mix so instead of just saying hi why

don't we consider version of the program

that says both high and by and I claim

now that that back sl0 that null

character is going to be ever more

important now if we've got two strings

in memory so that c knows how to

distinguish one from the other so let me

go ahead and just get rid of these two

lines for the moment let me recreate

string s equals quote unquote double

quotes High Let me give myself another

one and because I'm just playing around

I'll choose very short variable names

string T equals quote unquote by

exclamation point and then let me just

print them both out uh let me go ahead

and print out percent s back sln comma s

and then print F percent s uh back sln

and then T so very simple demonstration

of just these two variables make high do

SL high and of course it prints out two

lines one after the other what's

actually going on underneath the hood

well let's go back to the computer's

memory High I think it's going to be I

claim pretty much the same so s I'll

claim is in the top left followed by the

back sl0 and that's important now

because by probably is going to end up

there and Visually it wraps just by

nature of how I've drawn this grid of

bytes but it's continuous B ye

exclamation point null AKA back0 this is

Now helpful to print F because now

printf knows where one begins and ends

by way of that special null character

but we can poke around now too what else

can I do here how about this how about I

go into my code here back to BS code and

let me go ahead and say something like

well if I've got two of these uh strings

you know let's put them in an array

let's kind of do this sort of arrays in

arrays sort of inception style here so

string words bracket two so give me an

array of two strings is what I'm saying

here in code even though we've not done

it with strings yet we only did it with

ins and now let me do this the first

word AKA words bracket zero will equal

as before high and now words bracket one

will equal quote unquote by bite so by

and now I've done the exact same thing

but again I'm just avoiding having s t

QR and like all these different

variables in my code I just now I'm

treating them as one single array of

strings how do I change my code down

here well if I want to print the first

word I do words bracket zero and if I

want to print the second word I do words

bracket one this is not a useful

exercise at the moment because I'm just

making my code more complicated but

again it allows us to post poke around

and see what's going on because there is

that high and by but watch this if I

really want to be cool I can do this

let's print out percent C percent C

percent C back sln and then here percent

C percent C percent C percent C so four

of those and now here's where things get

interesting words is an array of strings

but again if I may what's a

string an array of characters so just

use the same logic if words is an array

of strings you get at the first string

with words bracket zero how do you get

at the first character in the first

string bracket Zer words bracket Z

bracket one and lastly words bracket Z

bracket two and now down here words

bracket one but the first character is

there words bracket one the second

character is here words bracket one the

third character is here whoops third

character is here and words bracket one

one the fourth character is here like

this is not how people program this is

only for demonstration sake my God it's

so tedious and verbose already but if I

make high now do SL high now I'm like

manually Reinventing percent s if I

forgot it existed using percent C alone

but you can indeed manipulate arrays in

this way but because strings are arrays

of characters you can manipulate strings

uh in this way too any question now on

this syntax

any questions here no no all right well

let's go ahead and propose that we solve

a couple of other problems we might not

have as before but first a quick visual

of what's been going on underneath the

hood here if here again is where we left

off on the screen high and by uh back to

back here is really how I just treated

these things s bracket 0123 and then t0

1 2 3 4 but really once I put them in an

array the picture becomes this words Z

is the whole High words bracket one is

the whole by but if I really get into

the weeds and start indexing into

individual characters in those strings

all I'm using is new syntax in order to

represent these same values here

questions then on these

representations before we Forge

ahead no

yeah does the new line character say

that one

more ah really good question does the

new line character take up any space uh

it does when so far as printf is

concerned but I'm not storing the back

slash n in my strings printf is being

manually handed that thing

instead all right so let's go ahead then

and consider how we might solve some

problems that have Arisen now with these

strings as follows here suppose I let's

do this let me go back to VSS code here

and let me go ahead and open up a new

file called how about uh length uh. C

and let's consider for a moment how I

might actually figure out what the

length of a string is which is distinct

from the length of an array I claimed

earlier you cannot figure out

dynamically what the length of an array

is but I can figure out the length of a

string specifically because of this

implementation detail of that null

character so let me go ahead and do this

let me include cs50.h in this second

program here let me include standard i.h

as before and let me do this int main

void and in the first thing I'll do is

just get a string from the user I'll ask

the user as always for their name so

I'll call get string and say what's your

name question mark as always and then

down here if I want to figure out the

length of this string and print the name

the print the length out on the screen

well I can kind of do this similar in

spirit to the average where I'm

accumulating something let me go ahead

and initialize n to zero let me give

myself huh it's not not a for Loop

because I don't have a I don't know in

advance how long it is but what if I do

this

while the value at name bracket n does

not equal single quote back

sl0 crazy syntax at the moment but it's

just the culmination of these various

building blocks let me just finish the

thought here

n++ and then down here let's just print

out with print F and percent I that

value of n

so I claim this is going to show me the

length of any string I type in whether

it's high or by or David or anything

else I initialize a variable to zero and

that's good because that's where you

start counting in general while name

bracket 0 does not equal back0 what is

this saying well if name is the string

the user typed in and name is just an

array as you noted then name bracket

zero is going to be the first character

and I'm asking the question well does

the first character not equal back0 and

if I type in David D is it's not so I

keep going and I add one to n then I'm

going to check name bracket one well if

I typed in David name bracket one is

going to be a a does not equal back0 and

so it's going to go again and again and

again but five steps in total later it's

going to get to the bite after David

realize wait a minute that is a back

slash n the Loop finishes and I print

out the total length arrays in general

do not have this null character however

strings do again strings are special

versus all of the other data types we've

talked about thus far but how could I

for instance uh do this differently well

let's actually Factor this out as a

function as I've I've commonly done but

rather than implement it myself you know

what it turns out what's nice about

strings being so common there are many

other people who have solved these

problems before and in fact there's a

whole string library in C it is used by

way of a header file called string.h and

what string H is is a library of string

related functions in fact you can see in

cs50's manual pages for C uh the

string.h functions at least those that

we recommend as most useful and in

particular if you poke around there

you'll see that there's a function

called Sterling means string length it

was named very succinctly just because

it's a little easier to type than string

length but Sterling tells you the length

of a string so how might I use this in

my code here well it turns out I can

simplify this quite a bit let me get rid

of my Loop get rid of my uh my counting

manually and do something like this int

n equals Sterling of the human's name

name and now I'll just use print f as

before with percent I back sln and

output the value of n but but there's a

bug at the moment what have I forgotten

to

do yeah I have to include the header

file at the top of the screen so let me

at the top of the code so let me also

include string.h at the top of my file

so that c knows that in fact sterl

exists let me go ahead and make length

as

before length uh or actually really for

the first time what's your name David

and hopefully I'm going to see in fact

five by contrast if I run it again and

type in high exclamation point now I see

three so Sterling is just one of the

functions in that library and there's so

many more in fact yet another library

that might be useful moving forward is

this one C

which relates to C data types and lots

of functions therein that can be useful

for instance if you review its

documentation in the manual pages online

you'll see that there are functions via

which we can solve problems like this

let me go ahead and propose here let me

see let's do um example here

involving how about uh checking if

something is uppercase or lowercase and

converting it uh to uppercase only let

me go back to vs code and code a program

called uppercase Doc in this file I'm

going to start by including now as

always

cs50.h I'm going to include standard i.h

and I'm going to add one other to the

mix which is

string.h now2 so I can access the length

of things as needed int main void comes

next and then within my main function

I'm going to go ahead and declare a

string called s I'm going to call get

string as before and I'm going to go

head and just ask the user for a string

called before I want to do it before and

after whatever the user types in is

before but I want to force everything to

uppercase thereafter let me now in this

Loop here do this let me print F quote

unquote after just so we can see this on

the screen and let me do four in I gets

zero I is less than stir Lang of s I ++

what am I about to do I'm about to

iterate over every every character in

the string from left to right from zero

on up two but not through the length of

s and how do I check if something is

lowercase so that I can actually force

it to uppercase well it turns out I

could do this literally if the character

in s at location I is greater than or

equal to Capital a Ampersand Ampersand

which means and instead of or which we

saw in the past s braet i is less than

or equal to little Z that means

logically in English that this is indeed

a lowercase how do I now convert it to

uppercase this character well I could

just literally print out the same

character but that would not be the

answer here because that's not changing

the value but what could I do instead

well let me actually pull up here real

fast the asky chart as before and let's

see if we can't glean some insight if I

pull up the same ask each chart and

suppose the human has typed in a

lowercase a that's

97 what letter I want to convert it to

uppercase a so what number do I want to

convert the 97 to per week zero so 65 we

keep coming back to that one what if the

user types in lowercase B I want to

change the 98 value to 66 and so forth

and any quick math how far part of those

so it's always 32 like uppercase to

lower case is always wonderfully good

design 32 away one from the other so

what does this mean well I think we saw

earlier that underneath the hood a Char

is just a number you can certainly do

arithmetic on it and here again if you

understand these lower level Primitives

what if I do this whatever s bracket I

is if I know on line 13 that it's up

that it's lowercase do I want to add or

subtract

32 so I want to subtract because I want

to go from like 97 to 65 or 98 to 66 so

indeed if you do some quick math that

gives you 32 so it suffices to just

treat chars as numbers subtract the 32

and printing it with percent C I think

will just convert lowercase to uppercase

if you now fast forward to like the real

world Microsoft Word or Google Docs if

you've ever chosen the menu option that

forces things to uppercase or lowercase

on occasion literally that's what

Microsoft and Google have done they

iterate over every character in the

document check if it's lowercase and if

so they subtract 32 from it and show you

the new value what if though it is not a

lowercase letter I think I can keep it

easy and just print out the current

letter unchanged if my goal is to Simply

Force things to all uppercase and that

letter then would be S bracket I so let

me go ahead now and make uppercase

hopefully no errors do/ uppercase and

I'll now now type in David with an

uppercase D but lowercase everything

else but now the after version is David

an aesthetic bug notice here I forgot to

include just for prettiness sake about

back sln at the end no problem I'll add

that let me fix my mistake make

uppercase SL uppercase enter daid enter

and voila and I I deliberately added

another space after the after just so

they would line up pretty even though

before and after have different numbers

of letters questions then on this

implementation of forcing something to

uppercase which in and of itself is not

all that enlightening but is

representative now of how you can

leverage these low-level

Primitives question

now all right well this honestly is

tedious my God like does Microsoft

Google everyone you have to literally

write out this code just to do something

simple well no that's again why we have

things like libraries and increasingly

now for problem sets projects and Beyond

well you just use libraries more often

off the shelf so as to solve problems

that surely other people have had before

you so how can I now use this uh Library

ctype.h well let me go back into my code

let me include this among my header

files here I tend just so I can skim

things easily I tend to alphabetize my

headers but that's not strictly

necessary but it allows me at a glance

to realize did I or did I not include

something I need now let me go ahead and

do this it turns out if you read the

documentation for the ctype library

there is a function wonderfully called

if is lower that takes in a character as

its argument essentially so s bracket I

and if that returns true a Boolean value

if you will well I'm going to force it

to lowercase but I don't have to do this

this math anymore turns out in the ctype

library there's also a function called

two upper that takes a character as

input like s bracket I and it just does

the math for you so that you can

abstract away the 32 thing and just know

that someone else has solved that

problem for you otherwise I can leave my

code unchanged down below because I'm

not changing anything else so if I do

make uppercase now and then do SL

uppercase daavid with just a capital D

and now it still works but if you read

the documentation further it turns out

that two upper is smart if you pass in a

character to two upper that's lowercase

it obviously converts its uppercase by

doing that math but if you pass in a

character to to Upper that's already

uppercase the documentation you would

see tells you that it leaves it

unchanged so I can tighten all of this

up I can get rid of the whole else I can

get rid of the whole if and arguably now

Implement a program that's just as

correct but better designed why fewer

lines of code easier to read Le lower

probability of mistakes assuming the

library is correct it just makes it

easier and faster for me now to write

code so if I now do one last time make

uppercase enter uppercase and type in my

name still working but now notice we've

whittel this down to far fewer lines of

code albeit using now this additional

Library questions then on how we did

this no well even even though this code

I dare say is correct it's not

necessarily welld designed just yet in

fact there's one line of code one

function call in this current

implementation that's more inefficient

than it needs to be and allow me to draw

our attention to this here line 10

wherein we're calling Sterling but we're

calling it inside of this for Loop

specifically inside of the condition and

why might that not necessarily be the

best

idea well is the length of the string s

changing ever I mean certainly not

within the span of this Loop and so here

we are within our for loop on line 10 11

12 and 13 asking on every iteration that

same question what's the length of s

what's the length of s what's the length

of s and in turn we're calling Sterling

every time even though we're getting

back the same answer so I dare say a

better solution here would be to maybe

figure out the length of s earlier on in

my code and maybe declare a variable or

perhaps do something that's

syntactically a little more elegant and

in fact a very common design in Loop

like this would be to declare not just

one variable like I but to actually

declare a second variable called n for

instance where n is just some number Set

n equal to the length of s but

thereafter inside of this condition

instead of calling Sterling of s again

and again and again what might I now do

I could instead just compare I against n

itself because n now will only be

calculated once when it's initialized

just as I is initialized to zero and

thereafter we're going to be compar

comparing I which is changing against n

which will not be so it's going to be

marginally more efficient by Design now

with that said a good compiler could

also recognize that there was this

optimization possibility and maybe do it

for us but for now best to get into the

Habit best to develop the muscle memory

for making those better design decisions

yourselves questions then on how we did

this no all right a few final building

blocks for the day so we started by

talking about those command line

arguments that clang uses whereby

anything after the command that you type

at a prompt be it make or clang or even

CD in Linux any word thereafter or

something cryptic like- O is a

commandline argument it's an input to

the command it's different from a

function argument because a function

argument of course is an input to a

function but it's the same idea it's

just different syntax after the dollar

sign at the prompt well it turns out

that command line arguments are

something you can now use in your own

programs by

accessing uh words after the prompt and

let me propose that we invent we invent

this as follows let me propose that we

switch back to vs code here and I'll

open a new file here called greet doc so

in greet Doc is going to be a program

that very simply greets the user had we

written this last week we would have

done this include

cs50.h and then include uh standard

i.h and then int main void and then we

might do something simple like string

name equals get string quote unquote

what's your name question mark and then

we would have printed out as always

hello comma percent s and then plugging

in that name so this is the same program

we've implemented many times just to

make sure it works although nope that's

not quite the same program semicolon in

the wrong place this now is the same

program so make greet /g greet and I'll

type in my own name hello David so we're

back there now what's arguably a little

Annoying about this program if I type in

something else like Carter enter you

know I have to run the program wait for

the prompt type in my name hit enter and

that's fine but imagine if every program

worked like this like make suppose you

could only type make then you wait for a

prompt then you type the name of the

program you want to make then you hit

enter or worse in Linux when you have to

change directories as you might have for

problem set one what if you had to type

CD enter now type the name of the folder

you want to change into enter I mean it

just slows life down and so it just gets

annoying quickly so commandline

arguments just let you express your

whole thought all at once so how can I

do this well if I want to express the

notion of command line arguments in my

code I could do something like this I

could for the very first time go up and

get rid of this void which as of today

means this program takes no command line

arguments and I can change it to exactly

this int ARG C string argv with brackets

now it's cryptic admittedly and let me

zoom in but I think we can perhaps infer

now what's going on if main Now does not

have void as its input which means it

takes no arguments surely the spoiler

here is that now main will take command

line argument somehow any guesses as to

what argv is or will be

what might this

represent it's an array of strings right

by way of the syntax

yeah exactly it will be all of the

characters or really all of the words

that you type at the prompt argc as an

INT any

guess

argument count is what it generally

stands for though technically you could

call these things any anything but this

is the convention because I claimed

earlier that arrays don't keep track of

their own length if you want to know how

many words the human typed at the prompt

after your program's name you have to be

told not just the array of the words but

the length of that array the strings you

can figure out the length of using

Sterling but you can't figure out the

length of the array of strings the

collection of words that the human typed

in so how can I now use this well let me

go ahead and do this let me go ahead and

change this program now just to be print

F quote unquote

hello comma uh percent s back sln then

Arvy bracket 1 so this is not the best

version of my code yet but it's my first

make greet and now let me dog greet

David all at once enter hello David now

let me run it again dogre Carter enter

hello Carter you know it's a marginal

Improvement but I don't have to wait for

get string to prompt me to hit enter

it's just speeding things up you know is

fast one less command to type in but I

deliberately did bracket one but where

what's the beginning of arv it would be

bracket zero well what's that this is

sometimes useful though for now it's not

suppose I recompile my code and run this

program now greet David anyone want to

guess what's in argv

Z say

again greet enter hello. SLG greet so if

you want sort of inception style your

program to figure out what its own name

is or at least how it was executed at

the command line at the terminal you can

look at ARG v0 in general probably not

that useful probably better to start

looking at bracket one which was the

first word after the program name and if

there were more I could do this how

about RV bracket 2 let me add in a

second percent s let me recompile greet

let me do greet David maen enter and

that two now works taking in two words

at the prompt if I really want to be

smart at this now I could do something

like this though how about if the count

of arguments AKA AR C equals equals 2

then assume that the human typed in only

their first name and do print F uh hello

comma percent s back sln and then uh

argv bracket 1 else if the human did not

provide exactly two arguments the name

of the program and their own name let's

just print out a default value L they

forgot their name or they typed in two

names or three names let's just do uh

hello comma world as a default and we'll

just ignore what the human typed in if I

recompile this make greet I can dog

greet and David again enter oops uh

sorry what am I missing yeah so newbie

mistake else all right make greet again

dog greet David enter there's my hello

David

but if I omit my name I just get the

generic like a default value and if I

get a little curious and I type in both

names then I get ignored two why because

I just haven't built in support for arxy

of three I could do anything I want but

now we have access to these kinds of

building blocks all right what else

might I do here well it turns out there

might be some final features for us to

now execute um notice though that in C

despite what you might see in books or

online tutorials nowadays the two

official formats for defining a main

function are either this which we've

been using now for Two Plus weeks or now

this whereby you change the void to int

argc and then for now string argv and

then empty brackets and we'll see that

this two is an a simplification some

training wheels if you will but for now

those are the two forms even though you

will see in online tutorials and even

books some people use Maine in different

ways these are the two now to keep in

mind and I'll note that these command

line arguments are kind of all over the

place didn't probably expect to see this

word on the screen here and what does it

mean well it turns out that for decades

there's actually this program that comes

with Linux systems in particular called

coway why probably because someone had

too much free time once and decided to

write a program that creates asy art out

of a cow saying something textually on

the screen but you use coway Just For

Fun by way of command line arguments so

for instance let me propose that uh I go

back to vs code here not because I want

to write any code but I just want to use

my terminal window and let me uh

maximize my terminal window here and let

me go ahead and type in something like

how about c space moo so cow is not a

program I wrote It's been around for

decades but we installed it in vs code

for you in the cloud it takes at least

one command line argument what do you

want the cow to say I can say cow say

moo and hit enter and voila there's my

asky art of a cow saying moo on the

screen it can say multiple words so I

can say hello world and enter and now it

says hello world so this is just an

example of a silly program that uses

command line arguments but it takes

others too just like clang use this

convention of hyphens to change the

output of the program Dash something is

just a super common convention with

commandline arguments when you want a

very tur notation for some option like

output um in C I read the documentation

and it turns out there's a-f command

line argument that allows you to change

the uh the appearance of the cow if you

will so if I do coway dasf duck and then

some other word like quack it's no

longer a cow that command line argument

turns it into a tiny adorable duck

instead and then lastly just for fun

because I spent way too much time

playing with command line arguments uh

c-f dragon and then how about like raar

enter you can even get this on the

screen here so this too is just an

example of what you can do with these

command line arguments now that we have

this building block and there's one

final thing we can now do with code

there's one last feature today that

we'll introduce before we now connect

all of these dots to readability and and

encryption by talking lastly about

something called exit status it turns

out that whenever your main function

exits it returns a secret integer that

you can figure out as the programmer or

an advanced user what it was and these

exit codes exit statuses are typically

used to indicate errors so for instance

over over the past couple of years if

you us zoom and you ever got some kind

of error you might have seen a screen

like this it's usually not that helpful

maybe tells you to click report problem

or contact support but very often in our

human world on Macs PCS and phones you

see cryptic error codes like literally

numbers that probably only Zoom knows or

Microsoft or Google or whatever company

wrote the software you're using but that

number corresponds to a specific error

that some human somewhere knows might

very well happen these are used similar

L although under a different name that

we'll talk about later in the term uh on

the web as well have you ever seen this

maybe not character but number so 404

means

what so error yes but really not found

so why I mean this is the most Arcane

thing and we'll talk in a few weeks

about like what this and other numbers

mean but numbers are all around us in

technology and they very often mean

something to the technical people who

wrote the software less so to humans

like you and me why so many of us

recognize 404 is kind of weird that like

that's been around long enough that we

all know it but it really is just a

special number that represents an error

of some sort so it turns out the last

thing we'll reveal today about what

we've been taking for granted for two

weeks is what the int is in Maine we've

seen just a moment ago that the thing in

the parentheses which up until now has

been void which means no command line

arguments now in Arc string arv brackets

just means yes command line arguments

and we've seen how to access them so the

last piece of the puzzle honestly of all

the cryptic syntax the past two weeks is

just what int means int is always there

for Maine and it indicates that Maine

will always return an integer even

though you and I have never done so

explicitly usually Maine returns Zero by

default but it would be weird if you saw

an error message saying zero so zero is

just hidden you would never see it on

the screen but it's happening

automatically by way of how C is

designed so let me write one final

program here I'll call it for instance

status . C to show you these exit

statuses code of status. c and then up

here let me do something simple like

include

cs50.h then include standard i.h and

then int main uh let's do actually let's

use a command line argument in argc

string argv so that's copy paste but now

let's do this if argc does not equal to

why don't we do something like this

let's not just um default to hello world

like last time let's yell at the user so

let's say something like printf missing

command line argument so that they know

they screwed up and they need to run the

program again correctly else let's go

ahead and say print out uh as before

hello comma percent s and then plug in

argv bracket one so the human's name

from The Prompt now at this point let me

go ahead and run

statusstatus and I'll will type nothing

first first I get yelled at this time

I'll type it again/ status David and it

works properly but now let me show you a

somewhat secret cryptic command you can

type this at your prompt and it's just a

coincidence that there's another dollar

sign Echo dollar sign question mark

totally Arcane but it allows you to see

what exit status your program has ended

with so let me run this again the wrong

way/ status okay I get the error message

what was secretly returned I can't can't

see it there's obviously no error screen

but by typing Echo dollar sign question

mark I can see that oh my program

automatically by default return zero

however if I run it again correctly

status David enter this is the correct

version but if I run Echo question mark

status again it's still entered with

zero and long story short this is just a

missed opportunity when something goes

wrong why don't I return a value other

than zero zero by default means success

and it's always there automatically but

but you can control this I can go into

my code here and return one else if

something works fine I can return zero

by default and honestly if I omit the

return zero again zero automatically is

returned so let me go ahead and though

be explicit just so I know what's going

on make status

again/ status and let's do this

correctly with David enter hello David

Echo question mark uh Echo dollar sign

question mark zero so all is well

but now if I do/ status and nothing or

multiple things but not just David enter

I get the error message but now if I do

Echo dollar sign question mark waa there

now is the one so what does this now

mean this is in the graphical world we

would just show something like this on

the screen which is a little more

informative to the user but even in the

Linux world we don't have a guey

necessarily even for the programs we've

written you can check these exit

statuses and in fact more comfortable

more advanced programmers when they

write code that calls programs be it

coway or anything else you can encode

check what the exit status is of a

program and then decide did my program

work or did it not and now let's connect

the final dots uh before we adjourn for

some fruit snacks uh cryptography namely

one of the applications this week via

which you'll be able to send if you will

secret messages and better yet decre

secret messages this will be in addition

to perhaps analyzing the readability of

text using puristic like we identified

at the start of class 2 so a

cryptography is just the art the science

of encrypting information scrambling

information so that if you have a secret

message to send in so-called plain text

you can run it through some algorithm

and turn it into what's called Cipher

text thereby encrypting it and only

someone who knows what algorithm you've

used and what input you've used to the

algorithm theoretically can decrypt that

process and convert it back to the

original message so if we use our mental

model from last week here is a problem

here is an input and output the goal I

claim here is to take some plain text

like the message you want to send think

back to grade school if you ever passed

a note to a friend or to your crush

saying I love you it's a little awkward

if the teacher or someone else

intercepts the paper and in English it

just says I love you or whatever it is

it'd be nice if you had at least

encrypted it in some way but the other

person needs to know what algorithm you

used and what inputs you used to that

algorithm so that ultimately they can

decode the so-called Cipher text which

is the output so what go inside of the

box today well an algorithm as it

relates to cryptography is called a

cipher and a cipher is a fancy name for

an algorithm that encrypts text from

plain text to Cipher text the catch is

there needs to be not just the algorithm

there needs to be an input to it and so

for instance you might draw the picture

like this for the first time today and

we've seen this in code you can give

multiple inputs or arguments to

functions so in this black box can you

imagine passing in the message you want

to send and then some Secret so for

instance suppose that the simplest thing

I could think of as a kid was instead of

sending the letter A why don't I write

the letter b instead of the letter B why

don't I write the letter c so I can kind

of shift the English alphabet by one

space so a becomes b b becomes C dot dot

dot Z becomes a you can wrap around at

the end and let's assume no punctuation

in this part of the story so that's a

very simple algorithm add a value to

each letter and send the value as the

cipher text and now the teacher the

classmate they have to know that you use

not only this rotational algorithm also

known as a Caesar Cipher they also need

to know what number you use did you add

one to every letter two to every letter

25 to every letter now if they're super

smart and probably not the the young age

in this story they could also just try

all possibilities and that would be an

attack on the algorithm this is not a

sophisticated algorithm but it's enough

to send a message in class so if the two

inputs now are high as the plain text

message and one as the so-called key the

secret number that only you and the

other person know you might uh be able

to encrypt a message from one way to the

other and so in this case for instance

high would become i j exclamation point

in this version of the algorithm we're

not going to bother with numbers or

punctuation will only operate on a

through z be it uppercase or lowercase

so now if you were to receive a a slip

of paper in class with i j on it you

know you the recipient would know what

it is so long as you know that the

sender used one because you just reverse

the algorithm and you subtract one

instead the teacher you know they

probably don't know what this means and

they're not going to spend time hacking

the message so it just looks scrambled

to them and that's what we get from

encryption someone who intercepts it be

it in class or in the real world on the

Internet or anywhere else can't actually

figure out ideally what it is you have

sent the opposite of course is indeed

called decryption but the process is the

same we now pass in negative one and so

how about this why don't we end with a

demonstration here

u y JT xbt

dt50 there's a bit of a tell there if we

pass that in and do Nega -1 well how do

we get out the plain text originally

well if this is the cipher text and we

subtract one from each letter I think U

becomes t i becomes h j becomes i t

becomes s x becomes w b becomes a t

becomes s d becomes c t becomes s and

this was indeed cs50 have a duck on your

way out and some snacks in the

[Applause]

[Music]

lobby

[Music]

[Music]

[Music]

[Music]

oh

[Music]

all right all right this is cs50 and

this is week three already wherein we'll

take a look back actually at week zero

where we first began and in week zero

recall that everything was very

intuitive in a sense we talked not just

about representation of information but

algorithms and we talked about tearing a

phone book again and again and that

somehow got us to a better solution but

today we'll try to start formalizing

some of those ideas and capturing some

of those same ideas not in pseudo code

just yet but in uh actual code as well

but we'll also consider the the

efficiency of those algorithms like just

how good how welld designed our

algorithms actually are and if you

recall when we did the phone book

example where in I first had an

algorithm searching one page at a time

and then second when two pages at a time

and then third started tearing the thing

in half recall that we with a wave of

the hand kind of analyzed it as follows

we proposed that if the x-axis here is

the size of the problem like number of

pages in a phone book and the y- axis is

the time required to solve the problem

in seconds minutes page tears whatever

your unit of measuring is recall that

the first algorithm was this straight

line such that if you had n pages in the

phone book it might have this slope of N

and there's this one toone relationship

between pages and tears two pages at a

time of course was twice as fast but

still really the same shape the yellow

line here indicating that yeah it's n /

two maybe plus one if you have to double

back as we discussed but it's really

still fundamentally the same algorithm

one or two pages at a time but the third

algorithm recall was this one here in

green where we called it logarithmic in

terms of how fast or how slow it was and

indeed the implication of this algorithm

was that we could even double the size

of the phone book and no big deal one

additional page tear and we take yet

another thousand page bite out of the

phone book so today we'll revisit some

of these ideas formalize them a bit but

also translate some of them ultimately

to code and all of that now is possible

because we have this lower level

understanding perhaps of like what's

actually inside of your computer this of

course is your computer's Ram or memory

and recall that if we kind of start to

abstract this away your compter

computer's memory is really just a grid

of bites in fact we don't have to look

at the hardware anymore and we looked at

a grid of bites like this whereby each

of these bites could be used to store a

Char an INT a long or even an entire

string at that but let's focus perhaps

just on a subset of this because last

week of course we emphasized really

arrays storing things in arrays and that

allowed us to start storing entire

strings sequences of characters and even

arrays of integers if we want to have

multiple ones and not just multiple

variables as well but the catch is that

if you look inside of an array in the

computer's memory and for instance

suppose these integers here are stored

it's pretty easy for us humans to glance

at this and immediately find the number

50 you sort of have this bird's eye view

from where you're seated of everything

on the screen and so it's pretty obvious

how you get to the number 50 but in the

world of computers of course it turns

out that this is hardware and computers

for today's purposes can only do one

thing at a time they can't just take it

all in and find instantly some number

like 50 so perhaps a decent metaphor is

to consider the array of memory inside

of your computer really is a sequence of

of closed doors and if the computer

wants to find some value in an array it

has to do the digital equivalent of

opening each of these doors one at a

time now how can code do that well of

course we introduced indices or indexes

last week whereby we by convention call

the first element of an array location

zero the second location one the Third

location two and so forth so-called zero

indexed and this allowed us to now

bridge this conceptual world of like

what's going on in memory with actual

code because now we had this square

bracket syntax via which we could go

searching for something if we so choose

and it turns out if I now uh paint these

red instead of yellow it would seem that

we actually have a pretty good physical

metaphor here standing in place for what

would be a computer's uh array of memory

if for instance you're storing some

seven numbers like that and so today we

begin with a look of a specific type typ

of algorithm that is for searching like

searching is all over the place all of

us have probably gone to google.com or

some equivalent like already multiple

times per day and getting back answers

fast is what companies like Google are

really good at so how are they doing

that how are they storing information in

computers uh memory well let's consider

what this really is it's really just a

problem as it was back in week zero the

input though to the problem for now

might be this array of seven lockers so

that's the input to the problem inside

of which is a number and maybe for

Simplicity now we just want a yes no a

true false answer a bull that is to say

of whether or not some number like 50 is

in that array it's not quite as fancy as

Google that doesn't just tell you yes we

have search results it actually gives

you the search results but for now we'll

keep it simple and just output as part

of this problem yes or no true or false

we have found the number we're looking

for given an input like that array but

it turns out inside of this black box

that we keep coming back to there's all

sorts of possible algorithms and we talk

about this at a high level conceptually

in week zero with the phone book but

today let's consider it a little more

concretely uh by way of a game that some

of you might have grown up with namely

Monopoly and so behind these doors it

turns out we'll be hidden some

denominations of Monopoly money but for

this we now have two volunteers if you'd

like to greet the world hi I'm

Jackson yay hi my name is Stephanie

Steph and you want to say a little

something about yourselves years house

first year living in Matthews

nice and I'm a first year in Canada nice

well Welcome to our two

volunteers so why don't we do this would

one of you like to volunteer the other

to go

first all go first okay all right so

Stephanie's up first and behind one of

these doors here we've hidden the

Monopoly money 50 and so we'd like you

to find the 50 we'll tell you nothing

more about the lockers but we would like

you to execute a certain algorithm and

in fact I'm going to give you some

pseudo code for this and I'm going to

give you the name for it it's called

linear search and as the name implies

you're pretty much going to end up

walking in sort of a straight line but

how are you going to do this well let me

propose that in a moment your first step

will be to think kind of like a loop for

each door from left to right what do we

want you to do on each iteration well if

50 is behind that door then we want to

go ahead and have you return true and

sort of hold up the 50 proudly if you

will for the group otherwise if you get

through that whole Loop and you haven't

found the number 50 you can just throw

up your hands and disappointment false

you've not found the number 50 so to be

clear step one is going to be for each

uh door from left to right how would you

like to

begin yep oh and then yep there we go

yep oh and if you'd like to at least

tell oh good good acting here what have

you found instead it's not 50 but 20 oh

okay so step one was a fail so let's

move on to step two inside of the Loop

what are you going to do next I'm going

to move to the next door

okay almost okay almost sort of a 500

instead next Locker I would rather

take

no okay we're not telling the

audience oh okay so keep going this is

step three

now

oh

[Music]

man five okay few more lockers to

check a little sad

guys all right second to last

step this one kind of close all right

and finally the last clearly you've been

perhaps set up here let's go all right

so the number

50 and Stephanie if I may let me ask you

a question here so on the screen this is

the pseudo code you just executed

suppose though I had done what many of

us have gotten into the habit of doing

when you have a if condition you often

have an else Branch as well suppose that

I had done this now and I'm marking it

in red to be clear this is wrong but

what would have have been bad about this

code using an if and an else might you

say any

instincts

um then you would end up like cancelling

canceling the code before you found the

50 yeah exactly just be eternally sad

indeed when Stephanie had opened the

first Locker she had found 20 20 of

course is not 50 she would have decreed

false but of course she hadn't checked

all of the rest of the locker so that

would seem to be a key detail that would

this implementation of the SoDo coat we

actually do go through as we did and

only return false not even with an else

but just at the end of the loop such

that we only reach that line if we don't

return truer uh earlier than that well

let's go ahead and do this let me take

the mic from you if you'd like to take a

seat next to Jackson Jackson in just a

moment we'll have you come up Carter if

you don't mind reorganizing the uh

lockers for us but in the meantime let

me point out how we might now translate

that same idea to code pretty high level

pretty English oriented with that pseudo

code but really now as of last week we

have syntax via which Stephanie and soon

Jackson could treat this Locker the set

of lockers as really indeed an array

using bracket notation so we can now get

a little closer in our pseudo code to

actual code and the way a computer

scientist for instance would translate

fairly high level English pseudo code

like this to something that's a little

closer to C or any language that

supports arrays would be a little more

cryptically like this but you'll see

more of this syntax in the coming days

for I from 0 to n minus one this is

still pseudo code but that's kind of

like the english-like way of expressing

what we've known come to know as a for

Loop if 50 is behind doors bracket I so

I'm assuming for the sake of discussion

that doors now is the name of my

variable this array of seven doors but

then the rest of the logic the rest of

the pseudo code really is the same way

and so you'll find in time that

programmers computer scientists more

generally when you start expressing

ideas algorithms to someone else instead

of maybe operating at this level here

you now have in your vocabulary really a

new new syntax that you can be a little

more specific not getting so into the

weeds of writing actual C code but at

least now doing something that's a

little closer to manipulating an array

like this so Jackson would you like to

uh stand on up all right yes yes support

for Jackson here too

nice and here now I'm going to allow you

an assumption that Stephanie did not

have Stephanie clearly was really doing

her best searching from left to right

using linear searches we'll Now call it

but they were pretty much in random

order right there was a 20 over there

there was a one over there and then a 50

so we deliberately jumbled things up and

did not sort the numbers for her but

Carter kindly has just come up to give

you a leg up Jackson by sorting the

numbers in advance and we'd like you

this time much like in week zero to do

something again and again but this time

using what we'll Now call binary search

it's exactly the same algorithm

conceptually as we did in week zero but

if we translate to the context of this

array we might now might say something

like this the first step for Jackson

might be to ask ask the question if 50

is behind the middle door where

presumably he's done some mental math to

figure out what the middle is then he's

going to just return true and hopefully

we'll get lucky and 50 will be right

there of course there's two other

possibilities at least which would be

what 50 is with respect to these

doors yeah so to the left or to the

right alternatively so if 50 is less

than the middle door then presumably

Jackson's going to want to go left else

if 50 is greater than the middle door

he's going to want to go right much like

I did uh physically last week with the

phone book dividing and conquering left

to right but there's actually a a fourth

case let's put it on the board first

what else might happen here that Jackson

should

consider yeah oh it's not there it's not

there so let me actually go back and

amend my my pseudo code here and just

say Jackson if we don't hand you any

doors at all or eventually as he's

dividing and conquering if he's left

with no more doors we have to handle

that situation so that the def behavior

is defined find all right so with that

said Jackson do you want to go ahead and

find us the number 50 and walk us

through verbally what you're doing and

finding all right so it looks like this

one is the middle door so I'm going to

open

it but it's 20 not 50 oh sad okay what's

going through your head now so now I'm

looking because 50 is higher than 20 I

want to look to the right good um and

look for the new middle door which would

be here nice and it's 100 sad um but 50

is less than 100 so now we know to look

left which would be here and Tada nice

very well done this time around

too

so thank you first to our volunteers

here and in fact um since you're fan of

Monopoly as we're so informed we have

the Cambridge edition of Monopoly with

all your Harvard favorites here you go

thank you so thank you to our volunteers

for finding us 50

so kind of was more popular than we

expected so here we can translate this

one more time into something a little

closer to code and again still pseudo

code but here now might be another

formulation of exactly what Jackson just

did just using the nomenclature now of

arrays where you can be a little more

precise with your instructions and still

leave it to someone else to translate

this finally to code but here we have

same question at the beginning if no

doors left return false if 50 is behind

doors bracket middle so I'm assuming

here because this is pseudo code that

somewhere I've done the mental math or

the actual math to figure out what the

index of middle is for instance if these

are seven doors in an array this would

be location zero 1 2 3 4 5 6 so somehow

I've taken the total number of doors

seven divided by two to find the middle

that's three and a half we have to deal

with rounding but suffice it to say

there's a well- defined formula for

finding the middle index given the total

number of lockers divide by two and then

round accordingly so that's presumably

what Jackson did just by counting in his

head to find us door number three not

the third door the fourth door but door

bracket three so this is just saying if

50 is behind door is bracket middle

return true that was not the case he

found a $20 bill instead else if 50 is

less than the uh doors bracket middle go

ahead and now it gets interesting search

doors zero through doors middle minus

one so it's getting a little more to the

Weeds now but if middle is three this

one here well we want to now have

Jackson search if 50 had been uh if the

number had been less we want to start at

bracket zero and go up through this one

when we deliberately subtract one CU

what's the point of looking in the same

Locker again we might as well do zero

through middle minus one else if 50 is

greater than doors bracket middle which

it was what did we then do Jackson

intuitively searched for doors middle

plus one through door

n minus one and honestly it gets a

little Annoying having the pluses and

the minuses here but just think of what

it means this is the middle door and

Jackson then did proceed to search

through doors middle plus one because

there's no point in searching this one

again and then the last element in any

array of size n where n is just our

go-to number uh for the size is always

going to be n minus one it's not going

to be n it's going to be n minus one

because we always start counting at Rays

at zero so here then we have a

translation into pseudo code that's a

little closer to C of this exact same

idea and here we come full circle to

week zero like in week zero is pretty

intuitive to imagine dividing and

conquering a problem like this but if

you now think back to actual your iPhone

your Android phone or the like when

you're doing autocomplete and searching

the list it's possible if you don't have

many friends or family or colleagues in

the phone you know what linear search

just checking every name for the person

you're searching for might be perfectly

fine but odds are your phones being

smarter than that especially if you

start to have dozens hundreds thousands

of people in your contacts over the

years what would be better than linear

search well perhaps binary search but

but but there's an assumption a

requirement which is what why was

Jackson ultimately able to find the 50

in just like three steps instead of a

full seven like

Stephanie because the array was sorted

and so this is sort of a teaser for what

we'll have to come back to later today

well you know how much effort did it

take someone like Carter how much effort

does it take your phone to sort all of

those names and numbers in advance

because maybe it's not actually worth

the amount of time now someone like

Google probably somehow keeps the

database of web pages sorted you could

imagine it being super slow if when you

type in cats or something else into

google.com if they searched linearly

over their entire data set ideally

they're doing something a little smarter

than that so we'll formalize now exactly

this kind of analysis and it's not going

to be so much mathy as it still will be

in intuitive but we'll introduce you to

some some jargon some terminology that

most any programmer or computer

scientist might use when analyzing their

own algorithms let's formalize now what

this kind of analysis is so right right

now I claim binary search better than

linear search but how much better and

why exactly well it all comes back to

this kind of graph so this recall is how

we analyzed the phone book back in week

zero and recall that indeed we had these

these formulas rough formulas that

describe the running time of those three

algorithms one page at a time two pages

at a time and then tearing the thing

again and again in half and precisely if

you count it up the number of pages I

was touching or the number of pages I

was tearing it's fair to say that the

first algorithm in the worst case might

have taken n total Pages it didn't

because I was searching for John Harvard

at the time which is somewhat early in

the alphabet but if I were searching

with some for someone with the last name

of Z I would have had to keep going and

going in the worst case through all n

Pages not as bad for the second

algorithm and that's why we do n divided

by two and even that's a bit of a white

lie right it's probably n / 2 + 1 in

case I have to double back but again I'm

sort of doing this more generally to

capture the essence of these things and

then we really got into the weeds with

like log base 2 of n for that third and

final algorithm and at the time we

claimed anytime you're doing dividing

something in half in half in half odds

are there's going to be some kind of

logarithm involved and we'll see that

today but today we're going to actually

start using computer science terminology

and we're going to sort of formalize

this imprecision if you will we are not

going to care generally about exactly

how many steps some algorithm takes

because that's not going to be that

enlightening especially if maybe you

have a faster computer tomorrow than you

did today it wouldn't really be fair to

compare numbers too precisely we really

kind of want to with a wave of the hand

just get a sense of roughly how slow or

how fast an algorithm is so the notation

here is deliberate that is literally a

capital O often italicized referred to

as Big O and so the first algorithm is

in Big O of n the second algorithm is in

Big O of n / two the third algorithm is

in Big O of log base 2 of n but even

that is kind of unnecessary detail when

using Big O notation you really don't

care about we'll see the smaller ordered

terms right we're not going to care

about the divided by two because you

know what the shape of these algorithms

is almost the same and really the idea

the algorithm itself is sort of

fundamentally the same okay and instead

of one page at a time I'm doing two but

if you throw millions of pages billions

of pages at me those algorithms are

really going to kind of perform the same

as n gets really large goes off toward

infinity and the same is true for

logarithms even if you're a little rusty

it turns out that whether you do the

math with log base 2 log base 3 log base

10 you can just multiply one by the

other to really get the same formula

this is only to say a computer scientist

would generally say that the first two

algorithms are on the order of eps the

third algorithm is on the order of log

end steps and we don't really care

precisely what we mean beyond that and

this Big O notation as we'll see and

actually let me let me zoom out if you

can imagine suddenly making the x- axis

much longer so more pages on the screen

at once it is indeed going to be the

shapes of these curves that matter

because imagine in your mind's eye as

you zoom out zoom out zoom out zoom out

and as n gets much much much bigger on

the xaxis the red and the yellow line

are essentially going to look the same

once n is sufficiently large but the

green line is never going to look the

same it's going to be a fundamentally

different shape and so that's the

intuition of bigo to get a sense of

these uh rates of performance like this

so here then is Big O here is perhaps a

cheat sheet of like the common formulas

that a computer scientist certainly in

an introductory context might use when

analyzing algorithms and let's consider

for a moment which of our first two

algorithms linear search and binary

search fall into these categories so

I've ordered them from sort of slowest

to fastest so order of n s it's not

something we've actually seen yet but it

tends to be slow because it's quadratic

you're doing n * n that's got to add up

to a lot of steps better today is going

to be n Lin even better is going to be n

even better than that is login and best

is soall order of one like one step or

maybe two steps maybe even a thousand

steps but a fixed finite number of steps

that never changes no how matter how big

n is so given this chart just to be

clear linear search let's consider the

worst case in the worst case how many

steps did it take someone like Stephanie

to find the uh solution to the problem

assuming not seven doors but n doors

yeah so on the order of N and in this

case it's exactly n but you know what

you know maybe it's arguably too n right

because it took Stephanie a couple of

steps like she had to lift the latch she

had to open the door maybe it's three

steps you had to show the money so now

it's 3n 2 N but there we don't really

care about that level of precision we

really just care about the fundamental

number of operations so we'll say yes on

the order of n so that might be an upper

bound we'll call this for linear search

and how about binary search in Jackson's

case or in general me and week zero if

there's end doors how many steps did it

take Jackson or me using binary

search in this case it was literally

three but that's not a

formula yeah so it's on the order of log

n and indeed if there's seven doors well

that's almost eight if you just do a

little bit of rounding and indeed if you

take log base 2 of eight okay so that

does actually give us three so the math

actually checks out and if you're not

comy with logarithms no big deal just

think about it intuitively uh logarithm

of base two is just dividing something

again and again so on this chart when we

consider Big O which to be clear allows

you to describe the order of an

algorithm's running time like the

magnitude of it but it also describes

more specifically an upper bound so in

the worst case for instance these are

pretty good measures of how good uh or

rather of how bad linear search and

binary search might be why well suppose

you're searching a thousand page phone

book and the person's name starts with z

the algorithm is still going to be on

the order of n steps why because it

might take you as many as all end steps

to find it now that's not necessarily

going to be the case in practice if I

use Big O as an upper bound well it

would be nice if there's sort of a

corresponding lower bound especially if

you want to consider not just best uh

worst cases but maybe best cases so what

might we use here well so this is a

capital Greek omega symbol so Omega is

the symbol that a computer scientist

uses generally to describe a lower bound

on an algorithm often in the context of

best case though not necessarily so a

lower bound means how few steps might an

algorithm take and here too same

formulas and we'll fill in these blanks

over time some algorithms might always

take a minimum of n squ steps or on the

order of n steps some might only take n

logn or n or log n or one so something

like uh linear search when Stephanie

started with linear search she didn't

get lucky this time on stage but what if

she had and the first door she opened

were 50 how much you then describe the

lower bound on linear search in this

so-called best

case using this list of possible

answers yeah yeah so Omega of one so in

the best case the lower bound on how

many steps it might take uh linear

search to find something might just be

one step why because maybe Stephanie had

gotten lucky and we had pre-filled these

lockers with the numbers in some other

order such that she might have open the

first locker and waila the number 50

could have been there so a lower bound

arguably could indeed be Omega of one

for linear search and how about now for

Jackson he used binary search so he

dived right into the middle of the

problem but what would be a lower bound

on binary search using this logic yeah

yeah so again Omega of one why because

maybe he just gets lucky and indeed

right in the middle of the lockers could

have been the number 50 it wasn't and so

more Germain in Jackson's uh actual

practice would have been the Big O

discussion but Big O and Omega upper

bound and lower bound just allow a

computer scientist to kind of wrestle

with what could happen maybe in the

worst case what can happen in the best

case and you can even get even more

precise like the average case or the

like and this is indeed what Engineers

might do at a whiteboard in a company in

a university when designing an algorithm

and trying to make arguments as to like

why their algorithm is better than

someone else's by way of these kinds of

analyses and just so you've seen it it

turns out that if some algorithm happens

to have an identical upper bound and

lower bound you can actually use a

capital Greek Theta as well and this is

the last of the the Greek symbols today

but a Greek Theta indicates a coinci

idents of both upper bound and lower

bound that is they are one and the same

that was not the case for our discussion

a second ago of linear search not the

case for binary search but you could use

the same kinds of formulas if it turns

out that your upper bound and lower

bound are the same so for instance if I

were to count everyone like literally in

this room 1 2 3 4 five six and so forth

you could actually say that counting in

that way is in Theta of n right because

I in the best case it's going to take me

end points at the uh people in the

audience in the worst case it's going to

take me n it's always going to take me

end steps if I want to count everyone in

the room you can't really do better than

that unless you skip people so that

would be an example off the cuff of

something where Theta is instead germine

are any questions now on Big O on Omega

or Theta which are now just more formal

tools in the toolkit for talking about

the design of our

algorithms any

questions

no seeing none yeah oh is this yes no

okay so we're good so let's go ahead and

translate this perhaps to some some

actual code let me go over to vs code

here and let's see if we can't now

translate some of these ideas to some

actual code not so much using new syntax

yet we're going to still operate in this

world of arrays like last week so let me

go ahead and create a program called

search. C by executing code space

search. c in my terminal and then up

here let's go ahead and include our

usual so include cs50.h so I can get

some input include standard i.h so I can

print some output we'll do int main void

which the meaning of which we did start

to tease apart last week the fact that

it's void again today just means no

command line arguments and let me go

ahead and do this let me go ahead and

declare just for discussion's sake a

static array like an array that never

changes and the Syntax for this is going

to be give me an array called numbers

using the square bracket not ation and

I'm going to immediately initialize it

to 20 500 10 5 100 1 and 50 reminiscent

of those same denominations as before so

this is a slightly new syntax that we've

uh perhaps not seen and the curly braces

here which are different from for loops

and while loops and functions just tell

the compiler please give me an array of

whatever size this is containing those

numbers left to right I could

alternatively use last week's syntax of

saying something like this let's see one

2 3 4 5 six seven denominations I could

alternatively do this and then I could

say numbers bracket 0 uh numbers bracket

0 equals 20 numbers bracket 1 equal 500

and I could do this five more times

that's just a little tedious if you know

the numbers in advance you don't have to

tell the compiler how many there are you

can just let it figure it out that your

numbers are be will be 20 500 10 5 100 1

and 5050 so this is how you statically

Define an array all right let me just go

ahead and ask the user now for a number

we'll call it n by using get int and

prompting them for a number so nothing

new there and now let me go ahead and

Implement linear search and the pseudo

code we had for this before used some

array like notation let me go ahead then

and start similarly four in I and it's

you almost always start counting at I by

convention so that's perhaps a good

starting point I'm going to do this so

long as I is less than seven not the

best design to hard code the seven but

this is just for demonstration sake for

now because I know how many numbers I

put in there and then I'm going to do

i++ so now I have the beginnings of a

loop that will just allow me to iterate

over the entire array and let me ask

this if the current number at location I

equals equals n which is the number the

human typed in then let's go ahead and

do something simple like print F quote

unquote found back sln and then per our

discussion last week to indicate that

this is successful I'm going to going to

return zero if I found it and if I don't

find it I'm just going to go down here

and by default say not found back sln

and just for convention whoops just for

good measure per convention I'll return

one or really any value other than zero

zero recall means success and any other

integer tends to mean error of some sort

irrespective of the number I'm looking

for so just to revisit the only thing

that's new here is the syntax for

creating an array of seven numbers these

numbers and then after after that we

have really highlighted here an

implementation of linear search I mean

this is the C version I dare say of what

Stephanie did on the board whereas now

the array is called numbers instead of

doors but I think it's pretty much the

same let me go ahead and open my

terminal window and run make

search seems to compile okay search and

let's go ahead and search for a number

we'll start with what we did before 50

and it's found let's go ahead and run it

again/ search let's search for maybe 20

at the beginning that one too is found

let's run it one more time searching for

like 1,000 which is not uh in among the

uh denominations and that one indeed is

not found so we've taken an idea from

week zero now formalized in week three

and just translated it now to code

questions on this implementation of

linear

search linear

search nothing oh so successful so far

today okay so let's see if we can't

maybe make this a little more

interesting and see if we can't trip

over a detail that's going to be

important in C instead of doing numbers

let me go ahead and do this we'll stay

on theme with Monopoly and I went down

the rabbit hole of reading the

wikkipedia article on Monopoly and the

original uh pieces or tokens that came

with Monopoly and it turns out we can

represent those with strings so I'm

going to create an array called strings

plural of whatever size I defined here

and the very first first Monopoly pieces

back in the day were a battleship that

you could play with a boot a cannon an

iron a

thimble and a top hat some of which you

might know from the game nowadays turns

out they've been changing these uh had

no idea over the years so here is now an

array of strings let me go ahead and

prompt the user now not for an integer

anymore I want to Now search for one of

these strings still using linear search

so let me create a string s set it equal

to string prompt the user for a string

to search for and then I think my code

here is almost the same except for one

detail I now have an array called

strings I now have a variable called s

but it turns out for reasons we'll

explore in more detail next week this

line of code is not going to work and it

turns out the reason has to do with what

we discussed last week of like what a

string really is and what is a string

again a string is an array and it turns

out though that equals equals is not

going to generously compare all of the

characters in an array for you just

because you use equal equals it turns

out it's not going to compare every

letter and so thankfully there is in the

uh string library that we introduced

last week a solution to this problem the

reason for the problem we'll explore in

more detail next week but for now just

know that when you want to compare

strings in C especially if you've come

into the class knowing a bit of Java or

python or some other language you cannot

use equals equals even though you could

in scratch you cannot in C so what I

have to actually do here is this I have

to ask the question does the return

value of a function called stir compare

or stir

comp equal zero when passed in the

current string and that user input so if

you read the documentation for this

function called stir compare you'll see

that it takes two strings as input first

one and second one it then someone

decades ago wrote the code that probably

uses a four Loop or a while loop to

compare every character in each of those

strings and it turns out it returns zero

if they are in fact equal turns out too

it will return a positive number or a

negative number in other situations any

intuition for why it might actually be

useful to have a function that allows

you to check if two strings are equal

if they're not equal what else might be

interesting to know when comparing two

strings are if certain values

are okay possibly maybe you want to know

just how similar they are um and that's

indeed an algorithm unto itself but stir

compare is a little simpler than

that exactly if you're trying to like

alphabetize a whole list of strings just

like your phone probably is for your

context or address book it turns out

that stir compare will actually return a

positive number or a negative number or

a zero based on whether maybe it comes

alphabetically first or later or or in

fact equal so that can be a useful thing

and that's just a teaser for a lower

level explanation that we'll see next

week so now let me cross my fingers and

see if I got this right let me go ahead

and do make

search did compile okay albeit slowly

do/ search and let's search for

something like the thimble and we see

that that's indeed found otherwise let's

search for something that I know isn't

there like a race car which was there

when I grew up but huh segmentation

fault core dumped like and actually some

of you have tripped over this error

before anyone want to admit seeing this

so yeah not something we've talked about

and um honestly not something I intended

just now but that too we'll see next

week any intuition for why my program

just

broke I didn't really change the logic

it's still linear search let me hide the

terminal so you can see all of the code

at once the only thing I did was

switched from integers to Strings and I

switched to stir compare here but

segmentation fault happened and the

teaser is that that somehow relates to

the computer's memory

yeah yeah and this is subtle but spot on

so 1 2 3 4 5 six elements total in this

array versus the seven number

of Monopoly denominations that we had

earlier and this is where see sort of

case in point this came back to bite me

the fact that I hardcoded this value as

to opposed to maybe separating it out as

a constant or declaring it higher up

kind of bit me here because now I'm

iterating over an array of size six but

clearly I'm going one step too far

because I'm literally going to iterate

seven times not six so it's as though

I'm looking at memory that's over here

and indeed next week we'll focus on

memory and that's just a bad thing so

odds are not even and seeing your code

from this past week if any of you have

had segmentation faults odds are you

touched memory that you shouldn't have

you maybe looped too many times you

might have uh used a negative number to

get into your array in general you

touched memory that you shouldn't have

and you touched a segment of memory that

you shouldn't have the fix though at

least in my case is simple just don't do

that so let me go ahead and recompile

this make uh

search do/ search and I'll search again

for uh race car and

and now it does not crash but it does

tell me it's not found so subtle but

something you might yourself have

tripped over already questions then on

what I just did intentionally or

otherwise yeah in

front return don't return Z return so

what

the a really good question so the

program will still work even if I don't

return zero or return one in fact let me

go ahead head and do that and just hide

my terminal window for a second let's

get rid of the return here let's get rid

of the return here however Watch What

Happens here uh let me go ahead and

recompile this make search Let Me scroll

up in my code here let me go ahead and

do do/ search and let me go ahead and

search for the first thing in the list

Battleship so I know that this should be

found I hit

enter huh interesting so it's saying

found not found but do you see why

logically in this case

exactly so the loop is still running so

there's a couple of solutions to this I

could for instance somehow break out of

the code here but that's going to still

result in line 18 executing I could then

instead just return here I don't

strictly need to return one down at the

bottom but I made this claim last week

that it tends to be helpful as your

programs get more sophisticated to at

least signify just like a real world

programmer error codes when something

goes wrong so returning zero in main is

the easiest way to signify my code is

done I'm ready to exit successfully

that's it but down here I could

absolutely still return zero because

it's not a huge deal it's not really an

error that deserves annoying the user

with some kind of popup that something

went wrong but return one is just a

lower level way of signaling H it didn't

really find what I was looking for and

remember from last week you can see this

as follows if I recompile this again now

that I've reverted those changes so make

search and if I do uh /search and search

for Battleship which is indeed found

recall I can execute this magical

command Echo dollar sign question mark

which you're not going to often execute

but it shows you what main returned if I

run search again and search for race car

which is not found I see not found but I

can also run this command again and see

that oh it returned one so now if you

fast forward a few months a few years

when you're actually writing code in a

company or for larger projects you might

want to be automating software you might

not want the human to necessarily be

running it manually you might want um

code to be automated by some nightly

process or something like that using

these exit codes can a program determine

yes or no that other code succeeded or

failed other questions on linear search

in this

way no all right well let's translate

this to one other feature of uh C here

by incorporating these ideas now into

one other program so I'm going to create

a phone book in C by doing code space

phone

book.car a phone book for an actual name

and getting back a number so I'm going

to go ahead and quickly include some of

the same things cs50.h so we can get

input uh standard io. so we can print

output and I'm going to preemptively

include string.h in case we need that

one as well uh int main void no need for

uh command line arguments today and let

me me give myself now an array of names

for this phone book so string names

equals and then in curly braces how

about Carter will be one person in the

phone book and David myself will be the

other so we'll keep it short so we don't

have to type too many names but this is

a phone book with two people thus far

suppose now we want to also store

Carter's phone number in mind so it's

not just saying found or not found it's

literally looking up our phone numbers

like a proper phone book well at the

moment there's really no way to do this

I could do something hackish like I

could put a number like

617495 1000 after Carter I could maybe

do something like 949 uh 468 2750 after

me but now you're kind of doing the

whole apples and oranges thing right

like now it's not strings it's a string

int string int all right so maybe I

could just make all of these strings but

now it's just a conceptual mixing of

apples and oranges like yes that's an

array of four strings but now you're on

the honor System to know that the first

string is a name the second string is a

number the third string is like you can

do it but it's a bit of a hack so to

speak so what might be cleaner than this

instead of combining our phone numbers

into the same array as our names what

else might we do that's perhaps a little

better say a little

Lou a 2d array uh possibly something we

could do I'm going to keep it even

simpler now because we haven't used

those by name even though that is we saw

last week technically what argv is what

else could I do if I want to store names

and numbers

yeahp yeah let me go with this

suggestion just it's a little simpler

rather than complicate things in

literally different dimensions let me go

ahead and do string well I could do int

numbers but you know what so that we can

support punctuation like dashes or even

parentheses or country codes I'm going

to do this instead I'm going to do

string numbers so that I can represent

Carter's number as quote unquote plus

one for the US 617 495 1,000 complete

with hyphens as is us convention and

then for mine I'll go ahead and do + one

949 how about 468

275 semi colon and now down below let's

actually enable the user to search this

phone book just like in week zero we did

string name equals get string and let's

ask the user for a name presumably David

or Carter or someone else and now let's

re-implement linear search so four in I

gets zero I is less than two and do as I

say not as I do I think we should beware

this coding but we'll keep it simple for

now i++ and then in this for loop I

think we have all of the ingredients to

solve this so if the return value of

stir compare of all of the names bracket

I comparing against the name that the

human typed in if all of that equals

equals zero that is all of the

characters in those two strings are

equal then I think we can go ahead and

say found just like last time but you

know what let's actually print Carter or

my phone number so found percent s and

we'll plug in numbers bracket I and then

just for consistency I'll return zero

here and down here how about I'll say

something like uh print F not found just

to be clear and then I'll return one as

well so just a recap here's all of the

code it's almost the same as before

except now it's useful I'm not just

saying found or not found I found a

number in Monopoly or I found a a piece

in Monopoly I'm looking up in one array

of the strings and then I'm printing

from the other array the answer so let

me go ahead here and run the compiler

make phone book enter okay that's

promising no errors do/ phonebook now

and let's search for for instance Carter

enter all right so we found Carter's

number all right let me do that again

phone book Let's search for David all

right we seem to have found David's

number all right let's do it one last

time phone book enter and now we'll

search for like John Harvard enter not

found

all right so I dare say albeit with

minimal testing this code is

correct would anyone now like to

critique the design does something rub

you the wrong way perhaps about this

approach

here and as always think about how if

the program maybe gets longer more

complicated how decisions like this

might unfold

yeah okay so if I is less than two so

technically I if I change the number of

people in this phone book I'm going to

have to update I and we've already seen

that I get myself into trouble so that's

bad design

goodes yeah so again I'm sort of

trusting myself not to screw up if I add

John or anyone else to the first array

but I forget to add their number to the

second array you know eventually things

are going to drift and be inconsistent

and then the just code will be incorrect

at that point so so sort of a poor

design setting me up for future failure

if you will other thoughts

yeah yeah really good we're assuming the

same order from left to right the names

go and from left to right the numbers go

but that's kind of just the honor System

like there's literally nothing in code

preventing me from reversing the order

for whatever reason or maybe sorting the

names like they're sorted now and maybe

that's deliberate but maybe it's not so

this honor System here too is just not

good right I could put a comment in here

to remind myself to uh you know note to

self always update arrays the same way

but like that's something's going to

happen eventually especially when we

have not two but three but 30 300 names

and numbers it would be nice to keep all

of the related data together and so in

fact the one new feature of C we'll

introduce today is one that actually

allows us to implement our very own data

structures you can think of a raay as a

very lightweight data structure and that

allows you to Cluster related data back

to back to back to back and this is how

strings are implemented they are a data

structure effectively implemented with

an array but with c and with other

languages it turns out you can invent

your own data types whether they're one

dimensional two-dimensional even or

Beyond and with uh with C can you

specifically create your own types that

have their own name so for instance

wouldn't it have been nice if C came

with not just Char and int and float and

uh uh long and and others wouldn't it be

nice if C came with a data type called

person and ideally a person would have a

name and a number now that's a little

naive and unrealistic like why would

they define a person to have just those

two Fields certainly people could have

disagreed what a person is so they leave

it to us like the authors of C gave us

all of these Primitives inss and floats

and strings and so forth but it's up to

us now to use those in a more

interesting way so that we can create an

array of person variables if you will

inside of an array called people just to

pluralize it here so how are we going to

do this well for now let's just

stipulate that a person in the world

will have a name and a number that we

could argue all day long what else a

person should have and that's fine you

can invent your own person

eventually at the moment I'm using just

two variables to define a person's name

and number but wouldn't it be nice to

encapsulate that is combined these two

data types into a new and improved data

type called person and the Syntax for

that is going to be this so it's a bit

of a mouthful but you can perhaps infer

what some of this is doing here so it

turns out C has a keyword called type

def as the name kind of suggests this

allows you to Define your own type

struct is an indication that it's a

structure it's like a a structure that

has multiple values inside of it that

you are trying to Define and then at the

very bottom here outside of the Curve

early braces is the name of the type

that you want to create so you don't

have discretion over using type def or

struct in this particular case but you

can name the thing whatever you want and

you can put anything in the structure

that you want as well and as soon as

this semicolon is executed at the bottom

of the code every line thereafter can

now have access to a person data type

whether as a single variable or as an

entire array so if I want to build on

this then let me go ahead and do this

let me go back to my C code here and I'm

going to go ahead and uh change just a

couple of things let's go ahead and do

this I'm going to go ahead and first get

rid of those two hardcoded arrays and

let me go ahead and at the top of my

file invent this type so type def struct

inside of it will be a string name and

then a string number and then the name

of this structure will be person and

best practice would have me Define at

the very top of my file so that any of

my functions in fact could use it even

though I just have Main in this case now

if I want it I could do this like person

P1 and person P2 but we know from last

week like that already is bad design if

you want to have multiple instances of

the same type of variable we should

probably use what

instead and yeah an array so let me not

even go down that road let me instead

just do this person uh will be the type

of the array but I'm going to call it I

could call it persons but in English we

typically say people so I'll call the

array people and I want two people to

exist in this array though I could

certainly change that number to be

anything I want how now do you put a

name inside of a person and then put the

number inside of that same person well

slightly new syntax today I'm going to

go ahead and say this people bracket

zero just gives me the first person in

the array that's not new but if you want

to go inside of that person in memory

you use a DOT and then you just specify

the name of the attribute therein so if

I want to set the first person's name to

Carter I just use that so-called dot

notation and then if I want to set

Carter's number using dot notation I

would do this plus one

617495 1000 and then if I want to do the

same for myself I would now do people

bracket

1name equals quote unquote David and

then people bracket one still number

equals quote unquote + 1

949468

2750 and now at the bottom of my file I

think my logic can pretty much stay the

same I can still on this line here

prompt the user for the name of the

person they want to look up for now even

though I admit it's not the best design

I'm just doing this for demonstration

sake I'm going to leave the two there

because I know I have two people but

down here this is going to have to

change I don't want to compare names

bracket I anymore what do I want to type

here as the first first argument to stir

compare what do I want to do here

yeah so people i. name yeah so I want to

go into the people array at the I

location because that's what my Loop is

doing it's updating I again and again

and then look at name and that's good I

think now I need to change this too what

do I want to print if the person is

found someone

else what do I want to print here if I

found the person's name

yeah say a little

louder perfect so people bracket. number

if indeed I want to print the

corresponding number to this person and

then I think the rest of my code can

stay the same so let me go ahead and run

make phonebook to recompile this version

so far so good/ phonebook let's go ahead

and type in Carter's name found all

right let's go ahead and run it again

David's name found let's go ahead and

run it one more time type in John

Harvard for instance not found in this

case so fundamentally the code isn't all

that different linear search is still

behaving the same way and I admit this

is kind of ugly looking like we've kind

of made a two-line solution like five

lines of code now but if we fast forward

a week or two when we start saving

information to files uh we'll introduce

you to files like CSV value CSV files

comma separated values or spreadsheet

files which you've surely opened on your

Mac or PC at some point in the past

suffice it to say We'll soon learn

Techni for storing information like

names and numbers in files and at that

point we're not going to do any of this

hackish sort of hardcoding of the number

two and manually typing my name and

Carter's name and number into our

program we'll read the information

dynamically from a file and in a few

weeks we'll read it dynamically from a

database instead but this is for now

just syntactically how we can create an

array of size two containing one person

each we can update the name and number

of the first person update the name and

the number of the second person and then

later search across those names and

print out the corresponding numbers and

in this sense this is a better design

why because my person data type

encapsulates now everything that it

means to be a person at least in this

narrow world and if I want to add

something to the notion of a person for

instance I could go up to my typed Def

and tomorrow add an address to every

person and start reading that in as well

and now it's not the honor System it's

not a names array a numbers array and a

addesses array and everything else you

might imagine related to a person it's

all encapsulated which is a a term of

art inside of the same type reminiscent

if some of you have programmed before of

something called objectoriented

programming but we're not there yet C is

not that questions on this use of struct

or this new syntax the dot operator

being really the juicy part

here any questions

yeah

on what line

number 16 so yes so syntactically we

introduced the square brackets last week

so doing people bracket zero just means

go to the first person in the array that

was like when Stephanie literally opened

this door that's uh that's doors bracket

zero but this is of course people

bracket zero instead today the dot is a

new piece of syntax it means go inside

of that person in memory

and look at the name therein and set it

equal to Carter and do the same for

number so that's all it's like open the

locker door go inside of it and check or

set the name and the number

yeah attributes is fine uh good question

in the struct can you set default values

short answer no and this is where C

becomes less featurable than more more

modern languages like Python and Java

and others where you can in fact do that

so when we transition to python in a few

weeks time we'll see how we can start

solving problems like that but for now

it's up to you to initialize name and

number to something

yeah really good question how can we

adjust or critique the design of what

I'm doing

this is one of the few situations where

I would say hypocritically do as I say

not as I do I am using pretty ugly lines

like this just to introduce the syntax

but my claim pedagogically today is that

eventually when we start storing names

and numbers or other things in files or

in databases you won't have this

redundancy you'll have one line of code

or two lines of code that read the

information from the file or database

and then fill the entire array with that

data for now I'm just doing it manually

so is to keep our Focus only on the new

syntax but that's it so forgive the bad

Design By Design today other questions

on

this all right that's been a lot already

why don't we go ahead and take our

10-minute break with snacks first we

have some delightful brownies in the

lobby all right we are back and up until

now it clearly seems to be a good thing

if your data is sorted because you can

use binary search you know a little more

some little something more about the

data uh but it turns out that sorting of

itself is kind of a problem to solve too

and you might think well if sorting is

going to be pretty fast we absolutely

should do it before we start searching

because that'll just speed up all of our

searches but if sorting is slow that

kind of invites the question well should

we bother sorting our data if we're only

going to search the data maybe once

maybe twice and so here is going to be

potentially a trade-off so let's

consider what it means really to sort

data in our case it's just going to be

simple and use numbers but it might in

the case of the Googles of the World be

actual web pages or persons or the like

so here is our typical picture for

sorting input uh for solving any problem

input at left and output at right the

input to our sort problem is going to be

uh some unsorted set of values and the

output ideally will be the same set of

values sorted and if we do this

concretely let's suppose that we want to

go about sorting this list of numbers 7

2 5 4 1 6 03 so it's all of the numbers

from 0 to 7 but they're somehow jumbled

up randomly that's going to be the input

to the problem and the goal is now to

sort those so that you indeed get out 0

1 2 3 4 5 67 instead so it turns out

there's lots of different ways we can

actually sort numbers like these here um

and in fact just to complement our

search example earlier could we perhaps

quickly get some eight volunteers to

come up if you're comfortable appearing

on the internet if you want to do one

okay uh two three four five 6 7 eight

how about all right come on

down all

right come on over here and I'll give

you each a number and if you want to

start to organize yourselves in the same

order you see the numbers on the

board okay so look up on the overhead

and organize yourselves from left to

right in that same

order and let's have the first of you

perfect if you want to come uh right

over here

how about right in line with this all

right and a few more

numbers all right number

two six

and perfect just the right number all

right no uhoh all right there we go

number three all right so let's just do

a quick check we have 7 2 5 4 1 603 very

good so far do you want to just scooch a

little this way just to make a little

more of room all right and let's

consider now who we have here on stage

you want to each say a quick hello to

the audience hi my name is Ryan uh I'm a

first year from Penny

Packer hi my name is Cel I'm a first

year at Strauss um hi my name is Lucy

I'm a first year from

greo hi my name is Shiloh I'm a first

year in

Wigglesworth hi my name is Jack and I'm

a first year in

St hi my name is Katherine I'm a first

year in stra hi my name is Michael and

my first year at Penny Packer hi my name

is Muhammad at my first here in massuse

nice welcome aboard all right so let's

consider now how we might go about

sorting Our Kind volunteers here the

goal being to get them into order from

uh smallest to largest so that

presumably then we can use something

smarter than just linear search we could

actually use binary search assuming that

they are already then sorted so let me

propose that we first consider an

algorithm that actually has a name

called selection sort and selection sort

is going to be one that literally has me

or really you as the programmer

selecting the smallest element again and

again and then putting them into the

appropriate place so let me go ahead and

start this here uh starting with the

number seven at the moment seven is the

smallest number I found so I'm going to

make mental note of that with a mental

variable if you will I'm going to move

on now oh number two is obviously

smaller so I'm just going to update my

mental reminder that two is now the

smallest effectively forgetting for now

number seven uh five not smaller four

not smaller one smaller and I'm going to

make mental note of that six not smaller

zero smaller I'll make mental note of

that having forgotten now everything

else and now number three is not smaller

so what's your name again Michael so

Michael is number zero he belongs of

course way down there but unfortunately

you are Ryan Ryan Ryan is in the way so

what should we do how should we start to

sort this

list where should number zero

go yeah do you want to say the

L yeah so let's just go ahead and swap

so if you want to go ahead and zero go

on where seven is we need to make room

for number seven it would kind of be

cheating if maybe everyone kind of

politely stepped over to the side why

because if we imagine all of our

volunteers here to be in Array like

that's a crazy amount of work to have

every element in the array shift to the

left just to make room so we're going to

keep it simple and just evict whoever is

there now maybe we get lucky and number

seven is actually closer to its

destination maybe we get unlucky and it

goes farther away but we've at least

solved one problem if we had n problems

at first now we have n minus one because

number zero is indeed in the right place

so if I continue to act this out let me

go ahead and say two okay currently the

smallest five no four no one currently

the smallest I'll make mental note 6 7 3

and now let me pause one is obviously

the now smallest element so did I need

to keep going well it turns out at least

as I've defined selection sort I do need

to keep going because I only claim that

I'm using one variable in my mind to

remember the then smallest element I'm

not smart enough like us humans to

remember oh wait a minute one is

definitely the smallest now I don't have

that whole recollection so I just am

keeping track of the now smallest so

number one what your name was Jack Jack

where should jack go probably there and

what's your name itel itel okay so Jack

and itel if you want to swap places

we've now solved two of the end total

problems and now we'll do it a little

faster if each of you want to sort of

start to swap as I find the right person

so five smallest four is smaller two is

smaller got to keep checking okay two

was smaller all right now I'm going to

go back to the beginning all right four

is small five is not six is not seven is

oh three is small where do you want to

go okay good I'm going to go back here

and I can be a little smart I don't have

to go all the way to the end because I

know these folks are already sorted so I

can at least optimize slightly so now

five is small six is small seven is four

four is smaller if you want to go in

place

there and now here things get

interesting I can optimize by not

looking at these folks anymore more cuz

they're obviously problem solved but now

five is small six is not seven is not

okay five you can stay where you are now

a human in the room is obviously going

to question why I'm wasting any more

time but with selection sort as I've

defined it thus far I still have to now

check six is smallest not seven and now

my final step okay they're all in place

so here too is this dichotomy between

what we all have is this bird's eye view

of the whole problem where it's obvious

where everyone needs to go but a

computer implementing this with an array

really has to be more methodical and

we're actually saving a step here if we

were really doing this none of these

numbers would be visible all eight of

our volunteers would be inside of a

locked door and only then could we see

them one at a time but we're focusing

now just on the Sorting aspect so let me

just before we do one other

demonstration here proposed that what I

really just did here in pseudo code was

something like this 4 I from 0er to n

minus one keeping in mind that zero is

always the left of the array n minus one

is always the right end of of the array

for I from 0 to N minus1 I found the

smallest number between numbers bracket

I and numbers bracket n minus one and

that's the very geeky way of expressing

this optimization I'm always starting

from numbers bracket I wherever I am and

then everything else to the right and

that's what was allowing me to ignore

the already sorted volunteers if though

my last line says swap smallest number

with numbers I I think that implements

what our humans were doing by physically

walking to another spot all right so

that then would what we'll call

selection sort let's go ahead and take a

second approach here using an algorithm

that I'm going to call bubble sort but

to do this we need you all to reset to

your original locations we have a little

cheat sheet on the board if you'd like

to go back to this position here and let

me take a fundamentally different

approach because I'm not really liking

selection sort as is because it's kind

of a lot of walking back and forth and a

lot of walking suggests like a lot of

lot of steps again and again so what

might I do instead well bubble sort is

going to have me focus a little more

intuitively on just smaller problems and

let's see if this gets me somewhere else

so if I just look at this list without

looking at everyone else seven and two

this is obviously a problem why because

you're out of order so let's just solve

one tiny problem first so seven and two

why don't you swap I know two is in a

better place now because she's

definitely lower uh less than seven so I

think I can now move on seven and five

problem so let's solve that seven and

four problem let's solve that s and one

let's solve that 7 and six let's solve

that 7 and zero solve that 7 and three

solve that okay done sorted right all

obviously not if you just glance at

these numbers here but we have

fundamentally taken a bite out of the

problem seven is indeed in the right

place so we maximally have n minus one

other problems to solve so how do I do

this I think I can just repeat the same

logic let me go over here two and five

good five and four no five and one no

five and six yes 6 and zero no six and

three no so so now we've solved two of

the problems and what's nice about

Bubble sword at least as this glance

it's nice and simple it's nice and local

and you just keep incrementally solving

more and more problems so let's go ahead

and do this again and I'll do it we can

do it faster two and four we know are

good four and one four and five five and

zero five and three five and six six and

seven good so we go back two and one ah

now another problem solve two and four

four and zero four and three

four and five five and six six and seven

and so notice too as per its name the

largest elements have bubbled their way

up to the top and that's what seems to

be happening just as we're fixing some

remaining problems so almost done one

and two two and zero two and three three

and four four and five five and six six

and seven almost done obviously to us

humans it looks done how do I know as

the computer for sure what would be the

most Surefire way for me to now oh it's

not done sorry uh that's a bug

okay one and zero okay one and two two

and three three and four three four and

five five and six six and seven okay so

now it's obviously sorted to the rest of

us on stage how could I confirm as much

as code right you're doing it with your

mind just glancing at this how would the

computer the code know for sure that

this list is now sorted

yeah let's do one more time and look uh

draw what

conclusion yeah let's do it one more

time even though it's a little wasteful

but logically if I go through the whole

list comparing pairs again again and

again and I don't do any work that time

now it's obviously logically safe to

just stop because otherwise I'm wasting

my time doing the same thing again and

again if no one's actually moving so I'm

afraid we don't have Monopoly games for

all of you but we do have eight stress

balls and a round of applause if we

could for our

volunteers if you want to put your

numbers on the Shelf

there so if we consider for a moment

thank you thank you so

much

sure thank you thanks sure so if we

consider now these two algorithms which

one is better any intuition for whether

selection sort the first is better or

worse than bubble sort the

second any

thoughts

yeah okay so bubbl swort seemed like

less work especially since I was

focusing on those localized problems

other

intuition selection sort versus bubble

sort well let me propose that we try to

like quantize this so we can actually

analyze it in some way and this is not

an exercise we'll do constantly for lots

of algorithms but these are pretty

representative of algorithms so we can

wrap our minds around indeed the

performance or the design of these

things so here is my pseudo code for

selection sort whereby as it's as per

its name I just iteratively select the

next smallest element El again and again

so how can we go about analyzing

something like this well we could just

do it on paper pencil and count up the

number of steps that seem to be implied

logically by the code we could literally

comp count like the number of steps I

was taking again and again left to right

we could also just com uh count the

number of comparisons I was making with

each of the persons involved and I was

doing it kind of quickly in selection

sort but every time I was looking at a

person trying to decide do I want to

remember that number is small as that

number I was comparing two values with

an equals equals or less than or greater

than sign at least if we had done this

in code so that tends to be the norm

when analyzing algorithms like these

counting the number of comparisons

because it's kind of a global uh it's

kind of a global unit of measure we can

use to compare different algorithms

entirely so think too that in the

general case when we have more than

eight volunteers more than seven doors

we can generalize our our array in

general as this is the first element at

bracket zero and the end of it is always

n minus one so arrays uh or doors in

this case or volunteers are always

numerically indexed from zero on up to n

minus one if there's n of them in total

so how do we analyze the code of

selection sort well how many steps did

it take me to find the first smallest

element or more precisely how many

comparisons did I need to make when I

walked left to right to find our first

smallest person which ended up being

zero how many comparisons did I do when

walking left to right if there were

eight people on

stage how many total comparisons did I

do like if there's eight people I

compared these folks then then like this

person this person yeah yeah so seven

total right because if there's eight

people on stage you can only do seven

comparisons total because otherwise

you'd be comparing one number to itself

so it seems like in the general case if

you've got n numbers that you're trying

to sort finding the smallest element

first takes n minus one comparisons

maybe n it's total steps left or right

but the number of comparisons which I

claim is just a useful unit of measure

is n minus one how about finding the

next smallest person how many steps did

it take me to find the next smallest

number which ended up being the number

one

yeah yeah so just n minus two why

because I'd already solved one problem

someone was already in the right

position it would be silly to keep

counting them again and again so I can

Whittle down my number of comparisons

for the next past n minus 2 the third

past to find the third smallest number

would be n minus 3 and then dot dot dot

presumably this story this formula ends

when you have just one final pair the

people at the end to compare so if this

is looking a little reminiscent of some

kind of recurrence from high school or

high school math or physics or the like

let me just stipulate that if you

actually do out this math and generalize

it that is the same thing as n * nus1 /

2 and if you're Rusty on that no big

deal just kind of commit to memory that

anytime you add up this kind of series

something plus something slightly

smaller plus something slightly smaller

Each of which differs by one you're

going to get this formula n * n -1/ 2 if

we of course uh uh multiply that out

that's really n^ 2 minus n all divided

by two if we keep multiplying it out

that's n^2 / 2 minus n /2 and now we

have kind of a vocabulary with which we

can talk about the efficiency the design

of this algorithm but honestly I don't

really care about this level of

precision like n^2 / 2us n / 2 as n gets

really large which of these symbols

which of these terms is really going to

dominate become the biggest influencer

on the total value of

steps right it's the square right like

it's definitely not n divided by two

that's shaving some time off but N

squared as n gets big is going to get

really big if n is 100 then n squar is

bigger if N is a million n squ is really

bigger and so at the end of the day when

we really just talking about sort of a

wave of of the hand analysis and upper

bound if you will let's just say that

selection sort as analyzed here it's on

the order of n s steps it's not

precisely n squar steps but you know

what n^2 divided two the intuition here

might be that well it's half of that you

n squ is what really matters as n gets

really really large and that's when you

start thinking about and trying to solve

the Google problems of the world when n

gets large that's when you have to be

smarter than just sort of naive

implementations of any algorithm so

where then does this algorithm fall into

this categorization here well n^2 it

turns out is on the order of n squ steps

in the worst case whether it's sorted or

not it turns out though lower bound if

we consider this same code suppose the

best case scenario like our eight

volunteers came up on stage and just

because they already sorted themselves 0

through seven suppose they just happen

to be in that state how many steps would

selection store

take to sort an already sorted list of

volunteers any intuition

yeah would it still be

n so for the first pass it would still

be seven for the first per uh pass

across the humans because even though

yeah I'm claiming zero is here I don't

know that zero is the smallest until I

make my way all the way over there doing

all seven comparisons okay fine first

pass took seven more generally n minus

one steps what if I look for the next

smallest element and the humans in this

story are already sorted 0 through 7

well yes the number one's here and I see

them first but I don't know they're the

smallest until I compare against

everyone else get to the end of the list

oh well that was stupid I already had

the smallest person in hand then and so

this pseudo code this implementation of

selection sort is sort of fixed like

this there's no special case that says

if already sorted quit early it's always

going to take n squ steps and so in this

case if we borrow our uh jargon from

earlier using Omega notation just to be

clear selection sword is also going to

be in this Incarnation on an Omega of n

s because even in the best case where

the list is already sorted you're going

to waste a huge amount of time

essentially verifying as much or

discovering as much even though we

humans of course could see it right away

so selection sort would seem to take

both N squared steps in the uh worst

case n s steps in the best case and so

you know what we can use our Theta

terminology for that here would be an

algorithm just like counting earlier

that always takes N squared steps no

matter whether the array is sorted or

not from the get-go all right so

hopefully we can do better and someone

proposed earlier that bubble sort felt

like it was using fewer steps well let's

consider that next with bubble sort we

had this pseudo code I claim whereby

let's focus on the inside of the code

first down here what was I doing for I

from 0 to n minus 2 that's curious we've

never seen n minus 2 before but I asked

this question if numbers bracket I and

numbers bracket I + 1 are out of order

swap them so that was when I was

pointing at our first two volunteers

here I saw that they were out of order

so I swapped them how come I'm doing

that again and again up to nus 2 though

instead of n

minus1 which we've always used up until

now as our rightmost

boundary any intuition for why I'm doing

this from 0 to n minus 2

yeah exactly because I'm looking at the

E person per this pseudo code here and

the E plus one person I better make sure

I don't go Step Beyond the at boundaries

of my array so if you think of like my

left hand when my back was two here

pointing at the current person at the

first position my right hand for this if

conditioner is essentially pointing at

the person next to them and you want to

iterate with your left hand all through

these people but you don't want your

left hand to point at the last person

you want it to point at the second to

last person but we know that the last

person is always at n minus one so the

second to last person just

mathematically is at n minus 2 so it's a

subtlety but this is like a Segal

waiting to happen if you implemented

bubble sort using n minus one you will

my right hand would go beyond the

boundaries of the array so just bad all

right so why am I saying this end times

Well we did it very organically with

humans but each time someone uh each

pass I did through the array someone

bubbled their way up to the end number

seven then number six then number five

so if on each pass through the array of

volunteers I was solving at Mo at least

one problem it seems like bubble sort

can just run n times total to solve all

n problems cuz the first pass will get

at least one one number into place

second pass second number into place you

might get lucky and it would do more but

worst case this feels like enough just

do this blindly end times and they'll

all line up together well technically

all right now we're getting into the

weeds technically you can just repeat it

n minus one times because if you solve

all n minus one other problems and

you're left with one like literally that

person's where they need to be just

logically if you've already sorted

everything else and you've got just the

one left it's already bubbled up so how

do we analyze this well in bubble sore

we might do something like this I'm

essentially doing n minus one Things N

minus one times now let me back up to

the pseudo code because this one's a

little less obvious this is where you

can actually mathematically infer from

your Loop uh how many steps you're

taking so this first line literally says

repeat the following n minus one times

so that's going to translate very

straightforwardly to our mathematical

formula do something n minus one times

this Loop just because I'm using four

Loop terminology it's framed a little

differently but but if you're iterating

from 0 to nus 2 you're iterating a total

of nus1 times and again the arithmetic

is getting a little annoying but this

just means do the following n minus one

times so do n minus one Things N minus

one times we can now uh run out the math

as follows we have the formula n minus1

* n minus1 we do our little foil method

here N2 -1 * N - 1 * n + 1 we can

combine like terms n^ 2 - 2 n plus one

but at this point when n gets really

large which term are we really going to

care about this is on the order

of yeah n s so at least ASM totically

ASM totically means as n approaches

Infinity gets really large turns out

that the upper bounds on selection sort

and bubble sort are essentially the same

now if we really nitpicked and compared

the total number of comparisons they

might differ slightly but as n gets

large honestly you're barely going to

notice the difference it would seem

between these two algorithms

but what about um the lower bound if the

upper bound on Bubble sword is also Big

O of n what about the lower bound here

well with this pseudo code what would

the lower bound be on bubble sort even

in the best case when all of the

volunteers are

sorted any

intuition in this pseudo code yeah in

the

middle

good question isn't bubble sorts

designed such that you wouldn't need to

compare numbers that have already uh

bubbled up that's what's happening here

in the middle implicitly I'm always

going from left to right but remember

that even when I screwed up at the end

and the last two people were out of

order I do always need to restart at the

beginning because the big numbers are

going that way and the small numbers are

coming this

way so that is true there are some

slight optimizations that I'm kind of

glossing over here let me stipulate that

it would still end up being on the order

of n squ but that would definitely shave

off some actual running time here but

what if the list is already sorted our

our pseudo code at the moment has no

allowance for if list is already sorted

quit early so we're going to blindly do

n minus one Things N minus one times

unless we modify our pseudo code as I

did verbally earlier I propose this

inside of that outer loop if you make a

pass across all of the volunteers and

your mental counter has made no swaps

you have to keep track with some kind of

variable well then you might as well

stop because if you do a whole pass and

make no swaps why would you waste time

doing it again expecting different

Behavior so to help visualize these

whereby now bubble sort can be

advantageous if the data is already

sorted or mostly sorted why because it

does have this short circuit detail at

least if we implement it like that how

can we go about um visualizing these

things a little more clearly well let me

go ahead and do this let me pull up here

a visualization of exactly these

algorithms thanks to a third party tool

here that's going to help us visualize

these sorting algorithms as follows

small bars represent small numbers big

bars represent big numbers and so the

idea now is when I hit a button here to

get all of the small bars this way all

of the big bars this way so just like

our volunteers but instead of holding

lighted numbers its bars representing

their magnitude so let's go ahead and

start with for instance select

sort and you'll see in pink is being

highlighted the current number that is

being selected and then pulled all the

way to the left so this is selection

sort and again it's selecting the next

smallest element but you can see here

all the more visibly that just like my

human feet were taking a lot of steps so

is this algorithm touching these

elements again and again and again and

this is why the N squ is really a thing

there's got to be some inherent

redundancy here like why do we keep

looking at the same darn elements again

and again we do in terms of our pseudo

code need to do so but it's this

redundant comparisons that kind of

explains why n s is indeed the case so

now it's done small bars here big bars

there and I had to just keep talking

there to kill time because it's

relatively slow well let me randomize

the array just so we start with a

different order and now let me click on

bubbl sore and you'll see similar idea

but different algorithm so now the two

bars in pink are the two that are being

compared and fixed potentially if

they're out of order and you can see

already that the biggest bars are

bubbling their way up to the top but now

you can also see like this redundancy

like we keep swooping through the list

again and again just like I kept walking

back and forth and this is n squar this

is not that many bars what 10 20 there's

like 40 or something bars I'm guessing

that's pretty slow already just to sort

40 numbers and I think it's going to get

tedious if I keep talking over this so

let's just assume that this two is

relatively slow had I gotten lucky and

the list were almost sorted already

bubble sort would have been pretty fast

but this was a truly random array so we

did not get lucky so indeed the worst

case might be what's kicking in here so

I don't I feel like it'll be

anticlimactic like holding in a sneeze

if I don't let you see the end of this

so here we go nothing interesting is

about to happen almost

done ah okay done all right so thank you

thank

you so still somewhat slow though how

though can we perhaps do a little better

fundamentally so we can do so if we

introduce yet another technique and this

one isn't so much a function of code as

it is concept and it's something that

you might have seen in the real world um

but perhaps not so obviously so so it

turns out in programming recursion

refers to the ability of a function to

call itself in the world of mathematics

if you have a function f if F appears on

both the left side and the right side of

a formula that would be a recursive

function in the math world too whenever

f is defined in terms of itself or in

our case in Compu in programming anytime

a function calls itself that function is

said to be recursive and this is

actually something we've seen already in

class even though we didn't call it as

much so for instance consider this

pseudo code um from earlier whereby this

was the pseudo code for searching via AR

search a whole bunch of doors if no

doors are left return false that was the

the additional conditional we added but

then if number behind middle door return

true and here's the interesting part if

number is less than middle door search

the left half else if number is greater

than middle door search the right half

this pseudo code earlier was itself

recursive why because here is an

algorithm for searching but what's the

algorithm telling us well on this line

and this line it's telling us to search

something else so even though it's not

explicitly defined in code as having a

name if this is a search algorithm and

yet the search algorithm is using a

search algorithm this pseudo code is

recursive now that could quickly get you

into trouble if a function just calls

itself again and again and again but why

intuitively is it not problematic that

this code this pseudo code calls

itself why will the algorithm still stop

yeah

exactly it has some exit condition like

if no do is less and more more

importantly anytime you search the left

half you're searching a smaller version

of the problem anytime you search the

right half you're searching a smaller

version of the problem literally half

the size so this is why in the phone

book obviously I couldn't tear the phone

book in half uh infinitely many times

because it was literally getting smaller

each time so recursion is this ability

to call yourself if you will it's but

what's important is that you do it on a

small smaller smaller problem so that

eventually you have no more problems to

solve or no more data no more doors at

all so these two lines here would be the

recursive elements here but if we go

back to week zero we could have used

recursion in some other way so this was

our Cudo code for the phone book back in

week zero and recall that we described

these yellow lines as really

representing a loop some kind of cycle

again and again but there was a missed

opportunity here what if I had reinvent

reimplemented this code to do this

instead

instead of saying open to middle of left

half of book and then go back to line

three like literally inducing a loop or

open to Middle right half a book and go

back to line three inducing another loop

why don't I just recognize that what I'm

staring at now is a algorithm for

searching a phone book and if you want

to search a smaller phone book like a

through M or n through Z we'll just use

the same algorithm so I can replace

these yellow lines with just this

casually speaking search left half a

book search right half a book this would

be implicitly and now I can shorten the

whole thing a recursive implementation

of the phone book pseudo code from week

zero and it's recursive because if this

is a search algorithm and you're saying

go search something else that's fine

that's recursive but because you're

searching half of the phone book it's

indeed going to get smaller and smaller

even in the real world or the real

virtual world you can see recursive data

structures in the wild or at least in

Super Mario Brothers like this let me

get rid of all the distractions here and

focus on this pyramid where you have one

block then two then three then four well

this itself is technically recursively

defined in the sense that well what is a

pyramid of height four well it's really

what how would you describe a pyramid of

height four is actually the same thing

as a pyramid

of of of height three plus one

additional layer well what's a pyramid

of height three well it's technically a

pyramid of height two plus one

additional layer and so even physical

structures can be recursive if you can

Define them in terms of itself now at

some point you have to say that if the

pyramid is of height one there's just

one block you can't forever say it's

defined in terms of a height negative 1

negative2 you would never stop so you

have to kind of have a special case

there but let's go ahead and translate

something like this in fact to code let

me go back to uh vs code here and let me

Implement a program called iteration

that refers to a loop iterating and let

me Implement a very simple pyramid like

that so let me go ahead and include the

cs50 library I'll include our standard

i.h in main void no command line

arguments today and let's go ahead and

do this let's declare a variable called

height ask the human for the height of

this pyramid and then let's go ahead and

draw a pyramid of that height now of

course draw does not yet exist so I'm

going to need to invent the draw a

function let me go ahead and Define a

function that doesn't have a return

value it's just going to have side

effects it's just going to print bricks

on the screen call draw and it takes in

an integer n as its input and how am I

going to implement this well again I

want to print one block then two then

three then four that's pretty

straightforward at least once you're

comfortable with loops let me go back to

the code here let me go ahead and say 4

into I gets zero I is less than n i ++

and that's going to iterate essentially

row by row and on each row I want to

print out one then two then three then

four bricks but I'm iterating from Z to

1 to two to three so I think that's okay

I can just say something like 4 in J

gets zero J Let's Be Clever about this

is less than I

j++ and now let me go ahead and inside

of this loop I think I can get away with

just printing out a single hash sign but

then outside of that Loop similar to

last week I'm going to print my new line

separately so a little non-obvious at

first but this outer loop iterates row

by row line by line if you will and then

the inner loop just Mak sure that when I

equals zero um let's see oh nope there's

a bug I need to make sure that it's j is

less than I + 1 so when I is zero on my

first line of output I'm going to print

out one brick when I is one I'm going to

print out two bricks and so forth so let

me go ahead and run make iteration

all right and now seems to compile

uhoh huh implicit Declaration of

function draw so I'm making week one

mistakes

again what say

again yeah the the Prototype is missing

I didn't declare it at the top that's an

easy fix and the only time really it's

okay and necessary to copy paste let me

copy the functions declaration there end

it with a semicolon so that clang now

knows that draw will exist make

iteration now it works thank you /

iteration we'll type in something like

four and there we have it our pyramid of

height 1 2 3 4 that looks pretty similar

to this albeit using hashes so that's

how we would have implemented this like

two weeks ago in week one maybe last

week but just using arrays but let me

propose that we could do something

recursively instead let me close this

version of the code and let me go back

to VSS code and open up recursion doc

just to demonstrate something

recursively and I'll do it incorrectly

deliberately the first time so let me

include cs50.h let me include standard

i.h let me do uh int main void and let

me just blindly draw a pyramid initially

of height one but now in my draw

function let me reimplement it a little

differently so my draw function this

time is still going to take a number n

but that's how many hashes it's going to

print so let's do four into I gets zero

I is less than n i

++ then let's go ahead and print out a

single hash mark here and then after

that let's print out the end of the line

just as before but now this of course is

only going to draw a single um row it's

going to print out one hash or two

hashes or three hashes but only on one

line let me now incorrectly but just

kind of curiously say all right well if

this draws a pyramid of height one let's

just use ourself to draw a pyramid of

height n plus one so the first time I

call draw it will print out one hash

then the second time I call draw it will

print out two hashes then three then

four so we're kind of laying These

Bricks down from top to bottom uh make

recursion uh oops I screwed up again so

let's copy the Prototype here let's put

this down over here semicolon let's do

this again uh make recursion all right

all good/ recursion and now let me

increase the size of my terminal window

just so you can see more of the output

and here we

have okay bad but thank you so we have

an infinitely tall pyramid and it's just

flying across the screen which is why it

looks kind of like a mess but I printed

out a pyramid of height one and then two

and then three and then four and

unfortunately what am I lacking any sort

of quick condition any kind of condition

that says wait a minute when it's too

tall stop all together so this is an

infinite Loop but it's not a loop it's a

recursive call and actually doing this

in en is very bad we'll see next week

that if you call a function too many

times you can actually trigger yet one

yet another of those segmentation faults

cuz you're using too much memory

essentially but for now I haven't

triggered that yet control C is your

friend to cancel and as an aside if

you're playing along at home or play

with this code later I actually cheated

here we have a special clang

configuration feature that prevents you

from calling a function like that and

creating a problem I overrode it just to

for demonstration sake but odds are at

home you wouldn't be able to compile

this code yourself but let me do a

proper version recursively of this code

as follows let me go back into the code

here let me go ahead and not just

blindly start drawing one then two then

three layers of bricks let me prompt the

human as before for the height of the

pyramid they want using our get in

function and now let me call draw of

height again so now I'm going back to

the looplike version but instead of

using a loop now this is where recursion

gets rather elegant if you will let me

go ahead and execute and code uh the

draw function as

follows per your definition if a pyramid

of height four is really just a pyramid

of height three plus another row well

let's take that literally let me go back

to my code and if you want to draw a

pyramid of height four well go right

ahead and draw a pyramid of height uh

three first or more generally n minus

one but what's the second step well once

you've drawn a pyramid of height three

draw an extra row so I at least have to

bite off that part of the problem myself

so let me just do for in I gets zero I

is less than n i++ and let me the

programmer of this function print out my

hashes and then at the very bottom print

out a new line so the cursor moves to

the next line but this is kind of

elegant now I dare say in that draw is

recursive because I'm literally

translating from English to code this

idea that a pyramid of height four is

really just a pyramid of height three so

I do that first and I'm sort of trusting

that this will work then I just have to

lay one more layer of bricks four of

them so if n is four this is just a

simple for Loop Allah week one that will

print out an additional layer but this

of course is going to be problematic

eventually why it's not done yet this

program how many times will draw call

itself in this

model infinitely many times

why yeah there's no there's no

equivalent of quit like if you've

printed enough already then quit well

how do we capture that well I don't

think we want this to go negative right

it would make no sense to draw a

negative height pyramid so I think we

can just pluck off as the programmer a

an easy case an easy answer a so-called

base case and I'm just going to do this

at the top of my draw function let me

just say if n is less than or heck less

than or equal to zero that's it go ahead

and just return there's nothing more to

do and that simple condition technically

known as a base case will ensure that

the code doesn't run forever why well

suppose that draw uh is called with an

argument of four four is of course not

less than zero so we don't return but we

do draw a pyramid of height three and

here's where things get a little

mentally tricky you don't move on to

line 20 until draw has been called so

when draw is called with an argument of

three it's as though you're executing

from the top of this function again

three is not less than zero so what do

you do you draw two okay how do you draw

two well two is not less than zero so

you don't return so you draw one got to

be careful here draw one and now we go

ahead back to the beginning how do you

draw One well one is not less than zero

so you don't return you draw height zero

how do you draw height zero wait a

minute 0o is less than or equal to zero

and you return and so it's kind of like

this mental stack this to-do list you

keep postponing executing these lower

lines of code because you keep

restarting restarting restarting the

draw function until finally one of those

function calls says there's nothing to

do return and now the whole thing starts

to unravel if you will and you pick back

up where you left off and this is

perhaps the best uh scenario we won't do

it in class but if you'd like to wrestle

through this on your own using debug 50

to keep stepping into step into step

into each of those lines logically

you'll see exactly what's actually

happening so let me go to my terminal

and do make recursion which is now this

correct version of the code do/

recursion let's type in a height of four

and voila now we have that same pyramid

not using iteration per se though

admittedly we're using iteration to

print the additional layer we're now

using draw recursively to print all of

the smaller pyramids that need come

before

it no question is can you only use

recursion with a void function no not at

all in fact it's very common to have a

return value like an integer or

something else so that uh you can

actually do something constructively

with that actual value other questions

on

this say a little

lad when is line 21 getting executed so

if you continue to UNR let me uh scroll

down a bit more so you can see the top

of the code so line 21 will be executed

once line 19 is done executing itself

now in the story I told we kept calling

draw again again again but as soon as

one of those function calls where n

equals z returns immediately then we

don't keep drawing again and again so

now if you kind of think of the process

as reversing then you continue to line

21 then a line 21 again then line 21

again and as the sort of logic unravels

and next week we'll actually paint a

picture of what's actually happening in

the computer's memory but for now it's

just it's very similar to the pseudo

code for the phone book you're just

searching again and again but you're

waiting until the very end to get back

the final result uh can Google Now whom

I keep mentioning by coincidence today

is full of programmers of course um

here's a fun exercise let me uh go back

to a browser I'm going to go ahead and

search for recursion because I want to

learn a little something about recursion

uh here is kind of an internet meme or

joke if I zoom in here the engineers at

Google are kind of funny funny see

why ah there we go yes yes this is

recursion and there's going to be so

many memes you'll come across now where

recursion like if you've ever pointed a

camera at the TV that's showing the

camera and you sort of see yourself for

the image again and again that's really

recursion and in that case it only stops

once you hit the base case of a single

Pixel um but this is a very funny joke

in some circles uh when it comes to

recursion uh and Google so how can we

actually use Google or rather how can we

actually use recursion constructively

well let me propose that we actually

introduce a third and final algorithm

for sorting that hopefully does better

than the two sorts thus far we've done

selection sort and bubble sort bubble

sort we liked a little better at least

in so far is in the best case where the

list is already sorted bubble sort is at

least smarter and it will actually

terminate early giving us a better lower

Bound in terms of our Omega notation but

it turns out that recursion and this is

not necessarily a feature of recursion

but something we can now leverage it

turns out using recursion we can take a

fundamentally different approach to

sorting a whole bunch of numbers in such

a way that we can do far fewer

comparisons and ideally speed up our

final results so here is the pseudo code

for what we're about to see for

something called merge sort and it

really is this tur sort the left half of

numbers sort the right half of numbers

merge the sorted halves right this is

almost sort of non sensical because I uh

if you're asked for an algorithm to sort

and you respond with well sort the left

half sort the right half like that's

being sort of difficult because well I'm

asking you for a sorting algorithm

you're just telling me to sort the left

half and the right half but implicit in

that last line merging is a pretty

powerful feature of this sort now we do

need another base case at the top so let

me add this if we find ourselves with a

list an array of size one well that

array is obviously sorted if there's

only one element in it there's no work

to be done so that's going to be our

base case but allowing us now in just

these what uh four six lines of pseudo

code to actually sort some elements but

let's focus first on just a subset of

this let's consider for a moment what it

means to merge sorted have so Carter has

wonderfully come up to volunteer here

just to help us reset these numbers

suppose that in the middle of the story

we're about to tell we have two sorted

halves I've already sorted the left half

of these numbers and indeed 2 4 57 is

sorted from smallest to largest and the

right half appears to be already sorted

0136 already sorted so in my pseudo code

we're already done sorting the left half

and the right half somehow but we'll see

how in a moment well how do I go about

merging these two halves well because

they're sorted already and you want to

merge them in order I think we can flip

down we can hide all but the first

numbers in each of these subl lists so

here we have a half that starts with two

and I don't really care what the other

numbers are because they're clearly

larger than two I can focus only on Two

And Z too zero also we know that zero is

the smallest there so let's just ignore

the numbers that Carter kindly flipped

down so how do I merge these two lists

into a new sorted larger list well I

compare the two on my left with the zero

on my right obviously which comes first

the zero so let me put this down here

and Carter if you want to give us the

next element now I have two sorted

halves but I've already plucked one off

so now I compare the two against the one

one obviously comes next so I'm going to

take out the one and put it in place

here now I'm going to compare the two

halves again two and three which do I

merge first obviously the two comes next

and now notice each time I do this my

hands are theoretically making forward

progress I'm not doubling back like I

kept doing with selection sort or bubble

sort back and forth back and forth my

fingers are constantly advancing forward

and that's going to be a key detail so I

compare four and three three obviously I

compare three and uh I compare four and

six four obviously I compare five five

and six five obviously and then I

compare seven and six six of course and

then lastly we have just one element

left and even though I'm kind of moving

awkwardly as a human my hands

technically were only moving to the

right I was never looping back doing

something again and again and that's

perhaps the intuition and just enough

room for the seven so that then is how

you would merge two sorted halves we

started with left half sorted right half

sorted and merging is just like what you

would do as a human and Carter just

flips the numbers down so our Focus was

only on the smallest elements in each

any questions before we Forge ahead with

what it means then to be merged in this

way so now here is an original list we

deliberately put it at the top because

there's one detail of Mer sort that's

key mer sort is technically going to use

a little more space and so whereas

previously we just kept moving our

humans around and swapping people and

making sure they stayed ultimately in

the original positions with mer sort

pretend ends that here's our original

array of uh memory I'm going to need at

least one other array of memory and I'm

going to cheat and I'm going to use even

more memory but technically I could

actually go back and forth between one

array and a secondary array but it isn't

going to it is going to take me more

space so how do I go about implementing

merge sort on this Al on this code well

let's consider this here's a array of

size eight if only one number quit

obviously not applicable so let's focus

on the juicy part there sort the left

half of the numbers all right how do I

sort the left half of the numbers I'm

going to just nudge them over just to be

clear which is the left half how here is

now a sub list of size four how do I

sort the left

half well do I have an algorithm for

sorting yeah what do I do here's a list

of size four how do I sort it what's

step one sort the left half so I now

sort of conceptually in my mind take

this sublist of size four and I sort it

by first sorting the left half focusing

now on the seven and two all right

here's a list of size two how do I sort

a list of size

two sorry I think we just keep following

our instruction sort the left half all

right here is a list of size one how do

I sort a list of size

one I'm done like it's done so I leave

this alone what was the next step in the

story I've just sorted the left half of

the left half of the left half what

comes next I sort the right half of the

left half of the left half and I'm done

cuz it's just a list of size one what

comes after this merge so this is where

it gets a little trippy because you have

to remember where we're pausing the

story to do things recursively again and

again but if I've just sorted the left

half and I've just sorted the right half

now I merge them together this is a

super short list so we don't need um uh

Carter's help here as before but I think

the first number I take here is the two

and then the second number I take

because it's the only option is the

seven but what's nice now is that notice

the left half of the left half is indeed

sorted because I trivially sorted the

left half of it and the right half of it

but then merging is really where the

magic happens all right again if you rem

if you rewind now in your mind if I've

just sorted the left half of the left

half what happens

next sort the right half of the left

half so again you kind of rewind in time

so how do I do this I've got a list of

size two I sort the left half the just

the five done sort the right half four

done now the interesting part I merge

the left half and the right half of the

right half of the left half so what do I

do four comes down here five comes down

here and

now notice what I have left half is

sorted right half is sorted if you

rewind in time where is my next step

three merge the two halves and so this

is what Carter helped me do before let's

focus only on the smallest elements just

so there's less distraction I compare

the two and the four two comes first so

let's obviously put that here now I

compare the new beginning of this list

and the old beginning of this list four

obviously comes next and now I compare

the seven against the five five

obviously comes next and now lastly I'm

left with one number so now I'm down to

the seven so even if you've kind of lost

track of some of the nuances here if you

just kind of take a step back we have

the original right half here still

untouched but the left half of the

original input is now indeed sorted all

by way of doing sorting left half right

half left half right half but with those

merges in between all right so if we've

just sorted the left half we rewind all

the way to the beginning what do I now

do all right so sort the right half so

sort the right half how do I sort a list

of size four well I first sort the left

half the one and the six how do I sort a

list of size two you sort the left half

just the number one obviously there's no

work to be done done sorting the left

half six done sorting the right half now

what do I do I merge the left half here

with the right half here and that one's

pretty straightforward now what do I do

I've just merged so now I sorted I've

just sorted the left half of the right

half so now I sort the right half of the

right half so I consider the zero done I

consider the three done I now merge

these two together zero of course comes

first then comes the three and now I'm

at the point of the story where I've

sorted the left half of the right half

and the right half of the right half so

step three is merge and I'll do it again

like we did with Carter all right one

and zero obviously the zero comes first

now compare the one and the three

obviously the one comes first compare

the six and the three obviously the

three and then lastly the sixth so now

where are we we've taken the left half

of the whole thing and sorted the left

and sorted it we then took the right

half of the whole thing and sorted it so

now we're at lastly step three for the

last last time what do we do merge and

so just to be consistent let me push

these down and let's compare left hand

to right hand noticing that they only

make forward progress none of this back

and forth comparisons two and zero of

course the zero so we'll put that in

place two and one of course the one so

we put that in place two and three we

merge in of course the two in this case

four and three we now merge in the three

in this case four and six we now merge

of course the four in place and now we

compare five and six we keep the

five

bug okay well pretend that the five is

on uh oh this is why all right so now we

compare the seven and the six sixth is

gone and lastly seven is the last one in

place and even though I grant that of

all the algorithms this is probably the

hardest one to stay on top of especially

when I'm doing it as a voiceover realize

that what we've just done is only those

three steps recursively we started with

a list of size eight we sorted the left

half we sorted the right half and then

we merged the two together but if you go

down each of those rabbit holes so to

speak sorting the left half involves

sorting the left half of the left half

and the right half of the left half and

so forth but this germ of an idea of

really dividing and conquering the

problem not such that you're having the

problem and only dealing with one half

clearly we're sorting one half and the

other half and merging them together

ultimately it does still lead us to the

same solution and if we visualize the

remnants of this now if I depict this as

follows where on the screen here you see

where the numbers originally started in

the top row from left to right

essentially even though this is in a

different order I divided that list of

size eight ultimately into eight lists

of size one and that's where the base

case kicked in and just said okay we're

done sorting that and after that

logically I then sorted I merged two

lists of size one into many lists of

size two and those lists of size two

into lists of size four and then finally

the list of size four into one big list

sorted of size eight and so I put forth

this picture with the little line

indicators here because how many times

did I divide divide divide in half or

really double double double so exponent

is the opposite oh spoiler uh how many

times did I divide so three concretely

but if there's eight elements total and

there's N More generally it really is a

matter of dividing and conquering login

times you start this and you can divide

one two three times login times or

conversely you can start here and

exponentially uh double double double

three times which is log n but on every

row every shelf literally I made a fuss

about pointing my hands only from the

left to the right constantly advancing

them such that every time I did those

merges I touched every element once and

only once there was none of this back

and forth back and forth on stage so if

I'm doing something log n times if I'm

doing rather N Things log n times what

would be our Big O formula perhaps N

Things log n

times yeah so n log n the order of n log

n is indeed how we would describe the

running time of merge sort and so of all

of the sorts thus far we've seen that

merge sort here actually is n log n

which is strictly better than n squ

which is where both merge uh both

selection sort and bubble sort landed

but it's also slower than linear search

for instance but you would rather expect

that if you have to do a lot of work up

front sorting some elements versus just

searching them you're going to have to

put in more effort and so the question

of whether or not you should just search

something blindly with linear search and

not bother sorting it really boils down

to can you afford to spend this amount

of time and if you're the Googles of the

world odds are you don't want to be

searching their database linearly every

time why because you can sort it once

and then benefit millions billions of

people subsequently using something like

binary search or frankly in practice

something even fancier and faster than

binary search but there's always going

to be this tradeoff you can achieve

binary search only if the elements are

sorted how much does it cost you to sort

them well maybe N squared if you use

some of the earlier algorithms but it

turns out n log n is pretty fast as well

so at the end of the day

these running times involve tradeoffs

and indeed in merge sort 2 I should note

that the lower bound on merge sort is

also going to be Omega of n log n as

such we can describe it in terms of our

Theta notation saying that merge short

is indeed in Theta of n log n so

generally speaking probably better to

use something like merge sort or some

other algorithm that's in N log n in

practice most programmers are not

implementing these sorting algorithms

themselves odds are they're using a

library off the shelf that themselves

have made the decision as to which of

these algorithms to do but generally

speaking and we're seeing now this for

the first time if you want to improve

time like use less time write faster

code you got to pay a price and that

might be your human time just takes you

more time to code up something more

sophisticated more difficult to

implement or you need to spend something

like space and as these shelves suggest

that too is one of the key details of

merge sort you can't just have the

elements swapping in place you need at

least an auxiliary array so that when

you do the merging you have a place to

put them and this is excessive this

amount of memory I could have just gone

back and forth between top shelf and

bottom shelf but it's a little more

interesting to go top down but you do

need more space back in the day decades

ago space was really expensive and so

you know what it might have been better

to not use merge sort use bubble sort or

uh selection sort even or some other

algorithm alog together nowadays space

is relatively cheap and so these are

more acceptable trade-offs but it

totally depends on the application the

very last thing we thought we'd do is

show you an actual compar comparison of

some of these sorting algorithms it's

about 60 seconds long and it will

compare for you uh selection sort bubble

sort and merge sort in parallel

simultaneously uh with some fun sorting

music showing you ultimately what it

really means to be an O of n s or better

yet Big O of n

Logan selection on the top bubble on the

bottom merge in the

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middle

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[Applause]

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all right that's it for cs50 we'll see

you next

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time

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e

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[Music]

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all right this is cs50 and this is week

four and if you think back a few weeks

ago already in week zero we started

talking about like what images are and

we talked about representation of images

as this grid of pixels and each pixel

has like some pattern of bits that

defines its color well it turns out

today we'll take a deeper look

underneath the hood at how things like

images and so much more is actually

implemented using just these zeros and

ones and how now as a programmer you can

actually harness that for better for

worse to better understand and better

manipulate what's going on inside of a

computer's memory using a language like

C in fact even this bowl of stress balls

that we keep happening is just a

photograph of course but if you think

back to week zero if you sort of enhance

enhance enhance this image like they do

in the movies it actually doesn't work

out the way you would think from

Hollywood as I keep continue to zoom in

and zoom in and zoom in on a screen like

this you'll see that yes it gets bigger

but if it gets too big what do you start

to notice the so-called pixelation and

indeed you can see the individual dots

so next time you watch some uh show or

movie on uh TV that has this sort of

notion of enhancing you know there's

actually a finite limit there you can

only enhance so far as there's actually

information there but once you zoom in

to a certain level like this like that's

all that there you're not going to see

the glint of the suspect in some crime

drama in their eye just because you've

enhanced the image there's only a finite

amount of information actually there but

we'll see today too that by

understanding what's going on inside of

a computer's memory we can start to

represent and even create and code more

interesting things so for instance here

is a bit map if you will which is a term

of art a bit map is a type of image and

it's a map of bits in the sense that you

have this coordinate system of up top

down left right at least in this artist

representation here and suppose that

maybe we all defi decide as a as as the

world that one shall represent the color

white and zero shall represent the color

black what might this map of bits this

bit map actually be can you see through

it yeah it is indeed a smiley face so an

amazing eye if I actually turn all of

the ones to White just to visualize this

you'll see indeed this is what was

embedded there but of course on our

computer monitors and phones we have

this grid of square is this grid of

pixels so indeed if you were to actually

see on your screen a smiley face like a

black and white one at that what's

probably going on underneath the hood is

just some pattern of zeros and ones and

maybe single bits one bit color if you

will where one here represents white and

zero represents black so if you kind of

like this thing it turns out you can do

pretty uh pretty beautiful pretty

interesting pretty artistically inclined

things if you go to this URL at your

leisure cs50. lart it'll actually

redirect you to a Google spreadsheet

that we've made in advance and we've

kind of Shrunk the rows and columns to

resemble a grid of pixels tiny little

squares all of which are white by

default not unlike this easel here that

we have a couple of volunteers working

away at in fact would you guys like to

come forward for a moment and say a

quick hello before we come back to you

uh hello my name is Daniel I'm from

Chicago welcome to Daniel and hi

everyone I'm Adam and I'm from chindon

Tobago nice well welcome to you both

thank you you'll see that in their hands

are actually a whole bunch of pixels uh

Post-it notes that we've handed them in

in advance so if you don't mind we'll

come back to you in a couple of minutes

and see what they've created if you will

on this grid of white paper much like

you could create on this Google

spreadsheet in fact feel free to send us

your Creations if so inclined uh via the

URL you'll get at cs50. lart now let's

come back to week zero where we Define

some of the building blocks for images

we talked about RGB which is just red

green blue and it's just one of the

systems a popular system via which you

can represent any color of the rainbow

using some combination of red and green

and blue and if any of you are

artistically inclined or have used

Photoshop or similar programs you might

typically have some means of selecting a

color by some grid like this but down

here notice there's explicit mentions of

the types of color systems in use R GB

and in fact here you see 0000 0 and up

here under new you see the color black

and that implies that if you have no red

no green no blue well that indeed would

represent by convention the color black

by contrast if we play around with

Photoshop or similar program if you have

a lot of red a lot of green and a lot of

blue for instance 255 255 255 like

really crank it up to the max value you

can represent with eight bits per week

zero well then it turns out you get the

color white here and we can play with

these numbers endlessly for instance if

we use 255 of red but zero green and

zero blue not surprisingly the square at

the top of the screen Becomes Of course

red entirely because it's all red and no

green no blue if we change it instead to

255 for green but zero for red and blue

of course we get green and then lastly

if we crank up the blue but leave red

and green as zero we of course get blue

but all this while down here highlighted

is something that maybe some of you have

seen before like some combination of

numbers and letters if any of you have

made personal web pages or use programs

like Photoshop you might have used these

so-called color codes so indeed the

world has this convention whereby using

six digits or sometimes three you can

represent a little more succinctly some

amount of red green blue and you'll see

here maybe by inference that if RGB is0

0 255 respectively perhaps where we're

going with this is that

0000 FF is just an alternative way of

expressing the exact same idea no red no

green and a lot of blue but why is that

and in fact we'll come full circle here

to introducing something that we could

have done in week zero but it doesn't

really solve a problem then but today as

we focus more on images and on memory

itself turns out understanding these

patterns is pretty useful so back in

week zero we talked of course about

binary and binary by implying two only

gives you two digits zero and one you

and I as humans almost always use the

decimal system in normal conversation

deck meaning 10 so we have 0 through n

instead if a human like us wants to

count up as high as 10 or 11 or 12 we

don't have a digit per se for 10 11 and

12 we start reusing digits so it's 1 0 1

1 1 2 and so forth but in other systems

not binary not decimal but systems

called heximal hex implying 16 there are

actually more digits than these which

might come as a surprise um it's not

pairs of digits like in decimal single

digits and frankly it doesn't really

matter what the digits are because at

the end of the day these are just

symbols that you and I immediately

associate with some notion of math but

just Strokes on the screen that

represent some have represent some

actual value so it turns out that by

convention

when you want more than n 10 digits 0

through 9 you start using letters of the

English alphabet A B C D E and F and you

can represent them in lower case it's

case insensitive so it doesn't really

matter you might see it in upper case or

lower case but this is how you can count

Beyond N9 not using decimal but using

indeed something called hexadecimal if

we get really technical this is also

known as base 16 and it's the same idea

as week zero where instead of using base

2 for binary base 10 for decimal use 16

as the base for heximal and so if we run

through just some simple examples here

in the world of heximal your columns are

just powers of 16 16 to the 0er 16 to

the 1 16 to the 2 and so forth but in

the world of hex we usually at least

thus far and today we'll see just pairs

of digits like this so here for instance

is the ones column and the 16's column

if we multiply that out so if you wanted

to represent the number you and I know

in in uh the real world as zero in

heximal it would just be 0 0 if you want

to represent the number one it would be

01 and from there we get 02 03 04 0 five

06 07 08 09 now things get potentially

interesting in decimal it would

obviously become 10 but in heximal it

just becomes 0 a and then 0 B which is

to say if I rewind after nine comes in

heximal if I pronounce it in in decimal

this is how you'd represent 10 this is

how you'd represent 11 12 13 14 and then

lastly in heximal the 16th value is f

which is just always going to represent

15 so where to how do we connect this to

some of the past math well once you get

to Zer F in heximal if f is the highest

you can count just like in decimal nine

is the highest you can count what comes

next if this is 15 I claim how do I

represent 16 in heximal with what

pattern of

symbols what pattern of symbols for

heximal yeah so one Z not 10 even though

you might read it like that as a typical

human but one zero because why well even

if this is completely new to you the

whole column system the places are

exactly the same intuitively so you need

a one in the 16's place and a zero in

the ones place and we won't count all

the way up to 255 but we count if we

count a little higher this would be 1 Z

AKA 16 in decimal this would be 1 one

AKA 17 in decimal and then 18 19 20 and

so forth dot dot dot and we can count

all the way up to FF CU if f is the

biggest digit in hexadecimal FF is

indeed as high as we can count and if

each F represents 15 well let's just do

the math like in week zero so 16 * F + 1

* f is how all of us learned to do uh

math in grade school even though not in

heximal that's of course 16 * 15 + 1 *

15 multiply that out you get 240 plus 15

and air go you can count as high as

255 using two heximal digits now this is

not the kind of thing where like this is

going to be an interesting exercise

mentally to ever convert in your head

generally you'll get used to the fact

that after nine comes a and the biggest

digit is f and you'll just start to see

patterns like this in the worlds of

Photoshop web pages in a few weeks and

Beyond but like why is heximal useful

like why are we complicating the world

and adding on top of decimal something

else well it turns out that a single

decimal digit like f the biggest one for

instance is 15 and here let me just

propose a bit of Mental Math how many

bits do you need to represent the number

15 in binary if you've got the ones

place two's Place fours and so forth how

many bits

total so fewer than five to count this

highest 15 I think but

close someone

else sing in hand yeah so four bits I

think suffice because if you want to

count as high as F that is to say 15 I

think if you have four bits you can do

that because if over here is the one's

place from week zero for binary this is

the two's place this is The Four's place

this is the e place do out some quick

math so 8 + 4 is 12 + 2 is 14 + 1 is 15

so it turns out that by convenience

heximal digits can just be represented

consistently with four bits or fewer but

four and four of course is half of eight

and eight is like everywhere like eight

bits is a bite which is again just a

convention we've seen and so the reason

that you see hexad desmal in the world

of Photoshop and eventually web pages is

it actually just Maps really nicely to

expressing binary numbers more

succinctly with a fixed number of digits

so for instance anytime you see 11 one

one one 1111 in the world as binary you

know what that's a little tedious to

both say and write you can represent

more succinctly

any uh group of four one bits more

succinctly in heximal as just F so 111

1111 in binary more succinctly and more

commonly now in the world of Photoshop

memory images and the like is

represented more succinctly as FF and

that's why because it just Maps really

nicely to four bits and so we can be a

little more succinct so any questions on

heximal which is just another way of

representing information but using the

same grade school approach yeah good

question if you represent 15 with f it

would use four bits so base systems are

really just a way for us humans on paper

or on screens to represent information

if F represents the decimal number 15

the computer underneath the hood has to

use four bits to represent it so one

heximal digit by convention always

implies four bits underneath the hood so

therefore if you have two heximal digits

like 0 0 that means eight zero bits

underneath the hood like for red or for

green if you see FF now we know that's

four one bits and another four one Bits

And if we do out the math that's 255

that's why in Photoshop

000000 FF means no red no green and

255 of blue and it's just way more

succinct than writing out like what 8

plus 8 plus 8 24 zeros and ones and it's

just cleaner than even using decimal

when you're using units of eight which

again computers just use everywhere so

it's just another system it's not one

you need to dwell on very much but again

it's fundamentally no different from

binary or decimal we're just using a

slightly different base no all right

well we had this blank canvas here and I

think uh are you two perhaps ready to

reveal for the world what you've created

do you want to go ahead and I'll I'll

swivel it around for you all right here

we go big reveal and today's pixel

art a round of applause if we

could very nicely done well thank you

both if you want to come up after and

tear this off and bring it home you're

welcome to and keep the Post-it notes

too well thank you to our volunteers

there let's now translate this to really

more technical worlds where we're going

to see and consider it more often

because in fact sometimes when you've

had error messages over the past few

weeks from clang the compiler you might

have even seen evidence of heximal we

didn't call it out it wasn't useful to

know at the time but it turns out a lot

of programs use and a lot of code uses

heximal for those reasons of more prec

more representation so for instance

where else might we see it well here's

that picture we keep pulling up of our

computer's memory and each of these

squares in this grid represents a bite

sort of top left to bottom right in the

computer's memory but again just an

artist's representation a few weeks ago

I claimed that each of these bytes can

be numbered of course like this is bite

zero at top left then bite one then bite

two then bite two billion if you have

two gigabytes of memory and so we could

just number them like this 0 through 15

on up 16 17 18 and so forth but per the

reasons earlier it's just more common in

computer systems and in software to

actually use hexadecimal just to

describe the locations of the addresses

of things in memory so instead a typical

programmer or computer scientist would

call these first 16 bytes 0 through F

just because but that's because it's a

predictable number of bits so if we keep

going beyond that you would get not 10

not 11 not 12 but in heximal 1 0 1 1 1 2

and so forth all the way down on the

screen to 1 F and if I you know shrunk

this down or had a bigger monitor we

would see eventually 255 bytes later

from the start 255 as well but there's a

potential problem here with using

heximal in this way there's an

ambiguity can anyone imagine like what

can go wrong if we use hex to just

simply describe locations in memory like

this

yeah yeah yeah like one zero might also

be 10 and you know maybe if you're you

know really thorough okay wait a minute

it can't be 10 cuz here's F over here so

it's obviously not decimal but why

create potential confusion especially

when you're collaborating building

something with someone we want to avoid

that ambiguity and so the convention

humans decided on years ago is that if

you want to make clear that a number is

in hexadecimal just by convention you

prefix all of the digits with Zer X the

X is not like another character it's not

like a 17th character it's just a human

conven

of putting 0x to imply here comes

heximal and now it's unambiguous so now

we see 0x10 obviously is not 10 as we

know it in decimal but rather it's the

number that comes after a single F so

it's really the number in decimal 16 so

0x anytime you see it that's just a

visual cue that what is ahead is

actually heximal so let's now start

playing around with this information so

here's a super simple line of code from

like week one where I'm just declaring

ing a variable n and I'm defining it to

be the value 50 and this is out of

context we probably need a main function

and all of that but let's just rewind to

week one where we actually saw code like

this and do something useful with a line

of code like this so let me go over here

to VSS code and in vs code I'll create a

program called how about addresses since

the goal of this uh the goal here is to

just play around ultimately with a

variable like n and let me go ahead and

do this I'll include how about standard

i.h I'll do int main void so no command

line Arguments for now in N gets 50 and

now so that we can do something mildly

useful with it let's just go use print F

and print out with percent I and then a

new line whatever that value of n is so

this is not going to be interesting per

se it's just week one stuff where I'm

defining a variable and printing it out

to the screen so let me go down to my

terminal window and do make

addresses no errors so that's good I'll

do do/ addresses and of course I should

see the number 50 here now what's going

on underneath the hood let's translate

now code to really what's going on under

under underneath the hood of the

computer so if this is our grid of

memory I don't necessarily know as the

programmer and I definitely don't care

as the programmer where exactly it's

ending up in memory that's the whole

point of using Code let the computer

figure this out but at least

conceptually I know that by declaring a

line of code like that the number 50

ends up somewhere in the computer's

memory and it's assigned the name n a

symbol n via which I the programmer can

refer to it and I very deliberately used

four of these squares for what

reason what might be the reason for

using four squares specifically yeah

yeah so an integer is four bytes at

least most of the time on Modern systems

an integer is four bytes on an older

computer it might just use one or maybe

even uh two bytes But Here by convention

we're almost always going to see four

bytes I don't know if it's going to end

up here it might end up over here but

for now who cares I just know that the

computer can store the the information

in this way underneath the hood so let's

now introduce another feature of C that

we haven't had occasion to use just yet

that's going to allow us to start poking

around the computer's memory For Better

or For Worse and this is one of those

situations where you're about to learn

acquire a a skill a power that can

actually come back to bite you because

once you know how to start poking around

a computer's memory you can do very

powerful things and next week we'll see

what you can build in a computer's

memory but you can also screw up pretty

easily and cause more of those

segmentation faults that a few of you

have already suffered so with that said

let's just stipulate that you know what

I don't care necessarily where the 50 is

in memory but I know it exists at some

address in memory and just so I have an

easy address to pronounce let's just

suppose it lives at ox123 so that's the

address in memory in heximal by

convention and that just happens to be

where it ends up when I write that line

of code but it turns out C has some

other operators we can use when we've

seen the asterisk before the star and

we've used it for multiplication but

today we're going to use it for

something more powerful and we're also

going to introduce an Amper sand which

allows us to do something as well the

Amper sand operator is going to allow us

to get the address of a piece of data in

memory like by literally putting

Ampersand before the name of a variable

C will tell us tell you what address

that variable lives at maybe it's ox123

maybe it's Ox 456 who knows but that

will give you back the answer the Star

does the opposite it's sort of means go

there so using the star otherwise known

as the D reference operator I can

actually go to a specific address if I

want and we'll see what this means in

code so how can I leverage this in some

mildly interesting way to start poking

around but eventually we'll use this

primitive to build more interesting

things so let me go back to save vs code

here and let me go ahead and do this

I'll clear my terminal to start fresh

and I'll introduce another format code

for printf percent p and for now just

take on faith that this it is percent P

because but percent p is going to allow

me to print the address of a variable if

I additionally tell C get the address of

n so I'm changing percent I to percent p

and that's just something you have to do

when printing addresses for now but I

need to change an ampersand in front of

the variable name so I don't print in

the number 50 I print out something like

ox123 and it's not going to be as simple

as that we'll see on the screen though

where it actually ended up in my code

spaces memory so here we go uh dot uh

down in my terminal make addresses again

to recompile and now dot SL addresses

should reveal not the value of 50 but

the address of 50 and there it is it's

pretty long it's not quite as simple and

pretty as ox123 but there's the ox

meaning here's a hexadecimal address and

it's 7f FC c784 a04 C suffice it to say

your codes space and even your Macs and

PCs nowadays have a lot of memory that's

why in part this address is so big not

as small as the thing on my slide so

this at the moment isn't that useful yet

but it introduces us to a concept that

we'll Now call pointers and pointers are

admittedly one of the more challenging

aspects of c and if in future life you

tell friends that oh I took a class

called cs50 and we learned C like you'll

probably get kind of a look at people

like why did you learn C or like oh c

was hard and it's largely because of

this topic which isn't to say that it's

that hard to wrap your mind around but

it's definitely very different and it's

not a feature that you can harness in

higher level languages that we'll see in

class two like Python and Java and the

like C is about as close to the

computer's Hardware so to speak that you

can get before things get actually scary

the so-called Assembly Language we saw

in week two when I had a link and

compile and assemble and all of that

like that gets really lowlevel and you

really have to be an expert with the

computer's CPU or brain to understand

that but with C you can actually poke

around the computer's memory and do

powerful things with that but again with

great power comes responsibility it's

very easy to break programs by misusing

memory or just having a bug that touches

memory in some way that you don't intend

so pointers at the end of the day are

pretty much what we just saw a pointer

is really just a variable that contains

the address of some value a pointer is a

variable that contains the address of

some value or more simply it's fine to

think of it as an address a pointer is

an address of something in the

computer's memory now what might we do

to uh to uh actualize this well here's

two lines of code it turns out by using

our two new operators today I can

declare an INT call it n and assign it a

value like 50 just like before if I want

to store the address of n in a variable

and not just print it immediately via

printf I can declare a variable for

instance called P but I could call it

anything I want like any variable but

because it's an address it's not int P

it has to be int star P so to speak and

the star here on the left hand side of

the equal sign is just a clue to C that

means p is going to be a pointer that is

p is going to be the address of what the

address of an integer now technically

it's still an integer itself right

because an address is just a number

whether it's 1 2 3 or ox123 so this is

really just a semantic difference so in

Star p just means that this variable

doesn't contain any old number like 50

it specifically contains a number that

is the address of something

else so how can I now use this well let

me go back to VSS code and let me

propose that we add a line of code like

that so instead of just directly

printing out that value let's go ahead

and Define a second variable called uh P

that's of type in Star P set it equal to

Ampersand n and then this time let's not

just print out Amper sand n let's

actually print out the value of P so the

only two new things here if I zoom in

are I've used not only the Amper sand on

the right to get the address of n I'm

now using the star on the Left To Tell C

that P is still a still a variable as

always but it's a pointer it is the

address of some other value like this

and I'm still going to print it with the

same format code percent P so that

doesn't change so let me go ahead and

zoom out and do make addresses and slash

addresses and there it is exactly the

same thing now in and of itself not that

useful yet but the fact that you can now

access the addresses of things in memory

means that we'll be able to build things

and construct things and Link things

together by knowing where they live so

to speak so any questions on this

technique thus far

[Music]

yeah a good question on line six must it

be starp and Ampersand and in this case

yes because what am I doing on the left

and I'll get rid of the equal sign for

now this would give me a variable called

P that's not an integer per se but

that's the address of an integer but

without the equal sign I'm not storing

anything in that variable so by adding

the equal sign and then Ampersand N I am

explicitly figuring out with Ampersand

what the address of n is which already

exists per line five and tucking it away

in this new variable called

P other questions yeah every time you

run good question every time I run the

program it uses up a different piece of

memory short answer yes computers though

long story short also have something

called virtual memory so if you run it

again and again you might actually see

the same addresses on the same Mac or PC

or cloud-based server but we'll see in a

bit where uh at a high level it's laid

out but it will always exist at some

address good question yeah some correct

Ampersand n is the address of N and in

Star p

is a pointer called p and honestly in an

Ideal World if C were made today and not

decades ago when humans were first

creating languages you know ideally we

would just have a data type called

pointer and then this would be a little

less complicated because it would

literally be what it says you know the

humans who invented SE didn't do this

but this is the idea so pointer is not a

legitimate word in the code it is a term

of Art in English but this is really

just the idea but the way you express

pointer as a data type type is a little

more cryptic as int star P here but

notice in line seven when I print out p

i don't use a star I don't use an

ampersand y I literally just want to

print the value of p and we've been

doing that since week one if you want to

print a variable just describe the

variable by its name no special syntax

any other questions on this thus

far uh what's the advantage of using

pointers with pointers we'll see today

some applications of them really the

idea is going to come to fruition next

week when we're going to create what are

called uh data structures in memory

where we can build not just uh for

instance uh one-dimensional data

structures like an array we'll see next

week we can actually create the

equivalent of two-dimensional data

structures or even threedimensional data

structures by using these addresses and

sort of linking things together and

we'll see the beginnings of that this

week but for now focus at least for now

on just really the syntax and what these

building blocks can do for us um does

the poin have to be does the p uh in

does the pop pointer have to be an point

to an integer short answer no and we'll

come back to this for now for the sake

of discussion we're only dealing with

integers like the number 50 uh you

mentioned strings or characters

absolutely we're about to go there soon

so you can use the address of anything

you want in the computer's memory so in

fact let's translate this now to just

the same picture just to help you wrap

your minds around what these two lines

of code really fundamentally are doing

so if I come back to my grid of memory

here let's plop the number 50 in the

variable n at the bottom right like it

was before so this is that first line of

code as before but with the new second

line of code as soon as I create P what

do I do well first remember that n lives

somewhere in the computer's memory

usually I don't care precisely where it

is but for the sake of discussion let's

suppose it's at ox123 which is easier to

say than where it actually ended up and

now what is p well p is just another

variable and variables live in memory

too so let me just hypothesize that P

lives up here and it turns out that P

once you assign it the value of

Ampersand N means that c will take a

look at the variable n realize oh it

lives at ox123 and what goes in the

value of p is literally

ox123 so again it's still an integer

which is confusing but it's technically

an integer being used as an address and

now just a a prompt here notice that

this pointer is pretty darn big it's

like eight squares what's the

implication of that because I did that

deliberately

how big must a pointer apparently be in

most modern systems would you say okay

good computers today are very big you

have gigabytes of RAM in your computer

you therefore need big pointers to be

able to point at memory that's

conceptually pretty far away so to be

clear how many bytes does a pointer

apparently take up well it seems to take

up eight in total integers by convention

nowadays are usually four pointers

though nowadays are typically eight in

this case so I'm drawing it in a manner

consistent with the reality even though

at the end of the day it's not really

that interesting what values are in here

in fact let's emerge from these weeds I

don't really care what else is going on

in my computer's memory at the moment

because I've only got those two lines of

Juicy code defining n and defining P so

let's hide all of the other squares and

honestly I mean it when I say that

programmers need to know that a variable

exists somewhere in memory and needs to

be able to get that address using like

the Ampersand but you're never going to

print F like I did the actual address

like it's not generally interesting

unless you're debugging your code but

you're not going to like start typing

out crazy Ox numbers in your code to

move things around you just need to know

that the computer can figure out where

things are so frankly by that logic who

cares that it's ox123 right tomorrow it

could be Ox 456 or something else so one

of the ways to think of a pointer is

literally as a variable that points at

something else and indeed in this case P

yeah technically it has an address and

yeah technically it's ox123 in the story

but honestly who cares I just need to

know that you using p i can get to the

value n and so what are these addresses

and in fact if Carter wouldn't mind

joining me up here for a moment what are

these addresses well just like in our

human world we have mailboxes even

though you might not check it very

frequently nowadays but to get physical

mail every uh home every business has a

unique address the uh science and

engineering complex is 150 Western

Avenue Austin Massachusetts 02134 USA

and theoretically that uniquely

identifies that building in the world

well here we we have two mailboxes um

over here we have a value n that happens

to live I'll claim it address ox123 and

then over here I claim there's another

address uh called by name p i don't

actually care where it is even though it

definitely exists somewhere in the

computer's memory but if this is p which

is a variable and that's n another

variable ideally this mailbox would be

twice as big because of the number of

btes using but Home Depot only had

identical size mailboxes but here is p

one variable there is in another

variable if I open up this mailbox what

should I find inside of it based on our

story thus far like what value will I

pull out dramatically in just a

moment yeah I think ox1 123 Now using

this you can kind of think of this as

like x marks the spot no pun intended

where I can now like walk around the

computer's memory and find my way to

that location by sort of following the

treasure map or if I want it more

dramatically uh thanks to our little

Yale foam finger here here you can think

of it more abstractly as p is just

pointing at n okay that's not going over

well so let's switch over to the Harvard

one so p is

pointing so p is pointing at n and so it

turns out we will be able to write code

now that will do the equivalent of me

walking over to n but for now Carter if

you want to reveal what's in the mailbox

we should see indeed the number 50 so

that's really all that's oh I feel

Carter's waiting for Applause so like

really well nicely done

thank you so that's just like a physical

metaphor of what's going on here in one

variable we have an address and that

variable by convention is called a

pointer in the other variable per week

one we just have a value like n and you

can yes follow the map and walk yourself

to that particular address and we'll see

how to do that in code but what's really

interesting is this abstraction that

pointers literally or really I guess

figuratively point at some other value

in memory all right questions then on

pointers in this form pointers point to

each other can pointers point to each

other so yes there's things called

double pointers we're not going to see

them anytime soon but using star star

you can express an address of an address

um but we won't see that just yet other

questions on

pointers yeah in

front our array so to summarize our

arrays then pointers so short answer

there's a relationship and we'll come

back to that in a little bit but arrays

are technically different from pointers

but we we're going to be able to blur

the lines a little bit by using one like

the other but let me come back to that

in just a bit of time all right so if we

have now this mental model if you will

of like what a pointer is in memory I

think we can start to peel back a layer

of uh simplification that we've been

assuming for the past few weeks since

week one so a string recall is a

sequence of characters and so if you

want to create a string that says Hi in

all caps and an next exclamation point

we do string s equals quote unquote high

and we can hard code it like this or we

could use get string but for now just

assume that I hardcoded it into my code

to always say hi in all caps with an

exclamation point well what does that

look like in the computer's memory well

let's stop looking at the entire memory

let's just focus on really what's going

on once you create a string called s and

store in it hi you know that a couple of

things are happening H and I and the

exclamation point are ending up in the

computer's memory we know from week two

that this thing the so-called null

character n AKA back slz is also being

added for you and it's somewhere in

memory at the moment I don't really care

where I drew it at the bottom right yes

it has an address but for now it just

ends up somewhere and in fact here's a

little visual cue as to how this happens

in C anytime you use double quotes to

give you a string you can imagine that

the double quotes are like a a clue to

not only store Hi exclamation point but

also put the null character there for

you and this is in contrast to what

chars if you want individual characters

what syntax did we use

instead so single quotes single quotes

do not add magically a back SL zero they

literally just store one character so

again strings have always been a little

special you get some extra an extra bite

for free so that you know where the

string ends and functions like stir

compare can then find their way there so

in memory it might indeed look a little

like this and if we assume that there's

going to be somewhere in memory these

things are going to be somewhere in

memory we can address them per week two

by way of the name of the variable so if

s is the name of the variable S braet 0

is how you would refer to the first

letter s bracket 1 s braet two and if

you really want s bracket 3 would get

you at the uh null character at the very

end but what is s so technically in this

line of code here not only is the

computer giving you memory for h i

exclamation point back sl0 we turns out

that s itself must take up some amount

of space right because s is the variable

and every time we' talked about

variables thus far I've given you a

rectangle on the screen in which to

store its value so let's assume for the

sake of discussion that the H is at

ox123 and I is at o x124 exclamation

points at o x125 and the null characters

at ox1 126 well what then is s well s is

just going to be some other variable and

I'll draw it somewhat abstractly without

all the other boxes up here and I'll

claim that the name of this variable is

s but it turns out what is s really how

do strings Really Work Well s is a

variable and has been since week one but

when you define it what the computer is

doing for you automatically is when it

knows you want to store Hi exclamation

point it puts that somewhere in memory

the computer then figures out for you

what's the address of the very first

character and it stores that address and

only that address in the variable you

created on the left hand side of the

equal sign and that's enough like to

represent a string with three letters of

the alph alphabet or punctuation you

don't need three variables you just need

one you just need to know the beginning

of the string why why is it sufficient

for a variable to Only Store the first

byes address and not all of the bytes

addresses exactly because of the design

of strings per week two we always null

terminate them so it suffices to only

remember the first bites address because

from there you can sort of follow the

breadcrumbs bite after bite after bite

and until you see the new line Sorry the

the null character you know that all of

those characters are apparently part of

the same string so this is what's been

going on in the me computer's memory all

since week one and in fact if we

abstract this away you can really think

of s as being just this really a pointer

to that chunk of memory so in fact what

do we have here well in the left to

recap on the code here on the left hand

side string that's what ensures that

we'll actually be able to store a string

in a variable called s we're going to

have on the uh right hand side though

the actual value so let me switch back

to VSS code here and let me change my

code to no longer involve integers alone

so I'm going to add the uh cs50 Library

just so that I can use some shortcuts in

there cs50.h and then in my main

function I'm going to go ahead and do

this string s equals quote unquote high

in all caps exclamation point and then

I'm going to go ahead and print out

using percent S as always back sln the

value of s so this program at the moment

not interesting at all it's just week

one stuff again/ addresses indeed prints

out high but it turns out that now that

I know this what's really been going on

underneath the hood all this time well

here's that same line of code that

defines the variable called s and it

turns out anyone want to guess what

string is actually a synonym

for string it turns out is kind of a

white lie we've been telling since week

one there is no such thing as string as

a keyword in in C it's technically a

cs50 thing

yeah it's a pointer to a character so

really all this time we've kind of been

lying to you there is no string quote

unquote it's actually Char star and if I

may it dramatically here go the training

wheels like okay that didn't land very

well so uh what have we been doing well

it turns out that string is a much

easier way conceptually to think about

what a string of characters is like my

God if we had to start in week one by

having you type char

like yeah you might get past it but like

this is just way too much ugly syntax

not intellectually interesting at all so

we abstract it away what a Char star was

in the first week of C by telling you

it's actually called string now string

is a term of art like C programmers

programmers are in any language we'll

use the word string to mean a sequence

of characters but in C it's not

technically a word unto itself it's

rather a synonym that we ourselves

created in some form so in fact how did

we do this well think back to just last

week last week I proposed that it'd be

really nice if we had a person data type

which the creators of C did not think of

decades ago but that's okay we can

Define it ourselves what did we do here

well using syntax like this recall that

we defined a person to be what to be

this structure this structure using the

new keyword last week struct means that

a person is just a name and a number and

it could have been other things we just

kept it simple but how did I associate

person with that structure well we

claimed that it was this value here type

def which as you might expect defines a

data type so what did we do as cs50 back

in week one without telling you well we

could have done something like this like

int itself is a little cryptic and maybe

we should have to keep things even

simpler said hey everyone turns out you

can Define integers in C and if you

wanted to do this well if you want to

create the keyword integer as a data

type you can just typ def it to int so

typed def creates the word on the far

right integer and U creates a synonym

for it in this case called int so what

did we do in week one without telling

you we have a line of code like this in

the cs-50 library that Associates quote

unquote string with more cryptically

charar and this is why in week one

onward anytime you use the cs50 library

you can write the word string as though

it's a real C data type and that's just

because we wanted to have this

abstraction these training wheels on for

the first weeks so we don't have to get

weeds of all this crazy memory stuff we

can sort of talk about strings at a

higher level but that's all they are

strings are the address of the first

character in that sequence of characters

questions now on any of these details

yeah strings

liar good question what about the

strings Library which we have used um

unrelated so it does not define the word

string everything in there actually

relates to char stars and so in fact if

you've used the CS5

uh manual uh which is just our

userfriendly version of the actual

manual pages for the official language C

you'll see throughout that now if you

start poking around or turning off less

comfortable mode you'll actually see

that we've changed any mentions of

charar in the official documentation for

these first weeks to just string to

simplify it but underneath the hood C

does not know the word string per se as

a keyword but it's absolutely a concept

that like every program in the world

knows about and in fact in other

languages in Python for instance there

will actually be a proper string

although it's not going to be called

string it's going to be called stir St

Str for short questions on these strings

here well let me propose there's one

other feature of this syntax that we can

now leverage as follows let me propose

that if we go back to the previous

version of my code here wherein let me

switch back to vs code in just a moment

I'm going to rewind in vs code to the

integer version of my code from before

and most recently it looked looked like

this before when we were using integers

only and not in fact strings at all let

me propose that there's this other

feature of C that we can use that

actually allows us to go to an address

so at the moment let me just rewind and

do make addresses to remind you what

this program do did when it was using

integers alone and there's that address

why because on line seven notice I'm

printing out the value of P which is a

pointer so of course it's going to look

like an address but let me zoom out now

and make one change and instead of

printing out P how can I use today's

second new operator not the Ampersand

but the star to actually go to that

address well what I can actually do on

this line of code is this if I want to

print out the actual integer 50 that's

in that variable or equivalently at that

address I can go to P here and not print

P literally because that's just an

address I can now say star p and star p

means go there more technically

dreference p that is follow the treasure

map to the actual address and do what

Carter did open the mailbox and print

whatever was in the mailbox which recall

was the actual number 50 so let me try

this let me recompile the code so make

addresses okay let me clear my terminal

window do/ addresses this time I

shouldn't see the ox anything I should

see just the number 50 in this case and

here to is kind of a unfortunate design

decision certainly pedagogically I would

say in C if I zoom in on this code star

is unfortunately being used in two

different ways in an Ideal World they

would have used three different symbols

to make this more semantically clear but

this is what we're stuck with so in line

six when you declare a pointer that is a

variable that stores an address you put

the type of variable that you want to

point at then a star just because and

then the name of the variable and on the

right hand side you actually get the

address of whatever using Ampersand but

when you want to go to an address you

want to dreference a pointer you don't

use int again and we've never done that

right once you declare a variable you

never again mention the data type but in

the world of pointers now if you want to

not print out P but go to whatever

address p is storing you use star P here

so a good visual indicator would be when

you declare a pointer that is make it

exist in your program you have to

declare the data type with the star but

when you use a pointer you just use the

star in an Ideal World this would be a

completely different symbol but again

this is what we have questions now on

that syntax

[Music]

yeah uh why can't we just do the Amper

sand here are you saying it was still a

little quiet so strictly speaking we do

not need line six so this is really for

pedagogical sake that I am um defining a

separate variable p and then printing it

out at this point though I'm just kind

of you know going in circles if you will

because more simple would have been what

I would have done in week one which

would be get rid of P alt together get

rid of P here and just print out n right

but today we're just giving you this new

building block these this new syntax via

which you can figure out the address of

something and then reverse the process

later and actually go to it as

well other questions on what we've done

here with these

pointers all right well let's context

switch back to the string now and see

what more we can do with this here in

the case of our strings here let me uh

refine this to zoom out let me delete

the integer related code here let me do

string s equals quote unquote high in

all caps let me go ahead and for the

moment include cs50.h at the top so that

indeed I can use the keyword s or string

rather and let me go ahead now and do

something more than I did last time last

time I did print F of percent s back sln

and then I printed out s and again I'll

recompile this just for clarity make

addresses addresses that just prints out

high so that's again week one stuff but

now that we have this other bit of

syntax we can do some interesting things

too so for instance suppose I want to

print out not s itself but what if I

want to print out the address of s like

at what memory location is s well I can

change my percent s to percent P which

now we know P is for pointer so percent

p means print out the value of a pointer

that is an address and here I can

actually print out s itself but why that

is we'll see in a moment let me do this

here go the training wheels string does

not technically EX exist but it does if

I'm using the cs-50 library but if I get

rid of the cs50 library as I'm

metaphorically doing by taking off the

training wheels I can't use the word

string anymore and in fact let me make

this mistake deliberately as you might

have accidentally in past weeks here is

the error message I get if I forget the

cs50 library use of Undeclared

identifier string did you mean standard

in it's trying to be helpful but it's

not because I didn't mean standard in so

indeed this is confirmation that c does

not know the word string exists at least

as a keyword exists as a concept but not

a keyword so I could fix this by adding

back the cs50 library but that's kind of

a step backwards educationally is

instead of a step forward what could I

do instead to fix this now if the

training wheels are now off

yeah exactly replace string quote

unquote with Char star instead so I'm

going to go ahead and change this to

char technically you can put the literal

star here the asterisk or you can put it

there or you can put it here by

convention is to do what I done from the

beginning put the star next to the name

of the variable as opposed to anywhere

else uh let me go ahead now and or sorry

I meant to add the spaces there you

could do this too but this would be the

most normal convention so now let's do

this make addresses compile is okay now

do slash addresses what should I see

high or something else feel free to just

call it

out so still high you say someone

else memory location a memory location

all right so could be one of the two

options right either I'm going to see

the string or I'm going to see a memory

address though I do in fact see a memory

address and this one's quite different

from the integer one but does anyone now

want to explain why you were correct why

am I seeing the address down here and

not hi it's subtle

yeah exactly because I left my percent P

there which means hey print F show me a

pointer but this is where printf is

smart and has been smart since week zero

humans who invented printf decades ago

uh wrote code that notices that okay

percent s means to treat the following

value not as just an address per se that

gets printed literally but printed as

with the mailbox demo is sort of a

treasure map that leads you to the

address of a character so simply by

changing one character percent P to

percent s and if I now do make addresses

again and slash addresses this now is

identical to week one but hopefully

makes sense because percent s is just a

clue to print f that means go to this

address in s print out every character

there and thereafter until you see what

the null character and then stop

printing anything more and this is why

hi has printed since week one today we

can see the address percent P but this

combination of having access to

addresses and the null Terminator is all

the information printf needs to actually

do something more useful by like

printing the actual

strings any questions now on this

approach to percent s yeah in

back oh so why is it traditionally being

used in this way honestly like the word

string has been around for decades it's

not a keyword you should be able to type

in C unless you're using a library like

cs50 um and so percent s just means

string so even though it doesn't exist

as a keyword percent s connotes string

and humans decades ago like today just

kind of know what that means so they

could have chosen any letter of the

alphabet but s sort of makes the most

sense all right well let's in back other

question good question before let me

zoom in I did not use a star before the

S why well it's subtle here but printf

is was invented years ago to know given

an address like in the variable s printf

knows to go there so if we looked at the

source code that some human wrote years

ago for C we would likely see the actual

uh asterisk that you're referring to

printf is taking on the responsibility

for going to S if you were to do uh star

s here instead an asterisk and then s

that would now be literally a character

because if I say p uh star s that means

go to the address in s and all you're

going to find there is a single

character what printf wants to know is

not what is the character there what is

the address of that character why

because printf needs to walk through the

rest of those characters looking for the

final null character and in fact let me

see with a bit more syntax if we can

highlight this a bit more let me do this

in addition to printing s let's try out

our syntax in another way let me print

out with percent s how about uh not s

here but let's print out some addresses

percent s back sln close quote and then

let's print out how about this the first

character in the string s would be

called s bracket Z but how do I get the

address of the first character in s well

I could technically just use today's new

primitive I can just add an ampon that

always gives me the address of some

value so when I end this thought and

clear my terminal window and run make

addresses still compiles when I run

addresses in just a moment any guesses

as to what I will see line by line this

will print out two

things and you don't have to remember

what the actual number was but at a high

level what will be printed now the same

thing twice why well when I run this

what I'm printing here and let me zoom

in at the bottom I ined see two really

long addresses but they're in fact the

same why well that's because again if s

is the address of a character as implied

Now by either the cs50 word string or

the actual phrase Char star well then s

is just an address by contrast per week

two s bracket 0 is a Char always has

been a Char a specific Char but if you

want the address of that Char you just

add the Ampersand well it turns out that

strings per the definition we keep

emphasizing is just the address of the

first character in a string so of course

if you do this you're going to see the

exact same thing and if I do this a bit

more generally you don't want to copy

paste but this is just for uh

visualization sake let me print out all

the characters so another another

another and let me change this to print

out the address of bracket 1 bracket two

and bracket three so all four characters

Hi exclamation point and the null

character notice I'm using percent P for

all of them so if I now do make

addresses and do/ addresses now notice

and this is kind of cool the first two

are indeed still the same but what's no

noteworthy about the other values on the

screen yeah they're consecutive each of

these is just one bite away even if

you're not good at hex yet and there's a

crazy number of digits here who cares

they're all the same except for the last

ones four four and then 5 six seven and

this confirms what I've been claiming

for weeks is that in an array all of the

characters are back to back to back

contiguous One Bites away so with just

this Ampersand with just this star like

it's actually a pretty cool tool in the

toolkit to have because you can start to

poke around what's actually going on

inside of the computer's memory and in

fact if we do this I can introduce one

other cool trick here if you will let me

propose that we can actually now do

arithmetic on pointers and you don't

have to you'll see a simpler way to do

this but now that you have perhaps this

underlying understanding of where things

are in memory and it's just addresses we

can actually do something kind of neat

we can do something like this uh let me

go back to how about uh the string

version of this with high and let me do

this instead let me um clean this up a

bit get rid of some of these lines of

code and let me do this let me print out

percent C percent C percent C let me get

rid of all these Amper Sands we're going

to roll back to like week two stuff just

to be clear when I compile and run this

version of the program and I'll zoom in

what should get printed on the screen

this is just week two stuff now no

pointers per se yeah

mhm just Hi exclamation point one per

line because I have all of these back

slash ends so let me do that let me go

down here make addresses enter okay

pretty good/ addresses and indeed Hi

exclamation point But now if you're

getting a little more comfortable and

it's fine if you're not yet today but

over the coming week or weeks as you get

a little more comfortable with the

equivalence of addresses with our

definition in the past of arrays and

strings and all of this you can start to

play around and I can do this instead if

I I want to print out the first

character in the string I could do like

week two s bracket zero like that will

always work and you can keep using that

that's not a cs50 thing it's just a

convenience in C but I could technically

print out not s because s is an address

but what would be the syntax I could use

to say print out the character at

s any

Instinct how can I say go to the address

in

s it's one of two possible answers

today so of our two new uh uh of our two

new operators today we have the

Ampersand and the star which one will

lead us to what is that an address so

the star so in fact if I want to print

out what is that address zero at the

address S I can just do star s and if

you really want to get fancy how do you

print out the second character that's

immediately to the right of it so to

speak well you can go to with the D

reference operator and do you want to

answer this

one s+ one Argo pointer arithmetic like

you can do math simple addition

subtraction whatever on pointers if you

want and you can do this here too so

star you want to pluck this one off too

how do I print out the last character

the

third s plus2 right because if you know

and understand that like a string is

just a sequence of characters every

character is just a bite and these bites

are back to back to back you can just go

wherever you want in the computer's

memory and here I can do make addresses

again/ addresses and voila we now have

high exclamation point so we haven't

printed out anything new but again just

by using these two new operators the

erson and the star you can figure out

the address of something and you can go

to the address of something okay

question in

back indeed it ends up being the exact

same and so I might have used this term

before the Amper sand technique where

sorry the square bracket technique where

you do s bracket 0o s bracket One S

bracket 2 that's actually what we would

really call syntactic Sugar like it

works and you can use it you should use

it it's nice and simple

but the square bracket notation

underneath the hood is essentially being

converted to this which this is not fun

right like this is when you want to show

off to your friends like you know how to

do cool stuff in code but this is not as

readable as just s bracket Z and one and

two but that's all that's happening

underneath the hood and so again this is

why in cs50 we spend time on some of

these lower level building blocks

because if you assume that indeed your

computer's memory is just this grid of

btes and you have now the code ability

in code to get an address and go to an

address you can start doing any anything

you want and you can poke around a

computer's memory at any location and

here in lies the danger like I'm kind of

on the honor System right now that if my

string is high exclamation point it's

kind of up to me to go to the first bite

the second and the third but I could get

kind of crazy now and if I want to see

what's going on in the computer's memory

I mean there's nothing stopping me from

doing like s+ 50 and let's see what's

there so make addresses do/ addresses Hi

and then okay nothing it seems well how

about 5,000 bytes away let's poke around

what's inside of the computer's memory

so make addresses again uh make

addresses addresses enter okay still

nothing there let's try

50,000 all right do make addresses do SL

addresses okay there we see it so you've

probably done this some of you by

accident because you probably went too

far to the left or to the right in an

array touching memory that you shouldn't

suffice it to say I should not go

blindly touching 50,000 bytes away cuz

who knows what's there and indeed in

your computer computer when a program is

running the computer uh segments it into

different segments of memory and if you

get a little too greedy and you touch

another segment of memory that

technically was not allocated to you by

Mac OS or Windows or Linux or the

operating system bad things happen and

you get a segmentation fault and that

means it's a bug in your code so you can

now do this and this means hackers too

can do things like this if they can

somehow inject code into your C program

maybe they can poke around the

computer's memory and indeed this is

kind of the technique whereby maybe a

really sophisticated hacker can jump to

this memory this memory this memory

looking for something like your password

or your financial information or

anything that's in the program but at

some other address there's nothing

stopping an adversary at least right now

from poking around if they can execute

code on your computer from doing this

kind of thing so there and again is the

power of C but also the danger and

you'll absolutely suffer more segals in

the coming days but ultimately the goal

is going to be to help you solve them

ultimately uh and fix things um but for

now I think that was that was quite a

bit so let me propose that we go ahead

and take our longer break here maybe 10

minutes and have ourselves some whoopy

pies in the transcept we'll be back in

10 all right so we're back and to recap

where we left off you now have this new

capability in code to do pointer

arithmetic like treat addresses as

numbers which they really are in heximal

or otherwise and like add them together

and kind of poke around a computer's

memory and it was asked during break

actually how we might further harness

this in the context of string so I

didn't change the code we wrote just

before break recall that we last broke

the program by checking out bytes 50,000

bytes away but let's not do that and

let's actually try printing out not

individual characters like I did per the

percent C but why don't we try printing

out strings and substrings if you will

so let me clear my terminal window let

me change all of these percent C's to

percent s percent s percent s and then

let me rewind to what we've been doing

since week one with strings which is

just print them out for instance with

that first line and the only difference

at the moment is that now I took off the

training wheels I got rid of cs50.h

wherein string is typ Def to char star

for you got rid of that so now on line

five I'm declaring S as being a Char

star which just means the address of a

character and print f is smart enough to

know that the end of a string is

wherever that null character is but now

that I can do pointer arithmetic notice

that I could do something like this if I

want to print out s i just print out s

suppose I do s+ one here and s+2 here

again after changing percent C to

percent s any intuition around what this

code will now print on the screen line

by

line yeah

thoughts okay reasonable conjecture

maybe the memory address of H that of I

that of exclamation point but other

[Music]

thoughts yeah I think it's going to do

the latter it's going to print high in

the usual way because honestly line five

is this rather line six is the same as

like week one stuff except we took off

the training wheel of string and we're

calling it Char star but I think line

seven is indeed going to print out I and

line eight is just going to print out

because it'll be just the exclamation

point prti will still be smart enough to

know where each of those substrings a

portions of the string End by the same

logic as always but let me go ahead and

zoom out run make addresses enter

compiles okay/ addresses and now indeed

this is all a string is it's a sequence

of characters identified by its first

bite if you then start poking around and

tell printf to print at what's at the

next bite or the next next bite it's

going to do its same thing printing out

that character and everything after it

up until that null character so again

even though there's like a lot going on

we've introduced these two new operators

like there's nothing that's happening

today that hasn't been happening for

weeks but hopefully through this week uh

this week's lecture this week's problem

set and Beyond you'll start to realize

that now you just have more tools via

which to harness those lower level

implementation details so last week too

recall one other implementation detail I

claimed that you could not compare two

strings quite as easily as you could

compare to uh integers for instance and

I told you to use a different function

instead that you probably used one or

more times with the past problem set how

are you supposed to compare strings

apparently yeah so string compare stir

comp that additional function that we

said H you just have to use it for now

but you might have a little intuition

already as to like why we have to use

Stir compare and we can't just use

equals equals to compare strings like

any intuition for this already why was

Stir compare necessary last week

equ perfect equals equals would compare

literally the two memory addresses

instead of the actual strings character

by character and unless the memory

addresses are literally the same so you

compare the that exact same memory

address two different strings probably

are not going to be considered equal

even if to us humans they indeed look

equal so let's see this let me go ahead

and close addresses. C and actually

before I do one last mention one of the

powerful things about Pointer arithmetic

as an aside is that c and really the

compiler is smart enough to know how

many bytes to keep adding and adding and

by that I mean this right now we got

lucky because a string is a sequence of

characters and by definition every

character is is a single bite you can

poke around and do s+ one to get the

next bite S Plus 2 to get the uh the

third bite however if we weren't dealing

with strings suppose we were dealing

with integers that were in an array back

to back to back if you wanted to get at

the next integer you could still do plus

one or plus two to get at the next or

the next next integer you would not

start to get into the weeds of doing

plus four and then plus eight you don't

have to know or care how big the data

types are in the computer C and the

compiler will figure that out for you

based on the data type in question so

keep that in mind if ever doing this on

a different data type uh than chars all

right so let me go ahead and open up a

file that I wrote in advance most of and

let me hide my terminal window and show

you this so here is a program called

compare. C whose purpose in life is to

compare two strings I'm back to using

the cs50 library because at least for

now in probably a couple more weeks it

is so much easier to get input from the

user using cs50's function get in but

we'll conclude today by taking off those

training wheels as well so you can see

how you can actually get user input with

nothing cs50 specific so line six and

seven pretty boring week one stuff get

an INT called I get an INT called J and

store them in two variables I and J

respectively if I equals equals J print

out the same else print out that they're

different let me just stipulate for time

sake I'm pretty sure this code is

correct this will get two integers from

the human it will compare them and tell

me correctly if they're the same or

different and I'll prove as much by

running make compare

slash compare and I'll type in 50 for I

50 for J and they're the same and now

I'll do how about 50 and say 13 and

those are different so let me just

stipulate this code is indeed correct

would have worked in week one also works

now in week four but let me now change

it to compare not two integers but as I

hinted maybe two strings instead so let

me go ahead and change this line of code

to maybe be string s equals get string

uh asking the user for s s uh then let's

change this second line here to be

string T just to keep the variable name

short for now and T is a good uh Choice

after s for something like this get

string prompt the human for T and then

let's change our I and J here to do the

wrong thing per the intuition earlier if

s equals equals T then print out the

same else print out that they're

different now if I want I could take off

at least some of the training wheels I

could change this to char star I could

change this to char star either is fine

I still need the Cs library though

because I'm using get string because

it's actually hard as we'll see today to

get strings manually without using a

library but I'll keep it using string

just for now with the library all right

make compare

again compare and now let me go ahead

and type in for instance Hi exclamation

point Enter and Hi exclamation point

Enter and they're different all they're

obviously not visually but they are

underneath the hood and you probably do

have the intuition for this already

whereby what's going on underneath the

hood is that we're comparing

accidentally the two memory addresses so

in fact let's go there let's consider

the memory and let me zoom out now so I

can just have more btes to play with so

the squares are a little smaller than

before just so we can fit more in them

and let me propose that when I declare s

on what was line six a moment ago it

ends up somewhere in memory like the top

leftand corner of my picture for

discussion sake and when I uh execute

that same line of code and get string is

called and I type in high exclamation

point we know from week one that get

string puts it somewhere in the

computer's memory and I'll propose that

it's in like the bottom left hand corner

of the screen here what happens after

that well I know even though I don't

generally care that Hi exclamation point

in the null character exist at some

address like ox123 12 4 125 126 for

discussion sake and what's in s same as

before break 0x1 23 so that's all that's

happening again on line six which is

pretty much the same as when we were

getting an S earlier but notice now with

line seven when I get a second variable

called T and I call get string again and

by coincidence as the human I type the

same thing well what happens here T gets

its own chunk of memory maybe at the top

right uh that second version of high

gets somewhere else in memory you know

the computer could be smart and notice

it's the same but C doesn't generally do

that for you it just plops it somewhere

else in memory and maybe it's at address

Ox 456 457 458 459 or wherever but you

can perhaps see where this is going

already T now of course contains the

address of that first bite and so in my

code on line nine when I compare s and t

for equality suffice it to say they are

not equal because of the way the uh

strings are laid out in the computer's

memory it's indeed looks the same the

same values are there but if we abstract

away further you can really see that s

and t are not the same themselves and so

how did we fix this or really how did we

avoid this last week without spilling

the beans and going down this Rabbit

Hole explaining like why you have to use

Stir compare well if I go back to my

code here let's do it now the right way

let me go ahead and include uh a line of

code that says string compare of s comma

T both as inputs and then if you recall

what does stir compare return when two

strings are equal there's three possible

return values so zero so one is for if

it comes alphabetically or atically

first or second but for now I just want

zero if I want to use S compare I do

need

string.h so string.h does exist that's

not a cs50 thing there's no keyword

string as a data type that's a cs50

thing but string.h does exist so I think

now with that change on line 10 if I do

make compare and slash compare and then

run again uh type again High exclamation

point High exclamation point I think now

they're the same and just as a uh second

check high in L caps maybe high in

lowercase those are in fact different

why well stir compare which was written

by some other human decades ago is just

smart enough to know that it should go

to S and go to T start comparing them

left to right stopping once it hits one

or both null characters and return zero

only if everything in s and and T are

exactly the same are any questions then

on this

here any questions on why we're using

stir compare all right if no yeah oh in

the

middle yes so so why does why is it not

the case with integers so it turns out

it's not the case with integers with

floats with bulls with uh doubles with

Longs like literally every other data

type works correctly strings though are

special they are useful enough in

programming and have been for decades

that the authors of printf and the

authors of stir compare and bunches of

other functions Sterling for that matter

just kind of treat strings special

because they're just useful right we

humans interact using language be it

English or anything else and so it's

just useful to have into the language C

just sort of uh first class support for

this notion of strings of human text so

the short answer is just because like it

just uh is necessar strings are

different they're implemented with this

address and the null character

everything else though is just a value

but a string again is a white lie it's

an address it's not a thing unto itself

good question yeah in front

oh really good question so in my code

here in vs code what if I do this

instead of stir compare and instead of

if s equals equals T what if I start

playing around using star s and star T

really interesting case to consider

let's go back to our sort of deductive

logic here so star the asteris operator

today means go there so when I've typed

in high once and then high again both

uppercase for instance what is at the

address s literally someone

else what is at the address s yeah uh so

not quite at the address not so not what

is the address what is at the address

ox123 H and what is at the address Ox

456 H also and so here you're kind of

cheating like you're comparing the first

character of both strings but not every

other one now you could be really

pedantic and here again this is like a

good use of uh codee but you could do

this if that and how about this

craziness so star

s+1 equals equals star t + one and and

you could do this for every character

manually but that's why stir compare

exists it does all of this for you but

that's why and that's the intuition so I

would encourage you too anytime there's

something kind of weird going on there's

I realize we might be um straining

credibility now we haven't told you that

many white lies and so most everything

that we've seen thus far can explain

pretty much all of the behavior up until

now from week one onward in C so let me

revert this back to the right way if s

compare of S&T equals equals z this now

is the right version of the code and now

here is again where you can play so let

me do this let me um clear my terminal

window just to tidy things up let me get

rid of all of this comparison stuff and

let's just see what's going on as you

are welcome to in your own code let's

print out for instance as we might have

in week one the value of s itself in a

new line comma s and then let's just

print out T just to make sure it

compiles and I'm not doing anything

wrong but this is not going to be that

interesting and frankly I don't need

string.h anymore because I'm not using

stir compare so make addresses addresses

there's my um oh sorry that's fun okay

not percent T percent s here too ignore

that let's do this again make a oh and

that's the wrong program okay dot SL

let's do make

compare compare and let's type in high

again and high again and and now we just

see the two strings I'm not comparing

but now we can kind of play around right

instead of printing out percent s which

prints the string how do I print the

address in S I just need to make a

slight change if I want to see not

what's at s but I want to see s the

address

yeah perfect so change percent s in both

places here to percent P so now printf

will treat it literally as an dress it's

not going to do any fanciness this with

a loop from left to right looking for

the null character it's just going to

print out S&T so let me clear my

terminal run make compare whoops let's

do make comparecompare enter type in

high type in high again and now you see

oh so this is interesting it's not quite

as straightforward as the other values

which were slight one bite away they're

almost the same but this one ends in b0

this one ends in F0 so they're indeed

separated by some number of bytes not

just one but a few because these strings

are indeed longer all right so once

you've seen this here how can we now

maybe leverage this to solve other

problems well let me propose that we do

this let me uh zoom out here let me

close compare and let me open up another

program I wrote part of in advance

called

copy.c so copy. C in theory makes a copy

of a string how on line eight I'm using

the same thing as before get string

storing in a string or char star and

asking the user for it then I'm not

asking get string again I'm just making

a copy super simply with line 10 here

string T equals s now intuitively I

think that's how I would copy a variable

right that's how we've copied variables

every week thus far and see but

something's going to go wrong in line 12

in English does someone want to explain

what you think line 12 does don't worry

about finding any bugs or mistakes but

what does line 12 seem to be doing using

two upper which which is thanks to the

ctype library which I've included the

header file for

yeah yeah right it's kind of like ugly

syntax but this would seem to be

capitalizing the first letter of T

specifically and just changing it so we

have t bracket Zer here because we want

to save the change and we're passing to

two upper the first character here so

this is how we did uppercase in the past

and now I print out s and t respectively

using percent s so this feels like it

should work I copied s and stored it in

t on line 10 and then I change T and

only t on line 12 but you can perhaps if

you're comfy thus far see where this is

going if I do make

copy copy and let me type in lowercase

Hi exclamation point this time just once

so I'm going to hit enter and watch what

we see for the value of s and

t huh the new value of SN T at the end

of my program seems to be what it seems

to be

the same high is capitalized both times

so what's the intuition then for this

why did this just

happen yeah and

back yeah I assigned S&T the same memory

address so it did copy s into T but C

takes this very literally what is s it's

an address what is T it's a copy of that

address if you want to copy the whole

string like a normal human would expect

hey you or someone has to do a lot more

work you have to go to that address copy

this character this one this one this

one and copy it to a new location in

memory that does not happen

automatically here for you in C it does

in some other languages those of you

who've programmed in certain higher

level languages this just works as you

would hope and that's one of the

benefits of python and other languages

that we'll soon see but for now it

literally takes at face value what this

is copy the address into this address

and I'll make that more clear by getting

rid of the string keyword which again is

just a type def this is technically an

address here this is technically an

address here so what's being being

copied is the value of that address not

all of the characters that might very

well follow it so I should make one note

to here I'm going to start getting more

in the habit of trying to avoid

segmentation faults because things could

go wrong here for instance online 12

previously I was kind of blindly naively

dangerously assuming that there will be

at least one character in s or t that

might not be the case if the user Just

Hits enter there's no characters to

uppercase and so this is reckless of me

and could theoretically create a sigall

so I should probably start to be smarter

and say something like this if the

length of T is greater than zero okay

now it's safe to actually capitalize the

first letter and that will decrease the

probability now of those segmentation

faults by just not making any

assumptions about what the human does

almost always the your programs will

crash when you've made a mistake yes but

the user gives you an input that you

yourself did not

expect so what does this all look like

in memory well let's go back to the Big

Grid this time focusing on the copying

of values and let's do this here's S as

in this new program just declared to be

a Char star uh here is where my

lowercase High maybe ended up in the

computer's memory that's probably at

ox123 12 4 125 whatever something like

that and that's of course what ends up

in s as a value when I declare T I do

get a second variable called T just like

before but when I copy s into T what

happens it's really just literally ox1 2

3 whatever the value of s is is now also

the value of T and so if we abstract

this away at a high level get rid of all

of those extra squares this is what s

and t now are they're indeed copies but

copies of each other not copies of the

underlying characters and so if you

follow those arrows um and try to print

them both out after capitalizing one or

the other you're going to unfortunately

end up capitalizing not just one of them

s but both of them s and T because

literally it's the same address any

questions then on this

visualization

yeah good question is this pass by

reference we haven't um we have not seen

in detail an example like that um right

now you're copying by value but

references will come into play and

remind me in a bit if I haven't used

that term yet but this is just copying

things by could have ended poorly value

other

questions no all right so with this in

mind like how do we actually copy things

properly for this we actually need

another building block so today we give

you two functions one of which is called

Malo one of which is called free and

these are used all of the time by like

every piece of software you and I use on

our Macs PCS and phones whether it's

written in C or some equivalent other

language Malo is for memory allocation

it's a function that you can use to ask

the operating system Mac OS Linux

Windows anything for some number of of

bytes one bite 100 bytes a gigabyte of

memory you can ask malog for however

much memory you want in advance it will

return to you the address of the first

bite of memory that it found free for

you unlike a string it is not null

terminated and so the danger with malok

is that it's on the honor System if you

ask it for one bite or 10 bytes you the

programmer in like a variable have to

remember how many byes you requested one

or 10 or the like strings do that for

you not when we're getting now to this

low level Mal just going to give you

some memory and it's up to you to manage

it free does the opposite when you're

done with some chunk of memory you can

free it by passing in that same address

and just hand it back to Mac OS windows

or Linux and say I'm done with this you

can let me use this for something else

later um as an aside if your computer

has ever like frozen or hung like the

whole thing maybe just spontaneously

reboots yet another reason for a bug

like that might be if you write a

program with a bug that keeps Mal

loocking Mal loocking Mal loocking that

is asking for more and more and more

memory but you make a mistake and you

never free it well eventually the

computer is going to literally run out

of memory and something's going to go

wrong and that's often when computers

freeze like they're just out of memory

it has the memory there but the program

was trying to use too much of it

endlessly so this too will be a mistake

that some of us will surely make in the

coming weeks but hopefully you'll now

see the solution so let me go back to uh

vs code here and let me propose that we

do the following I'll hide my terminal

window for a moment and I'm going to

introduce another header file up here

and I promise there's not going to be

too many more of these but this one is

called standard li. for standard library

and in this file are the Declarations

the prototypes for malok and free and a

bunch of other stuff as well it lets me

now manage my own memory so let's focus

now on line 11 line 11 is where I went

wrong before because conceptually I want

to copy the whole string but of course

I'm only copying modestly the individual

address so how do I copy the whole darn

thing well what I need to do is this

when when I declare T to be the address

of something in memory why don't I set T

to be the address of a free chunk of

memory so let me ask the operating

system give me this many bytes tell me

what the address is and I'm going to

store that in t initially just so I know

where there's free space for me so how

do I do that well quite simply I call

malok and then I pass in the number of

bytes that I need now for Hi exclamation

point I think I need three although wait

no I really need four because of the

null character but I don't think I

should be hardcoding numbers like this

cuz who knows what the human's going to

type in so I can actually use Sterling

of s and then plus one this will ask

malok then for however many btes

corresponds to the number of characters

the human typed in plus one for again

the null character so it's just being

smart and defensive rather than choosing

a number myself but now all T is is a

pointer if you will to some random chunk

of free space so there's nothing there

yet or there's you know bits there but

who knows what value they are they're

certainly not identical to what the

human type did in I now have to do this

so how can I copy one string into the

other well let me do this instead of uh

capitalizing something just yet let me

do this how about four in I gets zero I

is less than the length of

s uh and then i++ so I'm going to

iterate for the whole length of the

string and in here I'm just going to do

this the E character in t should be

identical to the E character in s so I'm

just literally copying from right to

left each and every character in s and I

can trust that there's enough memory in

t why cuz I asked for that many bytes

plus one now there's technically a bug

here I actually should probably do this

I should do plus one here or if you

prefer I should do less than or equal to

the Sterling but I think it's a little

clear to do the plus one why do I for

the first time want to go just beyond

the boundary of s and copy one more

bite yeah yeah I need the null character

like I could technically manually add it

with some additional line of code but I

might as well just copy it because back

sl0 is back sl0 so this time and

probably only this time it's reasonable

and correct to go just beyond the

boundary of your string so you copy the

null terminating character so that the

computer also knows where T ends and now

I think what I can do a little more

safely is this let me go down here and

say t braet 0 equals 2 upper of t uh two

upper of T bracket Z so same line of

code as before if I actually want to be

really safe I should probably do this so

if the stir L of T is greater than zero

so there's at least one bite there okay

now it's safe to blindly capitalize the

first character and I think that now

puts me in better shape so let me try

this now let me uh open up my terminal

make uh copy do slash copy I'm going to

type in Hi exclamation point in all

lowercase crossing my fingers this time

and now if I zoom in it indeed

capitalized only T and not s in this

case so pictorially let me switch over

here here is as before the variable s

pointing at high in all lowercase when I

call malok though that gives me a chunk

of memory that I'm going to store the

address in t of so if T is some other

variable as it is in my code and there's

some other available chunk of memory I

don't know where it is but let's assume

as always it's at Ox 456 4557 4558 N9 so

four bytes total what is now happening

well T is defined as pointing to that

because that's what Malo gives us the

address of the first bite of the free

memory and now with my for Loop I'm just

iterating over it copying the H then the

I then the exclamation point and then

for good measure the back slash Zer

instead questions then on this process

here a really good question um if I

omitted in my code the

uh plus one and I didn't do less than or

equal to so that I'm copying the fourth

bite odds are in this program because

it's so short you wouldn't notice that

there's an actual error but what could

happen is When I Call printf On T if

there's no null bite there it might

print h i exclamation point some random

value some random value some random

value some random value until it gets

lucky and there happens to be a zero

bite a null bite by chance for instance

so if you don't include the back sl0

some way that's going to happen and I

say some way I could even do this I

could technically just copy the length

of the string s and at the very bottom

here I could do something like T bracket

I uh sorry T bracket um

Sterling of T I could do this but this

is just not necessary like I could

manually add it at the end of the string

but again i' claim that it's just

simpler to borrow that is copy the one

that's already in s because it's the

same thing at the end of the day good

question other questions on this copying

correctly

[Music]

now all right is there any room for

improvement here well let me propose a

slight optimization this is kind of a

throwback now to week one turns out that

arguably my line 13 here wherein I have

this for Loop now that I'm doing things

in Loops again and again and using a

function like Sterling this is correct

it will iterate from zero on up to the

length of I a length of uh s plus one

but it's kind of stupid of me to write

this for Loop in this way why well

here's my initialization on the left

here's my condition in the middle and in

general calling a function inside of

your condition is probably not very good

design like why why is it bad for me to

be calling a function like Sterling in

this condition in the middle of my for

Loop

yeah yeah you're just calling it again

and again for no reason like the length

of s never changes so like why are you

wasting everyone's Time by calling

Sterling of s again again again again

just to check this inequality whether I

is less than that value so it turns out

if you haven't discovered this already

there's a slight optimization we can do

here that has nothing to do

fundamentally with strings or pointers

just with better design I can actually

Define two variables at once I could do

this let me remove this whole condition

and let me add a comma after I equals 0

set n or any variable equal to the stirl

of s + 1 and then after the semicolon

just ask the question while I is less

than n so it's almost the same but

notice now my condition in the very

middle of this Loop is at least

comparing two static values n never

change sorry one static value n never

changes all that changes is I but I'm

not foolishly calling sterling sterling

sterling again and again why well how

does Sterling work similar in spirit to

PR print F Sterling given the name of a

string looks at the first character and

then starts looking through the entire

string looking for the null character

and we saw this in week two counting up

how many characters are there so it's

just a waste of time again and

[Music]

again totally if you wanted to use n

multiple times you could absolutely take

it out of the for Loop put it right

after s is defined and reuse n and again

and again absolutely but in General

consider this when designing your for

Loops even though modern compilers like

cang can actually fix this problem this

inefficiency for you good practice would

be don't call functions unnecessarily

especially if the answer is always going

to be the same all right so what else

should I perhaps refine here well how

about I do one last thing and just

comment on what exactly could go wrong

here well a couple of things well

actually this is just silly too like

surely someone before me in the world

has had to copy a string before surely

there's a function like called stir copy

maybe like stir compare like stir Ling

and indeed there is so let me propose

that we actually get rid of this whole

for Loop and we actually just call a

function called stir copy no o just St

cpy and pass in the destination which is

T first and then the source that you

want to copy into the destination and

that takes the place entirely of that

whole Loop so again I demonstrated the

loop first just to be very pedantic

about it but that's wasting time you're

wasting time writing lines of code you

don't need to stir copy is what you can

use here instead and so this has now

always existed and what more can I do

well as one final point it turns out

that there's actually things that can go

wrong in this code even besides the

string being too short like if the human

Just Hits enter and there are no

characters I don't want to blindly

capitalize the first character that

doesn't exist that's why I added that if

condition but there's other things that

can go wrong and we introduced those to

you today it turns out that functions

like get string and functions like Malo

return potentially a special value and

wonderfully confusingly it's also called

null but with two L's all right so left

hand and right hand weren't talking so

well like decades ago NL is a back SL

zero it's a single character as it

always has been for a couple of weeks

now

n is technically a pointer it's an

address but it's address zero it's like

the top left hand corner if you will of

your computer's memory that just nothing

is ever supposed to go in by convention

so null is a synonym for zero but it's

specifically an address now why is this

useful well suppose that in my code here

something goes wrong with get string

suppose you're being a little crazy and

you type in way too long of a string

it's not just high but it's like an

entire essay of text and there's not

enough memory in the computer how does

get string signal to the programmer whoa

like that's way too big of a string I

can't fit it in memory well we never

told you this but all of this time it

turns out that that get string will

return this special value called null if

something goes wrong so to be really

careful now you should do something like

this if s equals equals literally null

then you better um exit the program

entirely and return like one or two or

three to signify that something went

wrong don't uh go any further similarly

with malok it's possible if you ask for

way too much memory that could fail

especially if you're asking now for

double the memory after the human typed

something in so if T equals equals null

then you know what but let's also return

one or some other value to just get out

before something crashes or freezes on

the human as well so honestly I tend not

to do this always in class because the

code just gets so bloated and

complicated but you absolutely in

practice need to start doing this

otherwise you will be responsible for

the freezes and the crashes and the

reboots that users in the real world

might actually encounter otherwise of

course if we get to the bottom of this

program now I should probably return

zero explicitly or implicitly to just

signify that everything is is

successful but there's one other thing I

haven't done we introduced malok but

what did I claim also

existed so free I'm also being a little

reckless now here I am not practicing

what I'm preaching I'm asking the

computer for memory via get string I'm

asking the computer for more memory my

via malok and I'm never technically

handing it back so really what I should

be doing at the very bottom of my

program to is freeing the memory I've

asked for so henceforth it is a rule a

law if you will in see whenever you

allocate memory with malok or certain

other functions as well you the

programmer must free it when you're all

done with it now this is a bit of an

overstatement because technically when

programs quit they'll free the memory

automatically so you're not going to

break someone's Mac or PC because you

necessarily have this bug but for

programs that are running all the time

like someone keeps a chrome their

browser open Microsoft Word or the like

bad things will happen if over time you

never never never call free and the

program keeps running so always get into

this habit here you you do not need to

free memory that comes from get string

because the cs50 library automatically

frees it for you but you anytime you use

malok henceforth as you did or I did

here you must free that by just passing

in the same address you got back

questions now on malok and

free questions

yeah oh really good question so free

just so what does free do so free um

just uh lets the computer know that you

are done with that chunk of memory which

means that if you have a another line of

code elsewhere that same memory might be

reused and can be used again and again

and that's going to be necessary

certainly for any long running program

you can't ask for memory constantly

you'll eventually run out so you need to

free it in this way other languages as

an aside python you get another

motivation in a couple of weeks for it

is going to be Python and certain other

languages manage all this headache for

you but in C the goal here is to really

harness these capabilities ourselves all

right so it turns out like almost

everyone in the room everyone in the

room myself included you're going to

screw up when it comes to anything

memory related if you haven't already

segals are in your future but hopefully

there's tools via which you can detect

these things and fix them proactively

and not just use print def uh or debug

50 or rubber duck we actually have

another tool we can equip you with now

that help you find some mistakes so let

me do this let me close copy.c let me

open a program I wrote In Advance called

memory. C that doesn't do anything

really interesting but it's going to

have two bugs in it notice that I've

included standard i.h as always I've

also included standard li. which is

necessary now for anything related to

malok Andor free and the like line six

it's a little weird what I've done here

but this is like the manual way of

asking for enough memory for an array in

week two how do we ask for memory for an

array you very simply say int X3 and

that gives you an array called X of size

three but if you do it manually Now

using Malo what you have to do is use

syntax like this you call malok you ask

for three things times however big an

INT is now we know it's four so you

could literally write 12 here but this

is more generic so three times the size

of an integer will give you 12

dynamically and what do Malo return the

address of the first bite you get back

what do I want to put that well I want

to put it in a variable now the variable

can't just be int X because that's a

number it's not an address per se if I

want to store this address in a variable

I could call it X I could call it P but

in Star X just means that X is now the

address of a chunk of memory

specifically a chunk of memory that's

big enough not for one but for three ins

in total all right now I'm just sort of

naively putting our old friends 72 73

and 33 at the first second and third

locations in memory but perhaps based on

week two or week four I'm clearly

screwing up up here in a couple of ways

someone want to identify at least one

bug what did I do

wrong yeah like this is now you know

amateur uh stuff like I should be zero

indexing not one indexing so this has

got to be 0 one2 ultimately and other

bugs that are maybe more week four

specific other bugs it's more subtle

yeah I'm not freeing the memory right so

I'm not practicing what I'm preaching by

freeing this memory now suppose these

are non obvious and honestly after like

an hour or two of this like this

shouldn't be obvious yet it will be over

time how could I find these bugs with uh

software as opposed to just staring at

the thing or asking someone for help

well let me propose this let me first go

ahead and run make memory to compile the

program and it seems to work look fine

there's no syntax errors at least

dotmemory notice seems to work fine too

now this program doesn't do anything

interesting there's no printf or

anything like that but it didn't crash

there's no segmentation fault but that

doesn't mean there aren't bugs latent in

the software and this is true sadly of

all of today's software like Chrome and

Microsoft Word and other programs surely

have memory related bugs that people at

Google and Microsoft haven't yet found

but there are tools at least to find the

most obvious of those bugs and we're

going to introduce you now to a program

called valren so valren it's a fairly

fancy program but we'll use it for very

simple ways we'll look at your code and

find memory errors as it's executing and

try to help you understand where they

are so let me go back to vs code here

memory seems to be fine you know I feel

like okay I'm going to submit this

homework all is good no error messages

that's no longer the case now you need

to poke a little more at your code to

see if maybe there's still some bug

there so let me do this

valren and then space dotmemory so just

like debug 50 you run it on a program

you already compiled valren I'm going to

run it on a program I already compiled

let me uh zoom in on my terminal window

so we can see more at once and enter all

right the output is crazy cryp IC for no

good reason there's lots of numbers and

equal signs it's a lot of clutter but

there is some juicy information here and

let me start from the top down invalid

write of size four so write means to

change a value read means to like access

a value and this is again esoteric like

a lot of our error messages are but it

looks like uh after a block of size 12

allock and then there's these weird hex

notation there's some mention of malok

but honestly the juicy part here is

memory. C line six that's probably my

fault so let's look at line six per that

output let me shrink the terminal window

look at line six okay 12 is now Germain

right if you did the mental math of the

size of an INT times three 12 is somehow

involved here but line six is now uh

happening next year that's where the

memory came from what is this let me

Zoom back in where is there invalid

right of size four like what's perhaps

going wrong here invalid right of size

four what does that mean it's like a

very technical way of explaining the bug

is actually one line later on line seven

as we already identified

yeah indeed and I I misspoke a moment

ago the bug actually arises here with

line nine so after the allocation of

memory I'm somehow writing four bytes

incorrectly and unfortunately the onus

is kind of on you to sort of think

through deductively like what could that

mean but I'm clearly touching four bytes

of memory in these few lines of code

that I shouldn't be and hopefully here

is the light bulb already went off ear

oh I'm not zero indexing okay that must

mean that X bracket 3 as you note is

just too far past the chunk of memory so

I'm invalidly writing to four bytes that

I shouldn't be so again it's not super

obvious this is not super userfriendly

but at least it does give you a clue as

to where that bug is so the fix there is

going to be quite simply to change the

one to a zero the two to a one and the

three to a two that'll fix that but

there's still a second error and let me

look at the cryptic output again Heap

summary some stuff there okay this does

not sound good down here 12 bytes in one

blocks are definitely lost in Lost

record one of one very Arcane output two

but clearly related to line six again

our allocation of memory now here too

it's not obvious what the solution is

but memory is lost AKA this is a memory

leak and now the deductions kind of up

to you why what is leak oh wait I didn't

call free and so the second solution

here is probably to free x at the very

end of the program and if you really

want to be you should probably check

like I proposed earlier if x is null

just get out now while you still can and

don't even touch those other lines of

code but if you get to the bottom return

zero but really the takeaways are I

fixed my zero indexing of the array to

avoid the invalid right of size four and

now I'm freeing the memory that I asked

for so there should be no leak lost all

right let's try this again make

memorymemory no visible errors yet but

let me now increase my terminal window

again do valren of dotmemory crossing my

fingers and now all Heap blocks were

freed no leaks are possible I don't see

any invalid rights there's still a crazy

amount of output but none of it is

erroneous it's not bad now I fixed my

memory bugs and so now my my ta my TF

they're not going to find them either

because at least valr has proactively

done that for me questions then on

valgren generally it's those two types

of Errors you might trip over there's

not twoo much else in the way of Arcane

output questions then on

this no all right well what else might

be going on so someone alluded to this

earlier what happens when you for

instance forget the null Terminator or

you generally start poking around memory

that you yourself didn't ask for or uh

looking at values you didn't put there

well let me go ahead and open this code

of garbage. C in honor of Oscar the

Grouch here of sorts and here is a

simple program if I hide my terminal

window that just does something kind of

arbitrary I first declare an array uh

called scores uh but I made it crazy big

like 1024 like that's a lot of uh

integers but so be it and then I iterate

over those integers and I print each of

those scores out so I'm using week two

syntax here but based on this program

what have I clearly not done that I did

do back in week two I've allocated the

array I'm printing the array but but

Bute yeah I didn't initialize any values

for that array back in week two we

didn't do 1024 we did like three and I

typed in like three test scores or

something like that he here I'm

allocating memory even more than that

just because I really want to be

dramatic with what I'm demonstrating but

I'm not initializing those values to

anything and so here it turns out in C

generally if you do not initialize a

variable or you do not initialize an

array with explicit values there are

going to be garbage values there so to

speak remnants of that memory having

been used before probably by some other

function of yours some Library function

or something else while your program is

running not a huge deal with a super

small program like this but for anything

sizable memory is going to be used and

unused and used and unused that is

macked and freed again and again there's

going to be lots of garbage values in

the computer's memory by default so if I

open my terminal window here uh let me

do make garbage uh let me zoom in on my

terminal so we can see the output when I

runt garbage theoretically I should see

1,24 integers but none of which have

been initialized now I'm going to get

lucky with some of them and it looks

like wow okay a lot of them are

initialized to zero and C does in some

context initialize memory for you to

zero at least at the beginning but not

again and again typically but if I start

scrolling backwards in time at this

array of size

1024 where did these values come from so

just random positive and negative

numbers interspersed among the zeros

well that's because I'm literally poking

around a random 1,24 bytes of the

computer's memory who knows what's there

so the lesson here is that garbage

values are indeed this like term of art

it means that a variable that you might

have uh defined that you might have

declared if you don't give it an

explicit value who knows what's going to

be there and the lesson here is just

don't do that always initialize

variables to something either yourself

or prompting the human for

it questions about garbage

values you'll see them sometimes if you

print things you shouldn't or touch

arrays beyond their

boundaries all right so maybe to make

this a little visual too it turns out

that a lot of things can go wrong

unfortunately with poers and we've seen

some of them and here's another program

that's a little contrived it's very

simple and it just is about manipulating

values it doesn't do anything useful per

se except demonstrate some of today's

Concepts so in main here let me propose

that we declare a pointer called X

that's going to store eventually the

address of an integer apparently here's

another one called y that's going to

store the address of an integer as well

here now I'm allocating enough memory to

fit one integer now technically it's

four we know that but size of in just

gives me that answer dynamically so it

will work on all systems

and I'm going to store the address that

Malo finds for me in X then I go to x

and put the number 42 there all right

why it's the sort of meaning of life the

universe and everything here but star X

again just means go to that address and

put a value there so why I don't know

but it's just uh correct at this point

but what about this line here star y

equals 13 unlucky in this case what's

bad about this line here star y It's a

combination now of today's Primitives

and that point here

yeah yeah we didn't ask the computer to

allocate any space so why was not

initialized with an equal sign at any

point to anything and so what is inside

y so to speak like a garbage value maybe

it's zero which isn't bad because at

least it's nice and simple but maybe

it's some crazy large uh positive number

some crazy large negative number either

way odds are if I go to this address or

that address randomly with star y bad

things are going to happen and so let me

go ahead and propose well let's not do

that let's let's actually do this

instead assign y equal to X and we've

done that before and then I can go to Y

now and change what was a 42 to a 13

again why this is just for educational

sake but for now this does not crash

because I only dreference y with star y

after actually giving it a value albe it

a duplicate value similar to our copy

example earlier so our friends at

Stanford have put together a wonderful

visual it's about 2 minutes long allow

me to dramatically dim the lights if we

could and play with what happens with

memory when you do bad things like

[Music]

this hey Binky wake up it's time for

pointer fun what's that learn about

pointers Oh goodie well to get started I

guess we're going to need a couple

pointers okay this code allocates two

pointers which can point to integers

okay well I see the two pointers but

they don't seem to be pointing to

anything that's right initially pointers

don't point to anything the things they

point to are called Pointes and setting

them up is a separate step oh right

right I knew that the Pointes are

separate or so how do you allocate a

point e okay well this code allocates a

new integer point and this part sets X

to point to it hey that looks better so

make it do something okay I'll

dereference the pointer X to store the

number 42 into its Point e for this

trick I'll need my magic wand of

dereferencing your magic wand of D

referencing uh that that's great this is

what the code looks like I'll just set

up the number

and hey look there it goes so doing a d

reference on X follows the arrow to

access its Point T in this case to store

42 in there hey try using it to store

the number 13 through the other pointer

why okay I'll just go over here to Y and

get the number 13 set up and then take

the wand of d referencing and

just oh hey that didn't work say uh

Binky I don't think de referencing Y is

a good idea cuz uh you know setting up

the point T is a separate step and uh I

don't think we ever did it good point

yeah we we allocated the pointer y but

we never set it to point to a point

T very observant hey you're looking good

there Binky can you fix it so that y

points to the same point as X sure I'll

use my magic wand of pointer assignment

is that going to be a problem like

before no this doesn't touch the Pointes

it just changes one pointer to point to

the same thing is another oh I see now y

points to the same place as X so so wait

now Y is fixed it has a point e so you

can try the wand of D referencing again

to send the 13 over uh okay here it

goes hey look at that now D referencing

works on why and because the pointers

are sharing that one point e they both

see the 13 yeah sharing uh whatever so

are we going to switch places now oh

look we're out of time but L thanks to

Professor Nick parlante of Stanford for

spending a huge amount of time doing

stop motion animation for that but

hopefully now you have a sense of what

two can go wrong when you misuse memory

in this way but at the end of the day we

really only have these four new building

blocks today like the star operator the

Ampersand operator malok and free and

really with that and the underlying

understanding of what your computer is

doing underneath the hood we have this

way now to really manipulate things in

memory For Better or For Worse and

eventually we'll see how we can build

things but we can also now use today's

Primitives to better explain some things

that we've been asking you to take for

granted over the past several weeks so

for instance let me propose that we uh

do one volunteer up here if we could

could we get one volunteer who's you

want to come straight up yep right in

the middle come on you'll have to take a

left right

there

all right so we have two empty glasses

here and two colors of liquid and we

have let me give you the mic if you'd

like to say hello to the

group hello um I'm Moen uh I'm in the

and first year all right welcome well

well welcome here I'm going to go ahead

and fill these two glasses with this

colored liquid um purple here on my

right let's fill up a glass

here yeah don't drink uh and now we'll

put some orange in here

and what we'd like you to do for the

audience if you don't mind is swap the

two values we've got a purple value and

orange value and I'd like the purple

liquid in this glass and the orange

liquid in that glass

please can I have another glass oh okay

good intuition but for the microphone uh

can I have another glass so you can and

just in fact I brought one here for you

why are you asking for this though cuz

if I just pour this into this and it'll

get mixed up right so obviously we need

like a temporary variable if you will so

here is your temporary

variable and you want yeah there's

yeah all right so pouring the value of

the orange glass into this temporary

variable if you

will all right and now pouring the value

of the purple glass into the former

orange

glass and

now the temporary value goes back into

the original purple glass and now I

think we give you a round of applause

for having done that very well

okay thank you all right so so it should

go without saying that in the real world

like that's how you do this and in fact

in code that's pretty much how you have

to do this although ask us some time for

a super fancy way of doing it without a

temporary variable turns out that is

possible using bits but for now let's

suppose that indeed this demonstrates

what is the reality in code if you want

to swap two values you need to have have

something like a temporary variable so

for instance on the screen here is a uh

the beginning of a function called swap

whose purpose in life is to as you just

did swap two values call it a and b so

orange and purple respectively are now

just a and b and integers to keep things

simple well here is the corresponding

code if I may to what you just enacted

as a human you declared a temporary

variable a called temp in this case

which was like me handing you The Empty

Glass and you stored the orange liquid

in it akaa you then changed the value of

the formerly orange glass to be equal to

the Purple by pouring one into the other

and then you did the opposite there now

at the end of this you still have a

temporary variable that's now empty so

it's temporary in literally that sense

like you just don't need it anymore but

it was necessary along the way so I dare

say this code is correct logically like

this will swap two values A and B thanks

to the use of that temporary variable

unfortunately though if I actually do

this in practice let me go over to VSS

code here and open a program I wrote in

advance called swap. C which does this

as follows in here notice I have my

prototype for a swap function at the

very top and let me scroll down to the

very bottom there is that exact same

code so I'm uh the same code for

swapping two values A and B which I'm

claiming for now is correct now if I go

back up here what is main going to do

for us main is really just meant to be a

demonstration of the correctness of your

algorithm so here I declare on line

seven and eight two variables X and Y

being 1 and two arbitrarily respectively

I then on line 10 just print out what

the value of x is and Y is just so I can

see it on the screen I then call the

swap function on line 11 and then I

literally print the exact same thing

again I print X and Y hopefully it'll

obviously be the opposite so I think

logically swap is indeed correct let me

do make Swap and then do/ Swap and I

should see X is one y is 2 and then

hopefully X is 2 Y is 1 enter but I

don't and it did work in the sense that

the code compiled the code ran so it's

not like some bug in that sense but

because I don't quite understand what's

going on underneath the hood at least as

of right now or prior weeks this code

here is indeed buggy in some way but

does anyone have an intuition perhaps

based on today's discussion is to like

why this code while logically correct

clearly works in reality apparently does

not work in

C any intuition

yeah perfect and to summarize here's

that term of art I prom

when you call a function and pass in two

arguments Like A and B you're passing

those arguments by value so copies of

those values effectively and so when

swap is actually called here sorry when

you pass in X and Y we call them A and B

but that's just a convention we could

call the parameters anything we want

what A and B are are indeed the values

of X and Y respectively but copies of

the values so this code here is very

successfully in vs code 2 swapping the

values of a A and B but as you note

because I'm passing them in by value

literally one literally two and not by

another term of art by reference AKA by

their addresses swap has no capability

in C to go to those locations swap the

actual locations just like we did

successfully in reality but I think we

really have the syntax already for

solving this if we consider that really

this is just an issue of scope and we've

talked a bit about scope in the past

whereby scope refers to the context in

which a variable lives and generally

I've claimed that a variable exists

between the most recent curly braces and

that's pretty much true for the swap

function because a and b i now claim

again exist only in the context of these

curly braces they have no effect on Main

up top which has different variables X

and Y but we can consider now what's

really going on underneath the hood and

here's that same picture of memory as

we've seen in the past if we zoom in and

see on these little black chips this is

a bunch of bites of memory if I create a

grid out of it just to kind of highlight

that we can address each of these bites

throw away the plastic circuit board and

focus only on those bites what's going

on underneath the hood when functions

are called in C which you've been doing

for weeks now well this rectangle of

memory if we kind of abstract it away

further is generally broken up into

different regions or segments like I

called them earlier and different things

get put in different parts of the

computer's memory and without getting

too into the weeds when you double click

a program on your macro PC or when you

do/ something on a Linux you are loading

your machine code into the computer's

memory from the computer's hard drive so

all the zeros and ones that compose

Microsoft Word or Chrome or whatever are

loaded into the computer's memory or RAM

and by convention it's put up top in the

so-called machine code area and that's

how the CPU has access to them quickly

at that below that are what are be going

to be our globals so Global variables

which we haven't used very much in C but

you can declare them outside of main at

the very top of your files if you have

globals they end up up there as well

just FYI and then there's this big chunk

of memory that we saw valgren mention

indirectly earlier called the Heap and

it's kind of like Heap literally like

it's a heap of memory that you can use

as you see fit and the Heap is where

malok grabs memory from so initially

there's nothing in the Heap it's just a

big chunk of free space anytime you call

malok Malo kind of carves out from the

Heap area more and more bites and Malo

keeps track of essentially which bites

have already been allocated so initially

it looks empty but different bites

squares if you will keep getting

requested again and again as a program

runs thanks to functions like malok and

it grows if you will conceptually down

so the more and more memory you request

from malok it starts up here but then

the next chunk you get is down here

conceptually the next chunk is down here

down here so it kind of fills the

available space in the computer's

overall memory but there's this other

chunk of memory called the stack and

just like a stack of trays in like

annenburg or a cafeteria kind of grow

upward so does the stack of memory and

it turns out the stack is where

functions have variables and have

arguments stored temporarily so whenever

you call a function and it has variables

inside of it or it has arguments there

too this is the chunk of memory and the

computer's overall block of memory that

are used for functions but anytime you

call malok it's memory up here right at

the end of the day they just had to pick

a direction top bottom and technically

it's an artist rendition you could

Circle this thing around any orientation

you want but you're just using a finite

amount of memory in this conventional

way Malo Starts Here functions start

here now you can kind of see where like

bad things can happen and indeed one of

the other reasons programs computers can

crash is if you ask for way too much

memory from the Heap by calling malok

many many many times or if you call way

too many functions or accidentally per

last week you recurse infinitely many

times you might have a segmentation

fault and that's because you're using

too much stack memory so this is bound

to be a problem eventually and the onus

is on the programmer to just minimize

the probability of doing that and really

avoid the possibility of doing that by

just checking return values like

checking if malok or get string return

null because you can proactively with

conditionals make sure that these two

things do not Collide by just making

sure that you get back non-null values

so let's consider the stack in the

context of Swap and what's really

happening here and Carter if you

wouldn't mind helping me animate the

screen here when I call the main

function of any program it is allocated

a slice of memory called a frame at the

bottom of this stack so if Carter you

want to go ahead and uh Advance here

here's like the First Slice of memory

that will always be used by main whether

it has commandline arguments or local

variables it just ends up here in memory

suppose now per our swap. C program that

main calls swap well where does the

memory for swap end up like right up

here so swap had two variable two

arguments uh A and B and it also had a

temporary variable so all of those end

up in here in memory and if you want to

go ahead and Advance again Carter once

swap is done executing whether it just

returns because there's no more lines of

code or you explicitly return this

memory is just freed up automatically

you don't call free you don't undo malok

this just all happens automatically and

has been since week one now technically

it's still there even though we've

removed it from the picture and there's

your first hint of garbage values right

like there's still zeros and ones there

and they're left in the original the

previous configuration and so the reason

you get random values in the memory is

because even though we hav't drawn swap

here there was stuff there a moment ago

it's going to be there the next time you

use that same memory now let's go ahead

and step through this a little more

methodically main has two variables

called X and Y one and two so let's

advance and represent X is one y is two

taking up these two chunks of memory

when we call swamp now swamp gets a new

slice of memory that then gives us three

variables A and B technically the

arguments and temp so what happens well

because functions automatically pass in

values by value or rather pass in

arguments by value X gets copied into a

y gets copied into B and then once we

start executing the algorithm all the

watered glasses well what happens here

so if I execute the first line of code

temp equals a temp gets a copy of a what

happens next a equals B so a takes on a

copy of B and now we do the final swap

in the glasses b equals temp b gets a

copy of temp now we don't have to change

temp because it's essentially empty

although there's the garbage value one

is always now going to be there until we

reuse that memory the important thing

though is that a and b have been swapped

but what obviously has not been swapped

as is Manifest is when swap returns X

and Y are untouched because copies

thereof were passed in so we need a

solution to this problem and if we

advance one more time if you don't mind

let me step over here but then call you

back in a second this code here is

logically correct this is what you did

but this is now a detail of C you can't

just swap the things by value because

you're only changing it in the scope of

the swap function function but I think

if we change it to this and add some

annoying syntax we can solve the problem

just like you can declare variables as

storing addresses you can declare

arguments to functions AKA parameters as

taking addresses this new version of

swap means that a shall be the address

of an integer B shall be the address of

an integer and now it gets a little

cryptic here temp is the same because

it's just an integer like it was in week

one nothing special about temp but if

you want to get the value at a you do

star a and that goes to the address

grabs the number one presumably if you

want to change the value of a you go to

that address you follow the treasure map

to the other mailbox and you set it

equal to whatever is at the value of B

you go to B as well last line you go to

B now and change it to be whatever the

temporary variable was which happened to

be the same as a so that's where the

final value gets swapped but here

there's a lot more like crisscrossing

metaphorically across the stage where

you're going to all of these different

addresses in the swap function to make

these changes so if we advance now to

the pictoral version of this here's the

same story as before with Maine and X

and Y are 1 and two respectively when

swap gets called now notice and I'll do

it with arrows here a is effectively

pointing to X B is effectively pointing

to Y if we really get into the weeds

these are actually like addresses but

who cares about the specifics it's

really just the concept here so now what

happens in temp gets star a star a means

start at a and go there Follow the arrow

if you will sort of shoots in ladder

Style and then that's one so we put one

and 10th all right star a equals star B

so let's do it from right to left star b

means Follow the arrow it's two and then

what do you do Follow the arrow it's now

two because you copy one to the other

from right to left and then lastly star

b gets temp so start at B go to B and

now store whatever the value is in temp

so just by having this basic new syntax

of like ampersands and stars and so

forth we can actually now go to places

and circumvent what is otherwise a

feature of C that these variables are

locally scoped but you can still access

things in other functions as well so

thank you so much for helping step

through this so we now have a

application of this that explains why

now in this version of the C code this

would actually now work so in fact let

me go back to my swap code here and let

me change the function ever so slightly

in vs code so let me scroll down leaving

main the same and let me change swaps

prototype to taking in addresses let me

go to a here let me go to a here let me

go to B here and let me go to B here as

well but nothing else changes this

change here in particular is enough of a

clue to see that means when you call

Swap and pass into values I'm expecting

addresses now not integers but now that

I've made this change I do need to go up

to Main and make one change does anyone

have the intuition for what now need

change in main so that I pass in X and Y

by reference that is by address rather

than than by value or copy oh yeah and

[Music]

back so close so on the swap line it's

not star that I want in front of the X

and the Y it's

instead what's the other one it's the

Amper sand why because if I want to

enable swap to go somewhere just like

Carter and I played this game with the

mailboxes I need to inform swap of the

address of X and the address of Y and

again per the beginning of today's class

Ampersand is the text via which we do

that so I add an ampersand here to get

the address of X Ampersand here to get

the address of Y and now this code lines

up with the picture that Carter just

helped us walk through and so when I run

make swap here I have a mistake oh what

did I do wrong not intentional but I

guess worth pointing out I screwed up

here it doesn't like Ampersand

X because of something on line three

which is way early in the code what did

I screw up yeah in the middle

yeah so this is why we you should not

copy paste even though it's necessary

for things like function prototypes if I

change swap at the bottom I need to

change its prototype so let me add the

star there add the star there or just

recopy paste it at the top of the file

now let me do make swap again let me now

do do/ Swap and I should now see X is

one y is 2 and hopefully X is 2 Y is 1

which I now do so the logic is the same

the algorithm is the same all the weak

zero stuff is the same except now and

four you just have a bit more

expressiveness via which you can tell

the computer exactly what you want to

manipulate and how any questions then on

this technique

here no all right well when we fix this

there's still going to be problems and

just so you've seen some terms of art

here this is bad whenever you have two

arrows pointing at one another certainly

if you might use and reuse more and more

memory and it turns out there's some

terms of art that might suddenly now

make sense especially if you've

programmed before bad things Can Happen

by this design but there's really only

this kind of design because it's a

finite amount of memory so at some point

bad things are going to happen no matter

what if a computer runs out of memory so

it's not that this was a poor decision

it's just sort of a necessary one given

finite amounts of memory in a computer

but a heap overflow so to speak is when

you actually overflow the Heap and touch

memory that you shouldn't up there stack

Overflow is when you somehow overflow

the stack and touch memory that you

shouldn't down there so with that said

these are really just problems that can

happen and there specific incarnations

of what are generally called buffer

overflows a buffer like in the YouTube

sense is just like a chunk of memory

that in the case of YouTube stores like

the next few seconds or minutes of video

but generally speaking a buffer is just

a chunk of memory that the computer is

using for some purpose be it the stack

be it the Heap be it an array in the

computer and so buffer overflows are

what happens when you just have logical

bugs in your code but with these

Primitives now in mind we wanted to

conclude with a final Revelations and

that's how some functions like these

here work the other thing in the cs50

library besides the type def for quote

unquote string is of course all of these

functions and we give you these

functions because honestly in C it is

hard it's annoying it's painful it's

difficult to get user input correctly

it's very easy when you don't know how

much how much the humans going to type

to write buggy code when it comes to it

and indeed it's really hard to store it

correctly without accidentally having

some kind of buffer overflow so for

instance let me show you a program here

I'm going to go ahead and write this one

from scratch so let me go ahead and open

a file called get. C where I'm going to

go ahead and mimic the idea of getting

integers manually without the cs50

library so I'm going to include standard

io. only I'm going to Define main as not

taking any command line arguments and

then I'm going to do something like this

give me a variable x with no value yet

and normally I would do something like

get int but let me take that away no

more training wheels for get int either

so let me just Define the int X let me

then just print out something like uh a

prompt and I'll just do x c just to make

it obvious to the human what we're

waiting for and now I'm going to use a

builtin c function to get user input I'm

going to call a function called scan F

which sort of scans the user's keyboard

for input I'm going to scan it for an

integer so just like print F I'm going

to use percent I because I expect an INT

and then I want to tell scanf where to

put the human's integer from the

keyboard it is not correct though to say

x because if I say x I run into the same

swap problem scanf no function can

change the value of X unless I pass it

not by value but

by reference so we're back to our

Ampersand friend and now it has like a a

treasure map to the actual location of X

and can therefore change it and so now

at the very end of this program let me

do something simple like let's just go

ahead and print out with print F uh the

value of x using percent I as always

plugging in X not Ampersand X this is

now week one stuff I want to print the

actual integer value of x so the only

change here is that instead of using get

in I'm now using this new function that

as of today exists called scanf so let

me go ahead and run get make get to

create this program doget and let's go

ahead and type in a value for x 50 enter

and it just works so it turns out get in

is pretty simple to implement however

notice what does not work if I type in

cat for instance cat gets converted to

zero and meanwhile get in recall will

reprompt the user if a human not type an

actual integer you get automatically

reprompt so that's one of the features

we for cs50 added to get in just to make

your programs more user friendly but

otherwise get in is pretty

straightforward to re-implement using

scanf unfortunately that's not true for

Strings because how do you know when you

write your code what word the human's

going to eventually type in how long

they're greeting like high is if it's uh

if their name is David or Carter or

anything else you just don't know in

advance how much memory you need so how

might we do this in with string well let

me go ahead and declare a string s

although you know what there's no cs50

Library so we do Char star s today

instead and that gives me not a string

per se but a pointer that will Point

presumably to a string ideally I would

use this get string but again we've

taken that training wheel away so now

that I have a pointer s suppose I prompt

the human for a value for S just like

before let me use scan F now and tell

the user that I expect to read a string

percent s from the keyboard

uh and store it in s now this is subtle

I don't technically need an Amper sand

here even though I did for an INT and I

would for a float and a double and a

long and a bull and a Char why do I not

need an Amper sand in this story to pass

by

reference because s is it's already an

address again strings are just special

strings now are always addresses so you

don't need to additionally add an Amper

sand here that's the only subtle

difference here but now if I go ahead

and print out at the very end what the

value of s is using percent S as before

this program looks like it's almost the

same as the int version but let's do

make get and okay so this is not good

all right so it doesn't like an

initialized value so let me make it

happy I said earlier to always

initialize my variable so let's

initialize it to null so that at least

something is there that's your good

default value nowadays now if I do doget

now we're good and let me type in

something like cat

okay cat is not X well let me try

another word maybe it's just cat is

wrong dog okay let me try David it just

doesn't seem to be working moreover it's

printing it as a zero what logically

though is the bug here scanf worked a

moment ago for integers but it's not

working for Strings and it seems to be

forgetting C A it's forgetting dog it's

forgetting

David

why what's Happening

Here think back to our yellow pictures

of

memory when I

yeah it might be reading just the null

itself because s is being initialized to

null and what step have I forgotten from

just like a few minutes ago what did I

not actually request of the computer

like actual memory to store the C A the

dog the d v d right there's nowhere have

I asked the computer for some amount of

memory and so technically it might be

reading it into some garbage location

and that's really the problem here S is

initialized to null now and so in fact

it is printing zero as null but I'm not

seeing any of the other letters because

there was nowhere to put them C A do D

ID because I didn't ask for three bytes

four bytes five bytes 100 bytes there's

no use of malok there's no use of an

array there's no memory allocated for

anything other than the pointer itself

and this is where honestly life gets

hard with scanf like I could solve this

problem in a couple of ways let me go

ahead and do this instead of declaring s

to be a pointer let me declare s to

actually be a an array of four chars and

now let me go ahead and recompile the

code so make get doget and I'll type in

cat now that now works why well I'm

allocating an explicit array of size

four enough for a one two three letters

plus a null character here's where to

someone's question earlier it turns out

then in some context you can treat

arrays as though they are Pointers

themselves see we'll sort of do the

conversion for you but for now just

assume that s is just an array of size

four and if you pass it into to scanf

that's like a treasure map that leads to

those four bytes so scanf can now

successfully fill it with c a d o but

but let's try this again let's type in

David and here okay we got lucky but I

technically touched memory that I should

not and in fact if I typed in a long

enough string and I don't think I could

do it very easily like by without typing

this thousands or hundreds of times

still okay but you'll notice that it's

forgotten the rest of it now right so

somewhere we went beyond the boundary of

the array and we just don't have enough

storage space for that entire thing so

what do you do in your program if you

don't know how long the person's name or

the the the animal name is going to be

what do you do 40 400 4,000 40,000 like

at some point you have to draw a line in

the sand and that's why like getting

user input is so annoying in a language

like C and that's why get string exists

what we do if you're curious is we look

at the user's input and we take baby

steps we look at it one character at a

time and every time we see another

character we actually call malok again

and say no I need more than one bite I

need two oh wait they typed in three

letters I need three instead of two oh I

need four instead of two and we have

this crazy loop essentially that keeps

asking for more and more memory but by

taking baby steps and honestly if you

all had to do that in week one my God

like we couldn't even write hello world

anymore and so that's why these training

wheels exist at least early on and

that's why in higher level languages

like in uh python you don't have to do

this at all it just works as you'd

expect so what more can we do well

you'll see in problems set for this

coming week if I open up an example like

this phone book. C you'll see that you

can manipulate files now that you have a

vocabulary for pointers it's going to be

new quickly but here we have an example

of how I have a program using some

familiar libraries here but as I claim

in my comment this saves names and

numbers to a CSV file all of my examples

thus far I type in some words I type in

some names and some phone numbers and

they disappear because we only store

them in memory but if you want to store

data in like a CSV file comma separated

values which is like a simple

spreadsheet like Excel and apple numbers

and Google Sheets can open you can

actually do this yourself so just as a

teaser for this week here on line n I'm

using a new data type not a cs50 thing

this is a c thing called file but if you

want to manipulate files you need to use

addresses that is pointers so here is me

creating a variable called file that's

going to point to an actual file on the

hard drive on the server or your Mac or

PC fop is going to be a new function

you'll use that will open open a file

and it will return effectively a pointer

there to in memory the file name I want

to open is phonebook .csv and in this

example it's going to be uh a pen mode

it will keep allowing me to add more and

more names and numbers to this file

here's some old get string stuff because

I'm not going to reinvent get string

with scan F but down here is a slightly

new function it's not print F but

fprintf and it turns out it's very easy

to print things not to the screen but to

a file with fprintf and it takes an

additional argument instead of starting

with the quoted string you'll have to

like say what file you want to write to

and fr print F we'll figure out how to

get the uh the bits into that file

passing in something like name comma

number so if I run this somewhat quickly

here let me do this let me pre-create a

file called uh phonebook do CSV and in

phone book. CSV I'm going to create a

temporary row here name comma number

just so that there's something in this

file and now let me go ahead and do this

and split my screen here if I have

phonebook.com

and phone book. C on the left let me

compile make phonebook which is the C

version phonebook and now I'm prompted

for a name and a number so I'll type in

David and then for instance + one

949 uh what is it 468 275 o enter oh

damn it bug uh pretend that didn't

happen I forgot to hit enter in the file

so let's do this again if I run the

program Again David and plus one 94 9

468 2750 enter it's been saved now to

the file and if I close this file and I

reopen code of phonebook.com

or the like and I've actually created an

actual CSV file uh if you're smiling

because I keep repeating my phone number

out loud I would encourage you to call

or text that number sometime it might

very well well be a an Easter egg of

source but via these functions here do

we have now the ability to write files

uh input and output and among the goals

then for this week as we'll see are to

actually play with images in the spirit

of something like Instagram filters or

the like and we'll introduce you for

instance to a file format called bmps

which to come full circle to the start

of class are just maps of bits but more

than just single bits for white and

black but rather colorful patterns as

well and we'll give you images like this

of the week's Bridge here across the

river at Harvard and you've run

after writing your own code in C and

understanding how the data stored in the

computer's memory you'll be able to

apply your own Instagram like filters to

make things uh grayscale instead or

sepia in this case you can even flip the

bits around so that the thing is a

mirror image you can blur things further

or if you really are feeling more

comfortable you can even write code that

finds the edges of the image and creates

works of art like these so all that in

more in problem set four we will see you

next

time

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all right this is cs50 and this is week

five which is going to be our last week

in see uh but what that means is that

we'll have

okay

but with this week with last week and

really all of the weeks prior have you

been hopefully acquiring if slowly and

with some challenge like some

fundamental building blocks that are

still going to underly everything we

continue doing in the semester even as

we transition to so-called higher level

languages next week indeed we'll

introduce python a very popular language

that does not have pointers does not

have memory management at this very low

level but that's really just because

someone else wrote the code that will do

that for you and it's going to make our

lives easier because it means when you

want to solve a problem concept up here

to just get real work done or build

something amazing you don't have to

really get into the same weeds as we

have been deliberately this week and now

last but the goal ultimately is that you

better understand and can better harness

than all that a computer can do even in

those higher level languages So today

we're going to focus particularly on

data structures how you might structure

your data in memory which really amounts

to building things digitally stitching

together ideas and Concepts in memory

using this new building block from last

week which of course are these pointers

pointers allow you to store addresses in

memory like in variables but with those

simple addresses we can sort of leave

these breadcrumbs we can point from here

to there and we can conceptually stitch

pieces of data together but there's

going to be different ways of doing that

and today we'll focus first on what's

generally known as an abstract data type

and similar to a type in C more

generally it does actually have some

properties in it but the underlying

implementation details of an abstract

data type are ultimately up to you that

is to say an abstract data type can

represent one thing and can do something

but how you implement it allows you some

discretion underneath the hood so for

instance in the world of computer

science a q is actually a technical term

this is a type of data structure that we

could theoretically build in code in C

or really any other language but a q has

properties just like cu's in the real

world for instance if you've ever lined

up for something to get food in a uh and

get food in a restaurant or go into a

store or wait for the airport to clear

you well you've lined up in a queue Q

being some sort of line but what's

noteworthy about Q's are specific

properties they are first in first out

data structures either virtually or in

the human world which is to say the

first person in the line should ideally

be served first at the restaurant or the

first person in the line should get

through airport security first by

contrast if it weren't first in first

out you can imagine how frustrated you

would be if you have this sort of

inherent unfairness in fact if you've

ever been in line at a store a

supermarket or the like and all of a

sudden they maybe open a new line and

now the person behind you gets to kind

of cut ahead and go forward that's

because they've broken the concept of

the cube so it has this inherent

potential for unfairness unless you

maintain this first in first out

property this would be true too for like

a to-do list just for productivity sake

if you're in the habit on paper or

virtually making a to-do list ideally

you probably want to go through that

list top to bottom so that you actually

get the first stuff you thought of done

first rather than always focusing on

your most recent thought now within the

world of Q's there's generally two

operations two functions if you will

that any Q would have either in the real

world or in the virtual NQ is usually

the technical term to mean adding

something to a q but specifically it

means adding it to the end of the que so

that someone isn't cutting in line for

instance to go up front and then DQ is

just the opposite when it's time for the

first person in line to be served the

time for the first person in line to go

through security they are dced so to

speak so technical concept ultimately as

it's implemented in code but it's really

just a real world concept and these are

in contrast to another abstract data

type that we might call a stack and a

stack much like a stack of trays in the

cafeteria has sort of fundamentally

different properties you can still add

and remove things from them but consider

what happens whenever they clean all the

trays in the cafeteria or the dining

hall they put one of the trays down here

and then the next one on top of it and

then the next one on top of it it it and

so forth but of course which tray do you

presume presumably take as a user of

that physical

stack the top one presumably right like

you're not going to fuss down here and

try to pull one out and so that would

seem to have an opposite property lifo

last in first out is what characterizes

something like a stack and that just

makes sense certainly in the physical

world of stacking all of those cafeteria

trays because it just makes more sense

to grab the most recently added one the

last added one first and at least in

that context the trays don't necessarily

care what order they're using used in

but even then you could imagine that

maybe there's some old dirty nasty tray

at the very bottom that like never gets

used because you're constantly

replenishing the stack so there might

very well be side effects of these kinds

of features um you might be familiar

with using Gmail for instance or really

any email program what you're looking at

in your inbox is technically a stack at

least if you've left the defaults

configured why you get a new email where

does it end up not like five pages of

emails later presumably right at the top

and the next one's right at the top

right at the the top right at the top

and so if you're like me you're guilty

of eventually losing track of some

emails why because you've pushed so many

more emails onto the stack that you sort

of lose track of the things you got

earliest last in first out though is

maintained the most recent email you get

might very well be the first one you

reply to but that's not necessarily good

for responsiveness to everyone else out

there uh similarly if you store like all

of your sweaters in a stack like this uh

the uh like a pile of black ones below

which is a red and then a blue stack

might be perfectly fine for keeping

things organized it's sort of the sane

way to do it if you just have a shelf in

your dorm room or home but what's going

to be a side effect of using a stack to

store your sweaters if they're these in

this way as opposed to a queue

yeah it's harder to get the red and blue

one so presumably you're going to much

more often wear for instance if you will

black instead there now the operations

for adding things to a stack are similar

in spirit but just different vocabulary

you push something on top of a stack um

even though it's more like in the tray

world you sort of place it or rest it

but pushing means adding something to

the top of the stack and popping means

some removing something also from the

top of the stack so it's not a matter of

enqing at the end and deqing at the

beginning with a stack everything's

happening on top you're pushing onto the

top and then popping off of the top now

when it comes to actual code how might

we Implement something like this well

let's just focus on really how you might

implement the data structure itself and

we won't Implement any functions you

might implement the notion of a stack

like this we've seen type def before it

just means Define a new type of my own

struct means here comes a structure AKA

a data structure of one or more

variables within and let's suppose like

last time we've had we defined already

like a person data type using a separate

type Def and every person has like a

name and a number or whatever let me

just stipulate that person exists

already so here you might have to

implement a stack an array called people

of some capacity maybe it's an an array

of size 10 or 100 or whatever this is a

constant here in this context capacity

and every element in that array is a

person structure and I now have this to

size now this almost seems like a

synonym for capacity but maybe

intuitively anyone want to propose why

I'm apparently maintaining separately

the capacity of the stack but also the

size of the stack why these two

distinctions

here yeah

perfect the capacity is how high the

stack could be like how much room is

there for those sweatshirts in my closet

for instance whereas size is just

literally at this moment in time how

many sweatshirts are in the stack it's

either capacity or fewer presumably in

total there so what is capacity well we

could implement this in you know perhaps

a familiar way I might just Define a

const somewhere else in my code of type

int that just defines it to be capacity

50 but what perhaps is going to be the

downside of implementing a stack in this

way of how using an array inside here

like what's a downside now of

implementing a stack using an array and

this size variable

within what's a caveat here

perhaps any hands

yeah okay so it's going to be harder to

reach elements that aren't the last one

that is the most recently added one so

there could be some sweatshirts so to

speak way down below so we've seen that

before too but at some point too a

limitation of this design is is what

it's going to be finite right I can

maximally fit in this example 50

sweatshirts or 50 emails or 50 cafeteria

Trace which is fine but at least it's

indeed finite and at least in the

computer's memory it might be nice to

use more and more and more maybe as more

things are getting added to the computer

so maybe I make this 500 or heck why

don't I make it 5,000 or 50,000 well

what's the tradeoff there if I want to

have enough room to grow seems like I

should just crank up the value of

capacity endlessly but why might I not

want to change the 50 to 500 or 5,000 or

50,000 what's the trade-off there

perhaps just intuitively

yeah okay you don't want to touch memory

that you're not supposed to be touching

and in this case it wouldn't be that

wouldn't be a risk per se unless you

indeed overflow the stack but there's a

related issue in asking for that much

memory what would another downside

be yeah okay exactly so if you've got a

capacity of 5,000 but you're only using

one of those elements it's it's awkward

to say it non-technically which is just

to say very very wasteful right that's

just bad design it's correct it will

work for up to 5,000 elements but my

gosh you're wasting

4,999 extra spots and that's not going

to end well especially if you're using

more data structures in memory like your

Mac your PC your phone is surely going

to run out of memory if you ask for that

much so it' be nice if there is a bit

more dynamism there whether it's a stack

or a CU both of which might be

implemented a little similarly in spirit

but let's conclude this abstraction by

comparing thanks to a friend of ours

Professor Shannon Duval of Elon

University who kindly put together this

graphical animation that's just under 2

minutes long that paints a picture of

these two types of abstract data

structures and then we'll dive in

underneath to how we might Implement uh

problems like these if we could dim the

lights

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dramatically once upon a time there was

a guy named Jack when it came to making

friends Jack did not have the Knack so

Jack went to talk to the most popular

guy he knew he went up to Lou and asked

what do I do Lou saw that his friend was

really distressed well Lou began just

look how you're dressed don't you have

any clothes with a different look yeah

yes said Jack I sure do come to my house

and I'll show them to you so they went

off to Jacks and Jack showed Lou the box

where he kept all his shirts and his

pants and his socks L said I see you

have all your clothes in a pile why

don't you wear some others once in a

while Jack said well when I remove

clothes and socks I wash them and put

them away in the box then comes the next

morning and up I hop I go to the box and

get my clothes off the top Lou quickly

realized the problem with Jack he kept

clothes CDs and books in a stack when he

reached for something to read or to wear

he chose the top book or underwear then

when he was done he would put it right

back back it would go on top of the

stack I know the solution said a

triumphant Lou you need to learn to

start using a queue Lou took Jack's

clothes and hung them in a closet and

when he had emptied the box he just

tossed it then he said now Jack at the

end of the day put your clothes on the

left when you put them away then

tomorrow morning morning when you see

the sun shine get your clothes from the

right from the end of the line don't you

see said Lou it will be so nice you'll

wear everything once before you wear

something twice and with everything in

cues in his closet and shelf Jack

started to feel quite sure of himself

all thanks to Lou and his wonderful

Q all right so sure so that paints a

picture of these two abstract data

structures but if we really were to dive

underneath the hood we could Implement

them in a number of different ways but

we really I think need some building

blocks via which we could solve problems

like those and we'll see today to some

others as well so let's rewind back to

week two where we imple we introduced

you to your very first data structure

that is an array and an array of recall

was just a chunk of memory whereby

elements were stored by Design back to

back to back it's an array of contiguous

memory specifically so with an array we

could certainly store not just one thing

but two or three or even more and so for

instance if we treat uh my computer's

memory as this sort of abstraction here

and pictured here are three bytes or

some uh uh multiplication thereof

suppose we're storing in the computer's

memory an array of size three storing

the digits one two and three well

remember that if we zoom out per last

week there's other stuff going on in

memory so even if we want to add another

number to this array that we didn't

think of when we first started the

program like the number four ideally we

would put it right here next to it

otherwise it's no longer an array so by

definition if we're using an array it's

got to end up right there after the

three but what else is going on inside

of your computer's memory well assuming

your program is of any length and you've

got other variables other functions

you've been running it for a while

there's a lot going on in your memory is

being used and reused so for instance

somewhere in memory might be immediately

adjacent to this like hello comma world

back0 the null character just because

maybe you have another variable

somewhere in there that is storing that

particular string alongside your

existing array of size three and all of

these Oscar the grouches here really

just represent what we called last week

garbage values like there's obviously

bits there because they don't disappear

they're always going to be inside of the

computer somehow implemented but we

don't really know or care what they are

they're the remnants of those bytes

having been used for other older

variables previous function calls or the

like but the problem clearly here is

that okay 1 two 3 is there but the H is

here and unless I want to start uh

taking a bite out of my string by

overriding the H with a four like we

just can't fit it right there and yet

even though there's Oscars all over the

place those are indeed garbage values

and therefore we could use that space

because it's technically unused we just

don't know or care what the values are

so where could I put 1 2 3 4 well my

gosh like I have all this memory down

here that's unused I could certainly

change those garbage values to be 1 2 3

4 but to do that I might need to do a

bit of work here right it's not just a

matter of just saying boom and it

happens now with c and with code I'd

have to do this a little more

methodically so let me abstract away

everything else that's a distraction let

me assume that there is indeed at least

four bytes available for four numbers

just down here though we could have put

them in a bunch of different spots

what's involved now in moving the one

two three to this new chunk of memory so

we can add the four well I think

conceptually we're going to have to copy

the one from old to new copy the two

from old to new copy the three from old

to new and then ultimately we can get

rid of the old memory those three

original bites could now look like Oscar

the Grouch and just be garbage values

for all intents and purposes but now I

have room for a fourth bite wherein I

can put the number four so this is nice

but what's a downside of this

approach what's a downside of solving

the problem in this way where the

problem at hand is just to grow the

array so to speak to increase its size

to fit one or more

numbers seems pretty

straightforward but

yeah okay maybe it's out of order but I

think that's okay because the order is

just matters that it's relative so so

long it's it's still contiguous back to

back to back in a different chunk of

memory I think we're okay there it's not

like I changed it for 4 3 2 one but a

reasonable hunch

yeah yeah like I don't really plan ahead

here like if I have to add another

number like five or anything else well I

might have to jump through these hoops

again maybe I get lucky and maybe

there's space there but not if I have

other variables and other things going

on that too might be used at some point

other thoughts yeah slow efficiency slow

effic in slow efficiency why have

to again yeah I mean it's just in it's

just inefficient it's sort of bad design

arguably why because I had to copy all

of my original work down here and as you

note if I want to add a fifth number I'm

going to have to copy it again and again

and again and do things end times again

and again now maybe that's necessary

we'll soon see for sure but it feels

like this is not going to end well

especially if the array isn't of size

three or four but 300 400 your computer

ends up spending so much time just

spinning its Wheels I mean honestly

better might be this like if this is my

same array physically incarnated Now 1

two 3 it's literally on the edge of the

shelf so there's no room for the number

four you know maybe where we could take

this story is well let's just find room

for the four like let's just put the

four for instance over here replacing

some available garbage value some spare

bite over here but now wait a minute

I've broken the definition of an array

right it's I can't have one two three

and then four over here so maybe there

maybe there could be a mechanism if I

put this thing on again where when you

get to the end of the existing elements

maybe I just somehow digitally point to

the fourth array and maybe we can kind

of stitch together all of these

different values in memory so that if

you follow the arrows so to speak we can

reconstruct exactly what the order is

even without having to find or make room

here or pick up all of these numbers and

move all of them over there so that's

perhaps the direction in which we'll go

here so let's see how we might get to

that spot as follows let me go ahead and

open up say VSS code here let me open up

a program called list. C in my terminal

and let me go ahead and whip up a

relatively simple program that just

demonstrates what we did back in week

two when we introduced arrays as follows

let me include uh standard io. so we can

print stuff out let me do int main void

no command line Arguments for now let me

give myself a an array uh called list of

size three and I'll just hardcode it to

keep it simple for lecture sake Each of

which is going to be an integer and now

just so we have some specifics to talk

about let me put it list bracket zero

the number one list bracket one the

number two and list bracket two the

number three so I'm just translating

into codee what we just had uh

pictorially on the screen and also

physically here with these numbers on

the desk now let's just do something

mildly useful for this how about we do

four in I gets zero I is less than three

I ++ let's just print each of these

numbers out just to make sure they're

indeed in memory as I intended so

percent I back sln comma I and then a

semicolon and I think that's it for now

so nothing interesting no problem solved

just yet just a proof of concept so that

now when I clear my terminal and run

make

list no apparent errors at the terminal

and so when I now do do/ list I should

see hopefully from left to right 1 two 3

but of course if I want to add a fourth

number now there's no mechanism for such

certainly in the code that I just wrote

I could go back in here and change this

to a four I could go down here and

change lists bracket 3 equals 4 I could

just manually change the code recompile

the code but of course that doesn't give

me any additional run way for the fifth

or sixth number so let me try to take a

different approach drawing some

inspiration from last week if I want to

allocate memory dynamically maybe

because I don't know when I wrote the

program how many bytes I want we have

another function as of last week that

does not require that you commit in

advance to a certain number of bytes via

what function can you just ask the

operating system for a chunk of memory

okay so Malo to allocate memory now an

array is just a chunk of memory and even

though since week two 've been using

this syntactic sugar this convenience of

just using square brackets and indexing

into it it's just making it easier to

manipulate a chunk of memory that's

contiguous all together back to back to

back so today just like last week we can

take those sort of training wheels off

and maybe be a little more deliberate in

how we allocate memory let me go for

instance and do this let me delete my

contents of my main function here go

back into Main and let me propose now

that I declare for instance how about my

list no longer as an array but as a

pointer so int star list and I'm going

to go ahead and initialize this to be

how about a chunk of three integers for

now so I'm still going to hardcode it

but I'm taking a step toward more

dynamism for now so let me allocate

three times whatever the size is of an

INT but it's usually going to be four

bytes as we know so this is really going

to be 3 * 4al 12 but it's a little more

Dynamic and now what can I do down here

well this is just a chunk of memory

so I can do literally list bracket 0 uh

equal 1 list bracket 1 = 2 list bracket

2 equals 3 and voila achieve the exact

same effect because again an array is

just a chunk of contiguous memory but

malok gives you any old chunk of

contiguous memory so you can rather

treat one like the other here now if you

really want to be cool you could do

something like this instead you could

dreference the address in list and go

there you could go down here and

dreference list + one and go there but

honestly no one really writes code like

this it's just too cryptic it's a little

too far over the line at least for most

people and so I think the syntactic

sugar as I keep describing it just the

more user friendly square bracket

notation does the exact same thing

figures out the pointer arithmetic and

puts each of these integers in the right

chunks therein now just to be super

pedantic let me make sure if something

went wrong so if list equals equals n

that means that something went wrong

like my computer is out of memory which

we should check for typically so let me

just immediately return one signaling

anything other than zero which means

success typically just to get out of

this program because something's wrong

but now let me propose that I've had a

uh well let's do this for in I gets zero

I less than three i++ though a better

design would always be to use a con but

I'm just doing this for demonstration

sake let's print out each of these ins

too and just make sure I didn't mess

anything up and let me open my terminal

window again let me do make list

again okay huh implicitly declaring

Library function Malo with type void

star something something implicitly de

declaring is the operative words there

what did I mess up

yeah yeah I forgot the header file in

which malok is declared I remember now

okay that's in standard li. and it's

fine to look stuff like that up if you

forget so let me include standard libh

now let me clear my terminal run make

list again okay now we're good /list and

now what did I do

[Music]

wrong oh okay not intended but teachable

moment what did I do wrong

yeah yeah I'm printing the values of i

instead of what is at location I in the

array so what I actually meant to do was

print this out thank you you so now let

me recompile make

listlist and now okay those are the

three values I was expecting not the

indices thereof now let me propose that

for the sake of discussion that I I

regret having only allocated space for

three integers and maybe I really should

have allocated enough space for four now

this is not how you would do this in

practice because presumably if you have

a change of thought just go back in and

correct the code but let me propose that

somewhere in here is a more complicated

program and time passes dot dot dot

there's a lot of other interesting code

there but at some point I might want to

give myself more memory so how can I do

this well let me just ask the operating

system now for four new bytes of memory

so that we can at least in version one

implement the ID on the board where I

just copied the three bytes into the new

four bytes and then added a fourth value

so I'm going to use malok again and I'm

going to say here's a new pointer I'll

call it temp TMP for short which is

quite common when you just need it

briefly I'm going to then call malok

again I'm going to say give me four

integers using size of let me again make

sure so if temp equals equals null

something went wrong so let me just

immediately return one and for good

measure before I return one one let me

free the original list so that I don't

leak memory so I'm not just immediately

returning one I'm being a good citizen

and remembering well if this Malo call

did succeed and indeed I got as far as

line 18 but then line 18 failed I should

free the memory that I previously macked

so again that's the rule of thumb if you

allocate it you should be the one to

free it even before you're about to quit

now once I've done that I think I need

to do what we did pictorially on the

screen where I need to copy the One the

two the three from the old array into

the new so how might I do this well let

me give myself a loop so for in I gets

zero I less than three i++ because the

size of the original is still the same

let me go ahead and treat the new chunk

of memory called 10 temp as an array

itself and so I can absolutely use these

square brackets just like before it's

just a chunk of memory I'm treating it

like an array and let me add to that

value whatever is at the original list

at location I as well so this again is

just this exercise of copying from right

uh from old to new step by step the one

the two and the three but I still need

one additional step if my goal at hand

now is to have ultimately a fourth value

here well I'm just going to hard coose

this for demonstration sake and I'm

going to go to the very last location of

temp which is of size four which means

the last element in temp is temp

bracket three because it's zero indexed

but there's four total spaces there and

I'm just going to arbitrarily for the

sake of discussion put the number four

there and that is what happened when we

proposed uh changing the final garbage

value there to that four but now I need

to do what the slide did for us sort of

magically on the screen I should now do

a couple of final things I should free

the original list which I've not done

yet CU I only called free earlier in

cases of error and that was just to be

safe I can now free the list and now if

I want to inform the computer that I

want list quote unquote my variable

called list to point at not the old

chunk like it originally did but the new

chunk I think I can just do this list

equals

TMP and again that's just saying that if

list is a pointer which it was cuz look

at the very top line here on line eight

on line six I declared list to be a

pointer uh to a chunk of memory temp

meanwhile is a separate pointer to a

chunk of memory so down here this line

33 is just a matter of my saying okay

now henceforth because I've already

freed the old chunk of memory my list

variable should Point not at this chunk

of three bytes but this chunk of four

bytes or really 12 in total now uh or

rather 16 now because we have four such

bytes questions now on this code the

point of which was quite simply to

demonstrate how we could Implement and

code this idea of Fairly correctly but

inefficiently allocating a new array of

sufficient size and then populating it

with a new fourth

value questions on what we've just done

here no

yeah

[Music]

good question at this point in the story

with line 33 do I not have two different

variables pointing at the same chunk of

memory short answer yes but here's where

the semantics are perhaps compelling

list is the variable that I intend to

use longer term and keep around in

memory and again assume that there's

even more code going on here that we

just didn't write yet so it's useful to

have that that variable temp was just

kind of a necessary evil because up here

it would not have been correct to do

this it would not have been correct to

say list on line 18 equals the new chunk

of memory because this would have

represented a memory leak if I pre uh

prematurely change temp to point not at

the old chunk but the new chunk at that

point no one's pointing at the old chunk

and so I've lost those three bites vren

for instance would yell at you for

having lost as many btes in memory so in

this case here I do leave this as temp

yes it's duplicative at this point but

it's it's not a huge deal if it was just

meant semantically to be a temporary

value but down here at the risk of one

more line of code I still want to to be

a good citizen free list and maybe just

for good measure return zero explicitly

but notice it's not doing it twice per

se on line 31 what am I freeing the

original address of list the three

integer version then I change what list

points at so it's pointing at a

completely different chunk of memory

this one of size four So eventually when

I'm all done using this memory for this

demonstration I still need to free list

but at this point in the story line 40

it's pointing at the new chunk of memory

which I similarly need to hand back to

the operating system by free

yeah when would temp equal null so let

me scroll back up slightly this is being

a good citizen and a good programmer

whenever it comes to using malok malok

can return null if the computer's out of

memory so this is maybe a much bigger

program you've got other things going on

in it and so you just don't have enough

memory available to be handed malok

needs to signal to you that there's some

error and so it will by convention per

the documentation per the manual pages

return null so this is just me being a

good citizen otherwise here's another

error that might cause your program to

crash with a segmentation fault if you

get back null but you assume that it's

good memory uh going to address zero AKA

null will crash your program

intentionally

[Music]

yeah correct if I were to change my

final line 40 here to be free temp this

would also work as well and here this is

really a matter of design it's a very

nitpicky thing we could probably debate

it but because at this point in the

story my main variable for remembering

where the list is is called list this is

sort of the more responsible way to do

it freeing the list just so that my

colleagues my ta doesn't sort of wonder

why are you freeing temporary ver memory

that you already freed like it just is a

semantic thing at this point but good

Instinct it would also work correct

maybe just not good design all right so

it turns out that this gets annoying

quickly as it did in the picture of

doing all of this duplication and even

though technically it's necessary to

copy those values if you need a newer

bigger chunk of memory

there is at least a function in C that

simplifies a lot of this for us and in

fact let me go ahead and do this instead

of using malok this second time on line

18 in addition to the first time I used

it on line six I'm actually going to try

and introduce another function called

realloc which as the name suggests tries

to reallocate memory for you and it

works a little differently from malok

realloc expects two arguments the first

one is what is the chunk of memory that

you want to try to grow or Shrink that

is reallocate to be a different size and

then you specify what size you would

want and indeed in this case I want four

times size of int and that will now give

me hopefully a new address of a chunk of

memory that's big enough to fit all four

numbers but what's wonderful about

realloc is that it will handle all of

the copying for me so in fact I'm going

to go down here I'm going to get rid of

all of this this extra for Loop and what

I'm simply going to do instead is this

once I can

trust after lines 18 through 23 that

rioc worked and it didn't return null

because I'm out of memory I can just say

okay just immediately remember that the

new list points at this new chunk of

memory instead and then I can still now

do this line but I can tweak the

semantics here and just say list bracket

3 the new Final a in uh um the new list

is for I don't need to free this here I

don't need to do this all I need now at

the bottom is the final for Loop to just

print out these values so in short even

though that was somewhat quick using

realloc just moves the entire copying

process that I implemented myself a

moment ago using a for Loop it just

moves it to realloc and lets it deal

with the copying for me it's no more

efficient but at least means uh I'm

writing less code which is more pleasant

and hopefully the people who wrote aloc

or realloc are smarter than me and they

just will introduce bugs with lower

probability

too all right that was a lot any

[Music]

questions good question why do you still

need to make list equal temp as I did on

line 24 so ideally I would do this

ideally I would just change this line 18

to be list that is to say call re or

actually even better ideally I would

just say realloc this list to be of this

new size but again things can go wrong

when allocating memory you need to check

a return value to see if it was

successful or not and so we need to use

a return value okay so let's not

introduce temp let's just use list but

here's where a memory leak might happen

in the off chance realloc fails and

doesn't have enough memory for your four

bytes therefore it returns by definition

null you can't overwrite the original

value of list with null

to then check it why because now who

remembers where the original Three byes

were if you prematurely change the value

of list you've lost you've uh leaked

memory in that sense and so that's why

let me undo this change I declare a

temporary pointer for the sole purpose

of making sure I can check the return

value and then once it's good now I'll

update the value of list so it's sort of

doing a Switcheroo by making sure first

that you have a new value to swap with

the old other question

on this code

yeah indeed realloc automatically frees

the previous memory for you and better

yet it's even smarter than that if you

get lucky and they happens to be space

right after your existing chunk of

memory so one two three garbage value

instead of one two three hello world

realloc won't even bother copying things

from old to new it will just say okay

I'm going to now reserve for you more

bites than you originally asked for so

it doesn't have to waste time doing that

copying and so in that sense this

version is now not only still correct

it's even better designed because we're

not wasting time with that for Loop we

might have to resort to it if there is

in fact hello world or something else in

the way but hopefully we'll get lucky

and save those steps other questions on

this this manipulation of code

here yeah in the

middle

what if you want to resize a

two-dimensional array so very similar in

Spirit uh whereby you can use the same

trickery let me wave my hand at that for

now just because I think that's going to

sort of significantly increase the

complexity but very same Primitives

ultimately a two-dimensional array is

essentially just a doubly long or

quadratically longer list of memory that

using square bracket notation is doing

some of that mental math for you but

it's fundamentally no different of

what's going on underneath the

hood all right so with that said and

that code under our belt even though

that's not going to be something you'll

frequently need to code yourself let's

propose now how we might go about

building some actual data structures

ourselves the new ingredient here being

this reality that if you want to resize

a chunk of memory so is to make room for

things we now have that ability memory

addresses and pointers just give us the

ability to like Point around at things

and move things around in memory but now

that we have malok and even realloc you

can imagine maybe rewinding and you

could Implement that stack that Q using

not an array per se because you have to

commit to an array size in advance but

if you implement your stack or your que

using a pointer and then malok and

realloc and maybe someone else writes

all that code for you perhaps now you

can imagine that okay now the stack can

grow or Shrink by using realloc

accordingly you don't have to preact pre

um preemptively say give me five bytes

or 50 or 500 or 5,000 you can say just

give me one initially and if I need more

I'll realloc realloc reloc and if you

keep popping things off the stack you

can realloc in the other direction and

ask for fewer and fewer bytes and the

operating system can take that memory

back as well so we now have this

building block let's see what we can do

with it so we've had a few pieces of

syntax in recent weeks all of which

we're going to combine now in just a

slightly more clever way so struct is

this keyword in C that lets us build our

own structure in memory like a

collection of two or three or more

variables like a person that we've seen

before the dot operator recall we've

used when you do have a struct like a a

person and you want to go inside of it

so like uh person. name or person.

number we did this a few weeks ago now

but the dot Operator just allows you to

go inside of a structure and get the

individual variables within and then the

star operator unfortunately has a lot of

uses now one was multiplication like my

God that was easy back in the day now

it's used to declare pointers it's also

used to dreference pointers so to make

one exist and then go to that address

unfortunately it's the same symbol for

all of those but it's all related but

with these three symbols it turns out

you're going to get one last one today

and my God it finally looks like the

concept it turns out there's a clever

way anytime you want to use the dot and

the star together that is to go

somewhere and go to an address and then

look inside of a structure you can

actually literally use an arrow symbol

on your keyboard it's not a single

keystroke it's a hyphen and then an open

angle bracket but at least it looks like

an arrow and we'll see indeed in code

today the things I was drawing

pictorially on the screen last time with

yellow arrows you can actually now

Express as well in code and so here we

have our next data structure called a

linked list and this is one of the most

useful powerful Concepts in C it's the

kind of thing that you can take for

granted in Java and Python and higher

level languages but today we'll see how

we or others can actually build these

things just using these same Primitives

so a linked list is going to allow us to

actually do what we you know used a foam

finger for last week allow us to link

together for instance these three values

maybe with that fourth value over there

and then if there's a fifth you know

maybe this other foam finger points even

farther overway to that fifth value the

key being that you can stitch together

fancier data structures without having

to like pick all of these up and find

new space you just have to at least

connect the dots somehow we just need to

somehow point from one to the other and

that's going to make things much more

efficient it would seem so how do we get

there so here's my computer's memory as

always suppose that I'm storing the

value one somewhere in there and it's at

ox123 address whatever and I'm storing

the number two somewhere else in memory

Ox 456 and number three at address Ox

789 this is not an array by definition

why even though it's the only three

things on the screen what makes this not

an array it's not contiguous so this

violates the definition of an array but

you know especially since they're

sequential it kind of looks to a human

like a list so it would be nice if there

were a data type called list and there

isn't in C there will be in Python but

you know what if I could somehow like

Stitch together these three values so I

can get from one to the next to the next

then I think we could achieve the idea

the concept of a list without this

really annoying constraint that they all

be contiguous as in an array so how do I

do that well at the end of the day I

only have memory at my disposal right

there's no more training wheels to take

off here this is what we've got

underneath the hood of a computer so if

all I have is memory I think the

solution to this problem of stitching

together those values in a list must be

to spend a bit more memory that's

literally the only resource we have

right now so let me propose that if we

want to create a list conceptually out

of three values that are in random

although pictorially pretty positions in

memory let me just add a little bit more

memory to the picture so in addition to

storing the one I'm going to leave my

space myself some room a little scratch

pad if you will to use some other bits

as well same for the two same for the

three and you can perhaps see where this

is going based on last week if I want to

somehow connect the one to the two any

instincts as to what I should write in

this box here that would lead me

effectively from one to the two

what could go here

yeah we could store the address of Two

And so specifically what would you have

me write

here perfect ideally I would just put in

this box another integer one that

happens to be represented in heximal but

that's just a base system it's just a

human thing for us to look at I'm going

to put the value Ox 456 here so let me

go ahead and reveal that Ox 456 goes

there you can perhaps see further where

this is going well if I want to get from

the two to the three I think I need to

put below the two the address of the

three which gives me Ox 789 now if three

is the end of the list I don't want to

let it be some garbage value because

that would imply that it who knows where

it's pointing I need some definitive

value and just what would your instincts

be if I want to make clear with some

special Sentinel value that the buck

stops here what do I put what my my

options be yeah so null not n per se but

n l which was the new keyword we

introduced last week which just

represents an empty pointer if you will

technically the address o x0 so

literally the zero address and what

humans did years ago they just decided

you know what nothing should ever live

at address zero in memory we're just

going to reserve that one special bite

to be a special signal a sentinel value

such that if you ever see a zero address

in a pointer it just means it's it's

invalid it does not exist now now we

write that though a little more

pleasantly for the eyes as just n l in

all caps and that's a key word in C as

well but of course last week I claimed

that who cares where things are in

memory and honestly like this quickly

gets tedious even worrying about these

values so let me abstract this away and

propose that if we want to remember

where all of these numbers are in memory

let's give oursel one final piece of

memory that just allows us to start the

whole process let me allocate on the

left hand side here not room for a

number like 1 two 3 just room for a

pointer that henceforth I think I'll

call list by convention and then store

in that one additional pointer a value

that just kickstarts the whole process

this is the sort of treasure map if you

will that you get handed and this has

the address of the very first actual

node in memory now technically we could

just start with this but it turns out

we'll see it's just a little cleaner to

use a simple single pointer that leads

to the things you care about as opposed

to just starting with the first element

why well if you ever want to get rid of

this element it'd be nice if you could

at least still hang on to an empty sheet

of paper that indicates that the list is

empty would be one argument for that so

again who cares about these addresses

now now with the wave of the hand let's

just abstract it away and there are our

pointers each of those addresses in the

rec uh the squares at the bottom are

simply pointing to the next element in

the list the jargon to introduce here

would be that now that we have these

integers 1 2 3 but they're in these like

wrappers if you will these structures

that have metadata that is additional

data that is related to but not the data

you actually care about this is data

this is metadata this thing here

rectangularly we'll call a node n o and

it's just a term of art that means it's

like a container in code for storing

some

values this then is a linked list and

this then is the sort of graphical

incarnation of like one node pointing to

the other in this case case they happen

to be by chance and by design of this

desk contiguous initially but there's no

requirement that they be such the one

could be over there the two over there

the three over there I would just need

more foam fingers to point at one to the

next questions on this concept of a

linked

list yeah and

[Music]

back can you say that

again a good question do traditional

arrays start with a pointer that's

outside of the structure short answer no

arrays are special in C and certain

other languages and the name of an array

is technically a symbol if you will that

the computer the program knows maps to a

specific location in memory it's just a

label a synonym for a memory address it

does not take up space so to be clear

the name of an array does not take up

space like that extra Square on the left

but you do need that extra Square on the

left when implementing a link list so

that you can determine if the list is of

size zero there's nothing being pointed

at or size three in this case we're sort

of taking on more responsibility

ourselves

yeah how do you point to the next

element can you

[Music]

elaborate ah good question if each of

these elements is pointing to the next

how is three point to the others short

answer it doesn't at least in this

design we have more technically what's

called a singly linked list and as the

arrows imply it only goes in One

Direction so if you somehow find incode

maybe a for Loop maybe a while loop

somehow you're sort of encode over here

you have no way in code to go backwards

unless we changed this to a doubly link

list where I add another box that lets

me have arrows in both directions or

maybe I just kind of make it uh uh

circular and I connect the three back to

the one which you can totally do but

that tends to you know make life harder

because now you have to figure out when

you're stuck in a loop in your data

structure but it's doable as well but as

is it's a dead end by Design other

questions on this design

here all right well how might we

implement this structure in code well

let me just connect the dots to

something like we've seen before here

like this is how a couple of weeks ago

we introduced the notion of a a person

and we claimed a person might have a

name and a number last week of course we

took off some of these training wheels

and a string is really technically a

Char star in both cases but really

there's no conceptual difference beyond

that but let's use this same Paradigm to

implement a node as I described it in

that picture so let me get rid of the

name and the number because that's

related only to a person and let me

rename this structure for discussion

sake to node that then invites the

question well what needs to go inside of

a node well minimally an

integer but this is now where we need to

think a little harder just conceptually

even if you have no idea how to type it

at the keyboard what else needs to be

part of a node based on these

rectangular pictures that we've

drawn what more do we need

yeah yeah we need a pointer to another

node so if I don't know how to implement

this yet you know it could be something

like you know pointer to another node

how do I do that well you know what but

it turns out you would ideally say this

if you know that the next node is itself

a node by definition well anytime we've

needed a pointer we just use the data

type and a star and I'm going to

arbitrarily but I think reasonably call

this second square at the bottom of

those rectangles next as the name of my

attribute here but node star just

connotes that the next variable is going

to be not a node per se but the address

of a node and that's exactly what we did

you had me put Ox 456 Ox 789 in that box

which is the address of another node so

the way we would Express this in code

would be node star next but we could

call the variable anything we want now

this is a bit of a white lie but we'll

fix this right now this code won't

actually compile C takes you pretty

literally recall and if you use some

term at the top of your file that you

don't Define until later in your file

you're going to see some error message

right we've seen this when I've messed

up and forgot to include the function

prototypes at the top of my code this is

related in spirit I seem here on my 1

two 3 fourth line of code I'm trying to

use this new term of art that I invented

here in my code called node even though

it's a CS term as well but nowhere Above

This it would seem did I even Define

what a node is it's not a data type and

C every computer scientist know what a

node is but it doesn't come for free

with the language so I need to do

something else like I need this word

here to come first so that I can use it

here and so we have this sort of Catch

22 like how can a structure be

self-referential that is point to

another version of itself if the word

doesn't yet exist so the solution to

this in C which we didn't need for a

person because there was no notion of

listing connecting as a list we need one

more keyword here that we didn't need

for a person and we reuse that keyword

here so kind of an annoying detail but

if we preemptively call this whole thing

struct node you can now refer to the

thing on the inside as a struct node

star but then you can shorten the name

of the whole thing from struct node to

just node sort of an annoying sequence

of steps but in short anytime you're

building a node a linked list in memory

this is just the Paradigm you use type

def struct the name of the thing you

want to Define like node you use that

name on the inside if you want to point

from one to another and then you can

shorten it down here to just be called

node questions then on this code

here questions on what we just did well

if I rewind just a moment to that final

picture what would be the upside to be

clear of having jumped through these

hoops and added this complexity if you

will what problem did we just solve by

linking together these three values to

be clear

[Music]

yeah making lists that

are that are not contiguous if you will

so making lists that are not contiguous

in memory the upside of which is that if

I want to add the number four to this

list it looks like I could choose from

any chunks of available memory on the

screen I just need to sort of point from

the end of the current list to wherever

that other one is in memory what I don't

need to do to be clear is copy the One

the two or the three everything can just

stay put which means TimeWise I can do

this much more quickly it would seem

without copying things again and again

and even without using realloc to let it

do all of the copying potentially for me

all right but as we'll start seeing even

more in the coming weeks every time we

benefit and solve some problem we pay a

price there's a tradeoff what is a

downside as you might perceive now of

using a linked list instead of an array

yeah as much memory yeah I mean we use

twice as much memory because now in

addition to storing the integers one two

three I also need to store a pointer for

each of those and honestly even this

picture is a bit of uh simplification

technically in most systems today each

int would be four bytes technically

today most pointers though would be

eight bytes I just didn't want to draw

this weird shape on the board where the

bottom square is even bigger than the

top square but technically we're using

even more than twice as much space for

these pointers so there's that trade-off

now thankfully decades after SE was

invented memory is generally much

cheaper nowadays and so it's okay to

sort of spend more of it if you need to

and it depends on what you want to

optimize for but that's absolutely here

a downside what's another downside of

having transitioned to in a uh link list

you can't index into it now I haven't

even tried in code but when you have a

linked list you can no longer use square

bracket notation because why well square

bracket notation just assumes the

contiguousness of memory location zero

is here location one is literally one to

the right location two is literally one

to the right one to the right these

things even though I've drawn it from

right to left to just keep things pretty

there are gaps here and this is just my

interpretation of this these gaps could

be big they could be narrow they could

be down here up here they could be

anywhere so long as we're linking things

together in this list the computer can't

just use bracket zero bracket 1 bracket

two anymore because it can't do simple

arithmetic and jump to like the middle

and now here's perhaps the worst price

we've paid if you don't have square

bracket notation or really you don't

have

contiguousness what algorithm did we

just sacrifice for this

dynamism if you rewind even back to week

zero and we gave it a name in week

three what algorithm can we not use now

if we can't assume that the memory is

back to back to back to

back binary search why because binary

search just like the phone book back in

the first week requires being able to

arithmetically jump right to the middle

right take the total length of it divide

by two and boom you're right there in

the Middle with some simple arithmetic

here they might be laid out again with

these big or small gaps there's no

simple math I can do to just jump

immediately to the one in the middle and

in fact again if this TV were bigger the

two could technically be in memory be

way down here or even way over here the

foam finger could be pointing in any

number of directions depending on where

malok put the thing there's just no way

to do binary search and so it would seem

that we've paid another price indeed in

terms of it performance we're now

talking about linear time again so

that's a regression now that's also a

lot things like feels like a good time

for some muffins and fruit out in the

lobby and when we come back we'll try to

solve the problem we just created so see

you in 10 so we are back and let's see

if we can't now take some of these

higher level concepts of like stitching

together these nodes in memory and

translate it to some actual code but

we'll do it step by step first before I

actually start writing it in vs code so

if Carter you wouldn't mind helping me

step through with some visuals let me

propose that line by line we solve some

of the problems that we've just created

for ourselves in building this thing in

memory so let's go ahead and first

consider how we could build a length

list containing the numbers indeed one

then two then three and let's translate

each of those steps to code and then

we'll put it all together into something

that actually runs so how about first

step here will just be this to declare a

pointer called list that's initially has

no value at least at this point in the

story list is the name of the variable

node star just means that this is

essentially going to be our little

square over here that points to the

beginning of the list of course it's

ideal If It ultimately has a value

because when we initially I'll call this

line of code it just gives us indeed

that square over here on the left but

it's got a garbage value because there's

no equal sign on the other side there so

let's propose that we do one more step

here and actually initialize it to null

so that if only we know that it's not

garbage it at least has some known value

and null is a good way of signifying

that at this point in the story The List

is empty indeed null indicates there's

no nodes in the list so that picture

would now look like this whereby let's

just draw instead of writing null

everywhere I'll just leave the squares

blank when it's not a garbage value per

se it's literally Ox Z or null all right

so that's it for building a link list of

size zero like we're sort of done then

but we want to now add a one and then a

two then a three so next step here might

be this if I want to allocate the first

of my rectangles on our previous picture

I'm going to call malok and I'm going to

ask for enough memory to fit a whole

node now technically I think that's

going to be like four bytes for the int

and eight bytes for the pointer even

though I did not draw it to scale on the

board so that's technically going to be

what 12 bytes but again size of node

just figures out how many bytes I

actually need dynamically that's going

to return to me the address of that

chunk of memory which apparently I'm

going to store inside of a temporary

variable called n for short for node but

let's see what this does pictorially so

when this line of code is executed I

first get on the left that variable n

it's got a garbage value by default

because I haven't executed the whole

thing from right to left meanwhile on

the right hand side of the expression

I've got now a node somewhere in memory

it happened to be free here this is

where malok put it for me but it does

have two garbage values initially but

because it's a node per my type def

earlier every node I proposed is going

to have a number and a next pointer so

we can see those labeled here but

they've got two garbage values initially

but all I care about initially is that

ultimately n is pointing at that chunk

of code so initially if we could back up

two

steps we have two steps so we have initi

one step forward we have this line of

code gives us this variable here which

has garbage when this side of the

expression is executed that allocates

the memory and then when we copy from

right to left the address of that chunk

of memory that's what gives us

conceptually this arrow and the garbage

goes away because it's a valid pointer

now of course there's still two garbage

values there because we haven't set this

node to store a number like the number

one so let's go ahead and execute one

other line of code like this which while

cryptic looking is just an application

of ideas we've seen in week four and

prior star N means to start at this

variable and go there Follow the arrow

is what the star or the D reference

operator does for us and then the dot

operator recall when we first introduce

structs like for a person struct allows

us to go at the number field or the next

field so if I do star n and then in

parentheses to make sure order of

operations is preserved do number and

then assign it the actual number one

which puts the one in the top of that

rectangle now admittedly this syntax is

not very user friendly it's annoying to

remember you have to the parentheses so

there's another Syntax for this whenever

you're doing two things like this in

code dereferencing a pointer that is

going to an address and then further

using the dot notation to go inside of

the structure you find that wonderfully

C gives us this syntax whereby you can

just change the star and the parentheses

and the dot to just be an arrow and

again it's not a single character on

your keyboard it's a hyphen and then an

open angle bracket but I kind of like

the semantics of this because this code

now pretty much matches the picture n

arrow leads you to the value that you

want to access or ultimately change in

this way there's one step though we've

forgotten of course which is that we

can't leave this garbage value here

because the garbage value is some

unknown value that effectively is

pointing who knows where and we don't

want to accidentally misinterpret that

garbage value as being a valid address

and risk going there so of course what

value should we put here instead our old

friend null just to signify that this is

indeed the end of the list and we could

do that with a line of code like this

and again we'll canote as much by just

leaving that empty box blank so now we

have a list of size one let's go ahead

and add the second number to it as with

these lines here list equals n allows us

to remember that indeed we have this

list here so if we can step one step

forward here's what the picture now

looks like and technically let's go one

step further here this is now really

what's going on in memory once my list

of size exists my main variable called

list is pointing at exactly that first

node at this point in the story I don't

need to know or care about the temporary

variable that I called n even though it

might very well still be there but

indeed this now represents that link

list let's now indeed add the number two

so with the same line of code as before

I'm going to allocate another node size

of node ideally I would be checking for

null here but we're doing the juicy

Parts only on the slides let's now go

ahead and depict that so what happens

with this this brings back our n pointer

which might have been there the whole

time but we're doing this step by step

it's a garbage value though because we

haven't yet copied from right to left

Malo of course gives us a second chunk

of memory which maybe ends up there with

two garbage values by default I've

omitted the labels now just because

they're still going to be number and

next respectively once we copy from

right to left the garbage value indeed

becomes an arrow Oscar disappears

because it's now indeed a valid pointer

pointing here now the values themselves

number and next are invalid garbage

values so here is where we can now start

using our new syntax like the arrow

notation or the star and the dot if you

prefer and we can change the value of n

Follow the arrow to number and that

becomes two similarly we can do this

again and set n arrow next so start at n

Follow the arrow access the next field

and set that equal to null now we're not

quite done yet because we haven't

actually linked things together so

here's now where things get interesting

how do I combine these two well let me

me propose this let me propose on our

next line here we actually update for

Now list equal to n that is to say

whatever address this is whatever it's

pointing at change list to be the same

address that is point at the same thing

so if n is pointing here let's change

list to point here and go ahead and do

that Carter if you could I don't like

this can you go one further step this is

bad what is wrong about my sequence of

operations here where I updated list to

point my new

node

yeah yeah we lost the pointer to the

other node so I don't even care about

the ordering 21 or one two the bigger

problem now as the lack of arrows over

there suggests is that I have a memory

leak I have orphaned my original node in

the sense that nothing is pointing at it

anymore now absolutely I could fix this

by adding some temporary variables I

could add it to the mix but at this

point in the story I have not done any

such uh recollection thereof so let me

back this up and let's go forward in the

slides this is where we left off a

moment ago I think I need to take into

account order of operations and I'm

going to keep this simple I'm not going

to care about the order of the numbers

for now I'm fine with a list that is two

and then one so with that said let me go

ahead and update I think this box here

to point at my original node so let's

see how we can do this in code okay n

arrow next so n arrow next should equal

the current list and this is a little

weird again but recall what list is list

is this pointer here that just contains

the address of the original address of

the list or equivalently it contains

this Arrow whatever it's pointing at so

what this means in this line of code n

bracket next means start at n Follow the

arrow access the next pointer and set it

equal to whatever list equals so if list

is pointing here then next should point

there as well this I think is safe

because now we have redundancy now we've

got two pointers pointing at the

original list and now I think we can do

another step whereby we update list to

equal n same line of code before that

got us into trouble but I'm doing it

second now instead of first when I

execute list equals n this now sets list

equal to the same thing that n equals

and so now I have successfully inserted

my new node containing two into the list

and in fact if we advance one more we

can just clear up the Clutter assume

that the temporary variable is gone from

the story now we have a linked list

where admittedly ordering is wrong it's

21 instead of one two but at least it's

linked correctly and I didn't orphan or

leak any

memory questions on this sequence of

steps

here yeah in

[Music]

back yeah spot on so this would fall

under that category of a stack if you

will although I've not called it that by

name because I just pushed the number

two onto this data structure if you will

and indeed it ended up at the beginning

of the list instead of the end and so

here's where we see a distinction

between an abstract data structure which

is where we began a stack is a thing

like the pile of sweaters that just has

push and pop properties and lifo access

like uh last in first out how do you

implement something like that in memory

well it would seem that you could

implement the notion of a stack here not

for sweaters but for numbers using a

linked list so long as you implement

insertion AKA pushing by prepending new

values to the list by prepending again

and again and if Carter you don't mind

hitting the keyboard one more time if I

wanted to add the number three now you

would could imagine prepending it to the

list why well honestly especially as

this list gets longer and longer I kind

of like the appeal of prepending these

elements why because even if this list

gets crazy long and way way out here you

didn't notice me following all of the

arrows earlier to do the insert if I

want to insert a fourth number a fifth

number a sixth number all I have to do

is like insert it here if you will point

it at the original uh start of the list

then update this pointer and done and I

would say that's like two steps give or

take it's not going to be end steps as

it would be if I had to upend the new

nodes to the end of the list now of

course we've sacrificed ordering of

these numbers they're literally in the

opposite order or whatever order they

were inserted in but that might very

well be okay depending on the goal at

hand all right thank you to Carter for

stepping through this what if now we

wanted to translate this oh sure thank

you it's all for you none for me in this

example so here we have perhaps a way of

translating this now to some actual code

and this will be the last of like the

sort of intense code here just to give

you a sense of how we can translate this

idea now to actual step so this is list.

C and VSS code here let me go ahead and

make a couple of changes up top let me

go ahead and how about uh declaring a

node using typ def uh struct node using

our new framing as before I'm going to

give every node a number as I proposed

and every node a pointer to the next

element which is going to be implemented

just as before and I'm going to simplify

the whole name as just node so all of

that is is the exact same type depth

that we proposed earlier now let me go

ahead and get rid of all of this code

which we wrote earlier and recall that

this was the most recent version that

was not a linked list this was just in

Array that we allocated and then

reallocated so this is sort of the old

way of doing things but it was

inefficient because we might have to

lean on a for Loop or lean on realloc to

copy everything around we're now going

to reimplement the notion of a list as

an actual linked list not as an array so

my main function now might do something

like this and I'm going to really just

copy the lines of code that we just

stepped through on the board so let me

give myself a uh special variable called

list that's going to be initialized to

null and this is just my pointer the

square on the left hand side of the

screen that represents the start of the

list and if it's null it means the list

is empty so done I'm done implementing a

linked list of size zero well now how do

I want to run this code well let me

propose for the sake of discussion that

this version of the program will take

command line arguments so I want to be

able to do something like this I want to

run this program ultimately and type in

three command line arguments like this 1

2 3 and I want my program in a couple

minutes to allocate one two three nodes

and Stitch them together just like the

visualization on the board uh I could

use get int but it's just going to be

faster if we use command line arguments

so again I'm just borrowing some

Concepts from week two but none of

that's possible yet until I change my

code here so let's do this in argc uh

string uh argv but you know what we know

that strings are not actually a thing

anymore so I can change my command line

argument definition to be what it really

is it's really charar but it's the exact

same thing as in week two just strings

are no more at least without the

training wheels on anymore like last

week and now let me do this uh for in I

equal 1 uh I is less than ARG C i++ so

what I'm doing with this Loop is I just

want to iterate over the command line

argument so I have one number at a time

from The Prompt um what else do I want

to do here uh well let's go ahead and

how about do this um let's get

a number so in number equals arv braet I

so a couple of notes Here one I'm

starting my for loop at one instead of

zero but I'm going up to RC RC is

argument count how many words are at the

prompt why am I starting at one instead

of zero though given my

goal why am I starting at one

yeah yeah so the first value in RV is is

actually the name of the program that's

obviously not a number so I want the

second value so I'm going to start

iterating over those command line

arguments at I equals 1 so that's all I

just want to get the actual numbers at

the prompt um unfortunately argv bracket

I is a string AKA Char star that is not

an INT so this line of code won't work

but can anyone think back to like week

two where we had a function for

converting strings to

integers anyone yeah so a to I is a

function that converts asky to an

integer assuming what you give it as an

argument looks like a number like one or

two or three so let me fix this let me

actually do the conversion if I were

really being careful I would error check

this make sure that there's no digits

just like you might have in problem set

two but for today's purposes I'm just

going to assume the honor System that

the user me is going to run the program

correctly all right so now that I have a

variable containing the number from the

command line let's just allocate a node

for it so let me do node star n just

like we did in the visualization and

let's malok enough space for the size of

one such node here I now need to just be

super safe so if n equals equals null

like if I'm out of memory you know what

let me go ahead and just immediately

return one here otherwise if that's not

the case let me go ahead and update the

number field of this new node which it

line 24 does exist because it did not

return null so I did not exit early with

return and let me just store whatever

number that human typed in first so the

return value of a to I which per line 17

is in my variable called number and then

let me go ahead and just prepend this to

the list let me go ahead and say that um

this next field first has a known value

null just so that we get rid of that

second garbage value and let me go ahead

and now prepend it to the list so if I

want to prepend it that means this new

node must have a next field field that

points to the current beginning of the

list and again the goal here is to preen

Preen Preen so whatever the current list

is let's change it so that this new node

points to that existing list and now

step two as before was to update the

actual list to point at this node so

recall in red on the screen before I

screwed up originally and I only did

this line by moving the pointer too

early if you will but I fixed that once

Carter helped me rewind and we got rid

of the red line which indicated error

and I just do n arrow next to change the

next field of this new node to point to

the existing list so I'm not orphaning

anything all right at this point in the

story I think my code is

correct not batting very well though

today but I think my code is correct but

the program doesn't do anything

interesting so it would be nice to kind

of now iterate over this link list in

memory whatever its order is and print

things out well how do we do that well

it turns out if you want to iterate over

a linked List the general Paradigm is to

do something like this to define a

temporary variable I could call it temp

but another convention that you might as

well see is called pointer PTR for short

but you can call it anything you want

and you can have a temporary variable

first point at the first node in the

list and then in some kind of loop like

a while loop you point it at the second

node in the list and then you keep

iterating you point it at the last node

in the list and then eventually you

iterate too far effectively pointing at

null at which point your while loop can

presumably terminate so how do I

Implement that idea of allocating a

temporary pointer that just points at

each node in the list and lets me print

out ultimately each of those numbers

well let's go back to my code here and

let me do this let me go ahead and

declare this temporary pointer which is

going to be a node star also why because

it's the address of a node the first the

second the third and I'm going to set

that equal to whatever the beginning of

the list is so that is going to be

equivalent to this version of the

picture here where pointer is just

temporarily pointing at the first node

in the list it's not pointing at list

per se it's pointing at the first node

in the list which list is also pointing

at itself all right once I've done this

I think I can translate this to code

that's a little new but it's

conceptually familiar perhaps now while

that pointer does not equal null so

while I have a valid pointer like my

finger or that arrow is pointing at an

actual node in memory well let me go

ahead and print it out so let me print

out with percent I back sln whatever is

in the current node at the number field

within and again this is going to have

the effect hopefully of first printing

the three and I think I just need to Now

update the pointer so that on the next

iteration it's pointing at the next

value so if this is where the story is

how do I update pointer to point at the

second element of the list well I want

pointer to point at the two and I want

pointer to eventually point at the three

well how do I do that well the way in

code I can follow these arrows is as

follows if I currently have pointer

pointing at this node but I want to

point it at the next node I can borrow

this pointer here so whatever this

address is in the first node aka the

next field I can copy that into pointer

because then pointer will point at

whatever this is pointing at by just

setting one equal to the other so once

I've done that the picture will

become this and how do I translate that

to code it while new syntax is

surprisingly straightforward all I need

do is say pointer after printing it

equals whatever pointer currently is but

grab its next field

instead and this is a very common

Paradigm when iterating over a link list

and you're using some temporary variable

like pointer you can simply set pointer

equal to pointer next and what that

means here is as follows if this is

pointer pointing from here down to here

pointer next is Follow the arrow grab

the next field so if you set pointer

equal to this thing that's the same

thing as pointing this at this same box

and indeed if I advance to the next

slide even though the arrows are

technically pointing at different parts

of the rectangles that's just for

graphic sake pointer is now pointing at

the second node and when I do this again

on my next iteration it points at this

and then this last step notice when I

keep doing pointer equals pointer next

this will become eventually this value

but what's this value in this link list

it's null technically so this Arrow will

eventually take on this value when I set

pointer equal to pointer next and at

that point PTR my temporary pointer is

going to be null so it might as well

look like this pictorially and what does

that mean for my Loop once pointer is

null because you've walked off the end

of the length list what's going to be

true of this Loop here started in line

32 any OB observations

here what's going to be true what will

happen now as soon as we hit the end of

the list yeah

sorry the loop is going to break out why

because line 32 which is constantly

asking well pointer does not equal null

well if pointer finally equals null

three steps later the four the while

loop is now done and so what I can do at

the end of this program once I've

printed out those values well first

let's go ahead and open my terminal

window let's make list okay a compile

do/ list and let me try the same values

one and two and three that's going to

again allocate one node two node three

nodes by prepending prepending

prepending each of those values and it's

then going to iterate over them from

left to right and so when I hit enter

now what should I see on the screen if

my code is

correct what will I see feel fre to just

call it

out 321 because I've prepended

presumably and here we go I indeed see

321 so the list is is backwards but all

of the elements are there now

technically if I ran valgrind on this

valgren would not be happy because I

have never freed any of my memory so I

should probably now have a second Loop

here that does something like this let

me again set pointer equal to list I

don't need to redeclare it because I've

already created this thing on line 31 I

just want to reset it to be the

beginning of the list again and now I

can do the same kind of thing while PTR

not equals null go ahead and do this

well I don't want to just do free

pointer and then do pointer gets pointer

next y my goal is to free all of my

memory but I think this is going to get

me in trouble pointer equals list just

gives me a temporary pointer that points

at the three and then eventually the two

and then the one how well while pointer

not equal null I'm freeing the pointer

so this is like saying to Malo free that

node free that node free that node but

what's the problem with what I've just

done

here this code is technically Bugg

[Music]

yeah exactly after you call free on

pointer You Are by social contract with

c not allowed to touch pointer anymore

it is invalid now it's still going to be

a number it's still going to be a

pattern of bits but it's invalid and

you'll very often get a segmentation

fault if you tempt fate in that way so I

can't free the pointer and then use it

literally the next line the solution

here kind of like our swapping of the

liquids last time was to maybe just have

a temporary variable so I can do a

Switcheroo and so a common way to solve

this problem to get the order of

operations right would be to do

something like this give yourself a

temporary pointer like node star next

set it equal to the place you want to go

next so one step ahead now you can free

pointer and then you can update pointer

to be that next value so essentially you

need need like two hands now you create

on line 41 another pointer that if this

is pointing at the first Noe the three

your new pointer is pointing at the two

temporarily so now you can tell malok

via free release this memory but I

haven't forgotten where I want to go

next and so I can now continue on so a

common Paradigm for just iterating over

these nodes and then freeing them a

couple of observations strictly speaking

I could have Consolidated this I don't

need two Loops to print the nodes and

then free the nodes I could do that all

at once but let's assume that there's

other stuff of interest in my program

and I don't want to just immediately

free it there's one other bug that I

should probably address here there is

still a potential memory leak up here

and this one is super subtle the valind

would help you find it notice that in

this Loop here when I'm calling malok

this line of code is fine if the first

line of malok fails and returns null

because I immediately return and I'm

done but what if the second call but not

the first or the third call but not the

first or second fail this line of code

has me returning immediately you really

need to to do some garbage collection so

to speak whereby you really need to go

in and free any nodes that you did

allocate successfully earlier honestly

that's going to be a pin in the neck we

won't do that here but probably what I'd

want to do is write a function called

free list or something like that and

call that function to free any nodes I

had previously created so it's not quite

at the finish line but the building

blocks are indeed here questions on this

code and I think it's safe for me to

promise that it won't escalate further

from

that questions on

[Music]

this no well let me show you one

alternative that you might prefer and

I'm pretty sure this isn't an escalation

it's just an alternative formulation

another way you can iterate over nodes

in a list could be this instead of a

while loop for instance let me actually

show you one other piece of syntax here

you could technically use a for Loop you

could give yourself a node pointer here

that initialized is initialized to the

list you can then check in your for Loop

that it's not equal to null and then you

can do your update as usual like this

either of these are equivalent even

though this one I suspect looks scarier

it's doing the exact same thing in one

line instead of two but there's no

reason we can't use four Loops instead

of while Loops to achieve the same idea

but I'll leave these two as

demonstrations of one approach or the

other but that's just like in week one

four Loops while Loops whatever looks

simpler to you even though admittedly

neither of these probably looks super

clean all right so let's take the back

to things more conceptual here up until

now we've been inserting elements into

this link list by prepending them let's

consider what the running time then is

of these operations so if I've got a

link list of size three or size N More

generally time has passed and I've added

a lot of things to it what's going to be

the running time for instance of

searching a linked list for some value

and I'll tell you already it's not login

because again binary search is off the

table as per before break so what might

the running time be of searching a

linked list for some value like two or

three or 1 or

50 what might the running time

be o of I heard it over here o of n and

y who was that oh in the middle here why

o of

[Music]

n exactly you're going to have to go

through every item in the list starting

from the left from the beginning which

is how we've been drawing things and

connecting the dots and in the worst

case the element might very well be at

the very end so it's going to be Big O

of n what about insertion how many steps

in terms of Big O notation has it been

taking me to insert elements into the

link list using this prepended

design yeah so it's technically constant

time Big O of one and again one is just

representative of any constant it could

technically be two steps or three steps

or even 10 steps or 100 steps but if

it's always finite and fixed then indeed

you can say it's in Big O of one now why

is that well over again no matter how

long this list gets so long as there's

memory available for me I can just kind

of uh create a little splice at the

beginning of the list to put in the new

node update the original list and I'm on

my way and it keeps getting longer even

though it might not be spread out in

memory so big O of one is possible with

these link lists if I indeed Preen

things of course if I Preen things

everything's going to get out of order

potentially and we're going to have

maybe the stack property instead of a q

property so we might want to do things

slightly differently so instead of doing

this

whereby we kept prepending prepending

prepending suppose we append to the end

of the list instead so if we now insert

the one the two and the three as we

might want to for a q to maintain that

fairness property we might start with an

empty list we might add the one we might

append the two append the three and so

it just is sort of laid out differently

in memory and again if I can come to you

in the middle what's the running time of

search again when the link list uses

this append

implementation yeah still Big O of n

because in the worst case you're going

to have to go through the whole list

just to find it and notice it doesn't

matter if you have an intuition now that

the bigger numbers might very well be at

the end you have no way to jump to the

end you have no way to jump to the

middle or do anything resembling binary

search every search has to start from

the left and follow the arrows again and

again all right so I don't think we've

done any better there and in fact what

is insertions running time now in Big O

when we're appending to the list in this

way as we might to implement a Q instead

of a stack what's the running time of

inserting a new

value Big O

of so not Big O of one in this case but

Big O of n because if I'm appending by

definition I have to start here and

Traverse the whole thing looking for the

end now this is a bit of an

overstatement right you could obviously

optimize this slightly by maybe adding

another variable that always points to

the last element sort of a cheat sheet a

shortcut that gets you all the way to to

the end that's totally fine it's not

doesn't really fit the traditional

definition of a singly linked list but

there's absolutely smart engineering

solutions to these kinds of problems but

as designed it would indeed be Big O of

end to insert to if you've got to go all

the way to the end and you're not using

a little extra memory to get yourself

there quickly well what if we want to

take things one last step and not just a

pen blindly because even though I

inserted one two three if I inserted

them in random order they would end up

in random order what if you want to

maintain a sorted list from smallest to

largest well then you might want to

insert numbers like this starting from

an empty list we might have a two then

we might try inserting a one but we want

to keep it sorted so now we're going to

prepend in our code but then you might

want to insert a four so you would

append the four because you're probably

going to look for the right spot to

insert it then we're going to insert a

three and this one's getting a little

annoying because now you have to like

iterate over the list look for the right

spot and then do a little smarter of a

splice but it's possible but you don't

want to Orphan the four for instance and

then ultimately we get back to this

question what would the performance be

of your linked list if you're trying to

maintain sorted order well search I

think is going to be Big O of n for the

same reasons as before what about

insertion big go of what for inserting

into a sorted linked

list yeah in the worst

case yeah it's still Big O ofen so it's

no worse than but it's not really any

better than a pending but we gain the

additional property of maintaining a

sorted list which might very well be

useful if you're sorting your contacts

in your phone or something like that

where it just maintain makes sense to

maintain sorted order now in the code

for online today if you take a look at

some of the final versions of code like

list 6. C and list 5.c as we'll post on

the website you can actually see code

that will solve all three of these

problems the Preen version that we wrote

live the appen version which we talk

through as well as this sorted order one

but I think I'll avoid showing it live

just because I do think that starts to

escalate quickly but I think we have

enough of a building block if we're

comfortable with prepending to at least

solve some real world problems with

these link

lists questions then on link list which

we'll now leave behind on their own but

now use this technique to solve fancier

problems but much less

code questions on linked list all right

so to C recap we've kind of taken a s

side step with link list like we have

this dynamism now where we can grow and

Shrink our chunks of memory without over

allocating or accidentally

underallocation as in the world of an

array we don't have to worry about

copying values endlessly because once

you allocate the node it can just stay

wherever it is in memory and you can

just maintain uh you can just Stitch it

together somehow but unfortunately we've

sacrificed what we started the class

with in week zero which was like binary

search divide and conquer which was like

gave us that log and running time which

was really compelling if you think back

to the demonstrations and the the

visuals can we get the best of both

worlds can we get the sort of uh speed

of binary search something logarithmic

but the dynamism of something like a

link list well we can actually I think

if we start to think not in a single

Dimension just the x-axis if you will

but two Dimensions such that our data

structures can maybe now have width and

height if you will and so a tree is

perhaps the right term here much like a

family tree if you have sort of your

elders up here in the tree and then the

branches below them for their children

and grandchildren and the like that's

actually what a computer scientist means

when they talk about trees not a tree

that grows up like this but really one

that typically is depicted growing down

although this is just an artist's

depiction no matter what but there are

certain types of trees in the world

called binary search trees that are

structured on paper and in visually like

a uh family tree but they have a special

property that lends themselves to

exactly that feature binary search so

for instance here is an array back from

week two and I've sorted a whole bunch

of numbers here in from 1 to seven we

know we can do binary search on this

structure if it's implemented as an

array but what feature do arrays to be

clear not have that link lists

do today's kind of a seesaw like what

did we just gain by adding link list

that arrays do not

allow

yeah

yeah you can insert more elements

without having to copy or moving

everything else around like right now in

this single Dimension if these values to

the left and or right are already used

then you have to move everything and

that's where we started today's story so

arrays kind of paint you into a corner

because you have to by definition decide

in advance how big they are well

couldn't we have some kind of array that

can still grow but still is contiguous

so we can do binary search in some way

well yes if we sort of rethink how we

Implement binary search Let Me propose

that this I've chosen these seven

elements in the array much like the

lockers from uh week two to be ordered

from smallest to largest I've

highlighted now in yellow the middle

elements here and if we were telling the

story of week two going left or going

right let me highlight in red the middle

elements of the left half and the right

half and then let me further highlight

in green the other elements in between

those and there's a pattern here as you

might notice whereby there's one yellow

in the middle and then there's the two

red and the four green there's kind of

an implicit structure there if you will

and what if I do start to think in two

dimensions and instead of laying out an

array of lockers like this on the x-axis

only what if I kind of like Slide the

four up and pull the uh the the one the

three the five down and kind of draw

this in two Dimensions instead well let

me do that as by separating these things

like this such that now let me propose

that each of these squares maybe it

doesn't have to be contiguous it can be

anywhere in the computer's memory but I

can't have these crazy gaps among them

how could I perhaps keep these things

connected conceptually what should I add

to the picture if you

will yeah branches say again so branches

metaphorically here and more technically

in the language of C maybe just some

arrows some pointers so I won't bother

drawing things as rectangles constantly

let me propose that we're now just

abstracting away what a node is but let

me claim that each of these squares now

is a node and a node might have a number

but it might also have a pointer heck

maybe even two or more pointers and let

me draw those now I don't care about

addresses like ox1 2 3 4 5 6 7 8 n

anymore let's just draw our pointers

with arrows but now let me propose that

we could very well think about this as a

tree storing what was previous

previously array data but now each of

these nodes can be anywhere in memory

and moreover even though I've kind of

painted myself into a corner visually on

the screen so long as there's more

memory in the computer I could put the

number zero over here I could put the

number eight over here and I'm if I'm

smart I could probably if I want to

insert other numbers like 2.5 or 1.5 or

values in between you know I bet we

could kind of make room by swiveling

things around and just kind of hanging

things off of these branches slightly

differently and so what does this gain

me well if I instead start to model my

data not single dimensionally but in two

dimensions and I connect those nodes

with these pointers what can I now do I

think I just gave myself back binary

search why suppose I'm searching for the

number five how do I find it Well just

like in a family tree where you might

visually start reading from top to

bottom I'm always going to start from

the so-called root of a binary search

tree this is just like the list pointer

that kicks off the whole link list

process

this is the so-called route all right

here I am at the number four I want to

find the number five what decision can I

make when I see that I'm currently at

the number four just like the phone book

from week zero where is five

not it's not to the left and if I were

had you know built a little mobile here

or something we could very dramatically

snip off this Branch this is this is

like very lowbudget animation these

nodes could like fall to the ground and

we're left with half of essentially a

tree but what do I now know it's

obviously the five to the right so let

me go to the right six is obviously not

the one I'm looking for but what do I

now know about the five well five is

less than the sixth so I can sort of

snip this off here because I know it's

not going to be down there and I can

follow the remaining Arrow here and

voila I just found it and now without

getting into the weeds of the math I've

got here what Seven Elements that's

roughly eight if I round up and if I do

some log base two I actually 1 two three

is the key detail here the height of

this tree

is three because I took a list of size

seven and IED it and IED it in order to

let it dangle in these two Dimensions

plus or minus one for rounding sake so

what do I get back I now have binary

search but it's not like H the middle of

the middle of the middle I now follow

these arrows in one of two directions so

each of these nodes now has an INT and

maybe a left pointer and a right pointer

but you can call them anything you want

and so I've gotten back binary search

and dynamism because if you want to add

zero or eight or 9 or 10 we can just

dangle them at the bottom of the binary

search tree so what would this look like

in code but we won't actually implement

it line by line well here was previously

our definition of a node for a link list

which was onedimensional if you will

even though it might bounce up and down

on the screen it was still just a line

if you will well let me get rid of the

single pointer in the linked list let me

make a little bit of room here in this

type death and let me propose that we

just add two pointers each of which is a

struct node star one will be called left

by convention one will be called right

by convention and so long as someone not

me not today not in class writes the

code that stitches together this data

structure too handling both the left

child and the right child so to speak I

think we can indeed stitch together that

two-dimensional structure and moreover

once you have this in memory you can

translate pretty elegantly to code

binary search itself using a principle

we talked about recently too here is for

instance a function that I'll write by

just clicking through steps called

search whose purpose in life is to

return a Boolean true or false the

number I'm looking for is in the tree

this search function therefore takes two

arguments the number I'm looking for

called number and then a pointer to the

tree the so-called root of the tree now

how can I Implement binary search in

code will recall our brief discussion of

recursion it turns out recursion is a

beautiful technique and honestly more

obvious technique when you have two

dimensional structures which finally

after five plus weeks we now do here's

maybe my first line of code here if the

tree is null then obviously return false

you've handed me an empty tree there's

nothing going on obviously the number

you're looking for is not going to be

here so that's my like safe base case to

make sure I don't screw up and recurse

infinitely well what else might be the

case well if the number I'm looking for

is less than the tree's own number and

now recall that trees a node star so

even though I'm calling it a tree it's

really the current node that's been

passed in so if the number I'm looking

for is less than the current nodes

number then I must know that the number

I'm looking for is to the left so to

speak so how can I solve that well this

is where the magic of recursion just

return whatever the answer is to calling

search again but on a sub tree if you

will this is the sort of equivalent of

snipping off half of the tree pass in

the left sub tree if you will with the

same number else if the number you're

looking for isn't less than the current

nodes number but greater than snip off

the other subtree instead and just

return whatever search says it finds in

the right subtree here and then there's

a fourth and final case what else could

be true

logically

[Music]

yeah perfect if the number you're

looking for equals equals the number in

this node then I'm just going to return

true and you might recall from our

recurring discussions of design I don't

strictly need to ask that explicitly

either there's no node it's to the left

it's to the right or you found it so I

can just Whittle that down as usual to

an else and this now returns my true so

here too this is where recursion once

you get comfy with it sort of gets

pretty elegant and cool in the sense

that wow even though there's a lot of

lines here I mean there's only a few

interesting lines a lot of it's like

Curly braces at that which strictly

speaking I could get rid of and so

recurs lends itself to Elegance when it

comes to traversing these

two-dimensional data structures as well

so that is in code how you might

Implement something like

search questions then on these

trees we have dynamism we can insert

more nodes to them they're faster

because we get B search back but but but

there's got to be a price paid any

downsides or question or

downside okay let me come back to that

in just one sec downside though what

price of we paid for this dynamism and

for this binary

searchability even though I've

abstracted it away in the

picture say

again we're using a lot of memory right

I'm kind of misleading you now because

I'm just drawing these little squares

with the simple numbers but there's

actually three things in there a four

byte integer an 8 by left pointer a 8 by

right pointer so we're already up to 16

20 bytes now to store individual ins

that's probably okay though if memory is

relatively cheap and voluminous as it

nowadays is but these are the kinds of

trade-offs and here too you see a hint

of why some people still do like and use

C and in fact it's so omnipresent

because when you have C you can really

fine-tune how much memory is being used

for Better or For Worse under the hood

as we transition soon to python these

decisions get made for you and you have

much much less control about how many me

how much memory is being used by your

program because someone else made those

designed decisions for you

question

is it bad if we don't know the parent

node uh not necessarily there's no

reason why you need to have pointers in

both directions however that can lend

itself to efficiency by spending more

space and having arrows go up too you

can actually save more time when

searching the tree in other context this

though would be the canonical way the

typical way to implement it um but

absolutely just like a doubly link list

that could help you solve other problems

too all right so turns out I'm kind of

overselling binary search trees there

are perversions of them so to speak

whereby they won't actually behave as

advertised for instance here's a a good

situation suppose you've got an empty

tree initially and you insert the number

two well it's got to go somewhere so it

might as well become the root of this

binary search tree and let's assume that

someone wrote the code to do this now

you want to insert the number one and

you want to maintain the searchability

of this tree well it's important to note

that binary search tree is different

from tree if you just got a tree in

memory there is no social contract with

where the numbers need to go they can be

completely random all over the place

binary search tree means that you can do

binary search means that any node here

is going to be greater than every node

here and less than every node here and

that's a definition it's a recursive

structural definition that must be true

to be a binary search tree or BST so if

we maintain that property ourselves let

me insert two let me insert one one

belongs there by that definition let me

insert three three belongs there by that

definition but I kind of got lucky in

that I in the story inserted two and

then one and then three let me propose a

sort of perversion of the algorithm

whereby we just get unlucky let me

propose that we insert in uh insert one

first and then we insert two well where

does two go well logically it goes to

the right because it's larger all right

now the user inserts three where does it

go okay it goes there logically and how

does this story un fold the user inserts

four five six it's wonderfully sorted in

advance by luck but this is a perversion

of the structure in what

sense it's still technically a binary

search tree but what does it look more

like it really is devolving if you will

into a linked list and so if you the

programmer don't Implement a binary

search tree with some kind of repairs

going on such that as soon as something

gets whoa a little too long in stringy I

think I can fix this it's going to be an

annoying line number of lines of code

which we're not going to write here or

or in a pet but we could kind of pivot

this thing right and we could just

rejigger things so that the two becomes

the new root the one becomes the left

child the three becomes the right child

but that's what like two three plus

lines of code it's possible it's doable

but it's it's extra work it's extra code

so unless you write that code though and

maintain balance of these trees just

because it's a binary search tree does

not mean its height is going to be log

base 2 of n the height could be log it

could be n in which case you don't get

those properties so when it comes to

looking up in a balanced binary search

tree yes it's log in but if it's

unbalanced if you don't add that

additional logic and those repairs so to

speak you could it could devolve into

Big O of N and this is a whole category

of algorithms and fanciness that you

would explore in a higher level course

on algorithms and data structures

there's lots of way to do that sort of

fixing that I'm alluding to in the

picture there on the screen

screen all right a few other data

structures if you will toward an end of

a sort of computer science Holy Grail so

log in is repeatedly a really good place

to end up we started in week zero when

we got log in we lost it this earlier

today by introducing link list but we

just got it back albeit at the price of

spending more space but the Holy Grail

so to speak when it comes to algorithms

would not be Big O of n certainly

definitely not n squar like our mer like

our bubble sorts and selection sorts and

not even Big O of Logan what's better

than all of

those big O of one constant time right

that's the Holy Grail because if we

could store huge amounts of data but

find it instantly in one step or two

steps or heck even 10 or 20 steps but

independent of the size of the data

structure that's pretty powerful I mean

that's the secret sauce of the Googles

and the twitters of the world trying to

get back results really really fast well

it turns out another abstract data type

or abstract data structure might be

something called a dictionary just like

the maram Webster Oxford English

dictionaries that you might know which

associate say words with definitions

well you can think of a dictionary

really abstractly as this like two

columns maybe on a spreadsheet of sorts

where the left column represents

something and the right column

represents something else like the word

is on the left and its definition is on

the right and that's almost literally

what a dictionary is on paper you've got

all the words and all the definitions

right next to it but more generally in

Computing a dictionary really just has

not words and definitions per se but key

value pairs this is a term of Art and

we're going to see this again and again

especially as we transition to web

programming keys and values key is what

you use to look for something the value

is what you find ultimately via that key

so that's the generic term there we've

seen key value pairs really in the past

in week zero we talked about your

contacts in your iPhone or Android phone

uh being an app that has a whole bunch

of contexts presumably alphabetized by

first name or last name or the like well

one one of those contact cards

ultimately has someone's number for

instance like John Harvard in this case

so in that type of application the keys

is the name like John Harvard that you

use to find information and the value is

the number that you find there or if

there's more information like where he

lives and uh email address and the like

the whole contact card could be the

value thereof the key is what you use to

look up John Harvard now back in week

zero oh and rather the corresponding

table then if we draw this in two

columns wouldn't be word and definition

or key value generically it would be

name and number for instance so we're

just slapping some new terminology on

this old contact problem well this is

the picture we drew way back in week

zero whereby I claimed that log of n was

really really good and indeed it was and

has been since but the Holy Grail would

indeed be something more like this in

this dashed Green Line constant time and

maybe not literally one step but a fixed

number of steps that even as the problem

gets huge and you go way way out on the

right of the X AIS the problem does not

depend on the side the uh the uh the

time to solve the problem does not

depend at all on the size of the problem

itself you can have a thousand contacts

or 100,000 contacts constant time means

it takes the same number of steps no

matter what well how can we get to that

point well there's a couple of final

building blocks today and there's one

called hashing and this is something

that will recur a few times but for now

hashing is all about taking as input

some value

and outputting a simpler version thereof

so for instance here's a a gratuitously

large deck of cards which are all the

more visible as a result and in a deck

of cards typically you've got like what

52 cards plus maybe the Jokers and

whatnot and each of those cards has a

number of sorts and a suit on it and

here are literally four buckets on the

stage and how might I go about sorting

these cards not just by number but also

by suit well you could certainly like

spread them all out and sort of make a

mess of things and just kind of reason

your way through it and get everything

in order according to suit and corded by

number but most of us even if you don't

have four buckets at home probably are

going to do something a little more

intuitive feels like an optimization

where if I find like the nine of Hearts

I'm going to put that into the hearts

bucket the King of Spades I'm going to

put that into the Spades bucket the jack

of diamonds over here and I'll do this

with the Queen of Diamonds and uh the

Ace of clubs here and the three here and

the 10 here and even though it's still

going to be 52 steps why am I and maybe

at home like why would you perhaps do

this step

first what's the value of bucketization

reduces the probability of errors or the

like and what I'm doing here to give it

a technical term is that I'm hashing the

values I'm taking as input a card like

this and I'm reducing it more simply

from a larger domain to a much smaller

range if you will so here's a domain of

like 52 possibilities I want to map that

to a range of four possible outcomes the

Diamonds the clubs the carts or the

Spades here and by doing that I'm just

shrinking the size of the problem so

hashing does that it's like literally an

F ofx type Arrangement whereby you pass

something in and you get back a simpler

known value well a hash function more

technically is the algorithm or even the

math or even the code that implements

that idea converting something bigger to

something smaller to this indeed finite

range of values and it turns out that

hash tables are a wonderful application

of arrays and length lists to try to

Leverage The Best of Both Worlds the

goal being theoretically to achieve that

Holy Grail of constant time and that's

going to be a bit of an overstatement

because you're not always going to

achieve it exactly but at least we can

get a little closer there too so with

hash tables you have something that

looks like this this is just an array

this is an artist rendition of drawing

it vertically instead of horizontally

but that's just a a detail graphically

and this array for instance maybe uh is

of size 26 and where am I going with

this well how does Apple how does Google

Store your contexts alphabetically in

your phone and search for things quickly

well they might they probably

alphabetize at least in English

according to a through z or if we

convert that to numbers it's like what

65 through whatever or really 0 through

25 suffices if we're using an array of

size 26 we start counting at zero and we

count up to 25 but let's abstract that

away as just letters of the alphabet so

maybe what Google and apple are doing in

your phone is storing all of the A's up

there all of the Z's down there and

everything else in between and so this

works pretty well if you start adding

your friends and your family so for

instance and I'll get rid of the letter

so it's to not distract uh alvus might

go in that first spot because a you

subtract the 65 maps to zero so we put

him in the first bucket the a bucket uh

maybe Zacharias ends up all the way at

the end there and then in the middle

might here be Hermione and if we do this

dot dot dot you keep adding all of your

classmates you might get a uh contact

database that has all of this data here

in now each of these nodes they're drawn

differently because this is just another

artist rendition these rectangles these

long rectangles represent a contact card

like John Harvard's that's got the name

maybe email definitely phone number and

things like that so this seems great why

how can I find Albus well I go to the a

bucket how do I find Zacharias I go to

the Z bucket how do I find Hermione I go

to the H bucket but but but I've done

this very deliberately what problem will

arise eventually assuming you have

enough classmates

yeah there'll be too many people too

many contacts for all of the available

spaces in the array there's still some

room here but I'm pretty sure if I think

back to this particular class uh we've

got not hermy but also Harry who's also

an H Hagrid who's also an H so where do

I put them I could just put them

arbitrarily in any of the open spots but

then you lose the immediacy of jumping

right to the H right to the a right to

the Z but now that we have link lists we

can kind of combine these ideas right

use an array to get to the first letter

of the name you care about and then if

you have a collision so to speak whereby

someone's already there you don't do

something stupid like put Harry down

here just because it's available or

maybe Hagrid down here just because it's

available because then you're losing the

immediacy of the lookup why don't you

just kind of stitch them together in a

linked list now what does this mean this

means for most of the characters here

you have constant time lookup you look

up alvas boom you're done Zacharias boom

you're done okay Harry Hermione Hagrid

it might be one two or three steps so

that's actually devolving into something

linear but here we make a distinction

today between theoretical running times

which we keep talking about and honestly

a clock on the wall running times that

actual humans care about this is way

faster than a linked list because you

don't have to search every name it's

even faster than a an array because you

don't need to do binary search

you can literally for most of the names

find them in constant time one step and

again it's not theoretically constant

because these if you only befriend

people who have H names it's going to be

a crazy long link list anyway so again

it really kind of depends on what the

nature of the data is here but this is

pretty close to constant time and in

fact how could we get even closer how

could we reduce the probability of

collisions for the H's or any other

letters how could we avoid putting too

many H names together

say a little

L okay yeah so we could add another

dimension if you will but let's not add

a third dimension per se but let's

indeed look at not just the first letter

of everyone's name but the first and the

second and in fact let's see if that

gets us a little uh further along so let

me go ahead and propose if you go

through the whole Harry Potter Universe

there's actually a lot of collisions if

we keep going and so we've got the L's

here the RS the S's and so forth well

let's clean this up here Hermione

originally went to the H location but

let's decrease the probability of

collisions there and everywhere else

instead of putting hermion Harry and

Hagrid Al together let's go ahead and do

this instead instead of labeling these

buckets A through Z let's just give

ourselves more buckets so in fact this

might be H well instead of H maybe this

should be ha and then this should be HB

HC HD h e HF now some of those are a

little nonsensical cuz I can't think of

names that match most of those but it's

deterministic at least we know the

bucket will be there which is important

even if it's empty and now we can put

Hermione here we can put Harry here but

uhoh we didn't do this perfectly well

Hagrid still Collide so let me come back

to you how can we reduce the probability

of Harry and Hagrid

colliding yeah so we could look at the

third letter okay so let me try that

instead of ha let's look at haa ha ha C

dot dot dot haq dot dot dot h h e q h e

r hes and so forth and now I think those

names and probably all the others we saw

are now much more cleanly distributed

there's much lower probability of

collisions unless two people have like

almost the same names or one is like a

prefix of the other but but but even

though we're now closer than ever to

constant time because the odds that we

hit a collision and have to devolve to a

link list or much lower what's the

downside that's not completely obvious

from how I've depicted this on

screen what's the price I'm paying here

yeah this is a huge amount of memory the

number of cells here in the array is now

what 26 * 26 * 26 for the first the

second and the third possible characters

all combinatorically combined here

that's a lot I didn't even draw them I

have the dot dot dot to evoke that

instead that's a huge amount of memory

this is a very sparse data set now and

odds are you're going to waste so much

memory even for the names like Hae ha

like HQ like I can't even think of names

so many of those buckets are going to be

empty not to mention the AAA and the zzz

and everything else in between so it's a

trade-off and it might be too expensive

a trade-off and so you might have to

tolerate something like the collisions

we had earlier whereby even though they

might very well happen at least you uh

are decreasing the probability by

perhaps having more buckets like this

and in fact if I rewind now to where we

might have gone with this here here's

how we might represent these nodes in

the tree previously in the past we've

had a person who had a string name and a

string number AKA now Char star and so

here now might be how in this hash table

we represent someone's name and number

as well as a pointer to the next element

in the list we let me rewind just to the

picture here we keep drawing different

shapes because again these are

abstractions who really cares if they're

to scale now we've got enough room for

the person's name not pictured on the

screen is herm's number that's somewhere

in this rectangle but yes pictured here

in this little square is a pointer to

the next node in the list so by storing

name and number maybe her address maybe

her mailing address whatever in addition

to a pointer allows each of these nodes

to be connectable just like the nodes in

a linked list but where they're starting

is in an array so the array gets us like

126 of or gets us um Narrows the problem

from size 26 to one gets us to the link

list in question hopefully it's a single

person or perhaps it has more than that

meanwhile what is the hash table itself

the hash table the whole thing is

literally just an array I've hardcoded

the simplest version of size 26 but what

do each of those boxes in the vertical

array represent just a pointer to

potentially a node a node in a length

list and if there's no one there if

there's no one in location y or X or the

like in that Universe well it's just a

null pointer signifying there's no uh

one there but if there is it's going to

be a pointer to a valid node from which

we can get to any of the others as well

and that so-called hash function just

like the one I did with the greeting

cards well it's just an a black box if

you will but implemented somewhere in

code like in C and so if you pass in

Albus what is the hash value of Albus

well in the first version of the story

with 26 buckets it should be a zero if

you pass in Zacharias it should be 25

and so just as my cards were being

hashed to one of 1 2 3 four values now

these names are being hashed to one of

26 possibilities or 26 time 26 or 26 to

the 3 power if you have more and more

granularity than

that questions on this implementation

now of this idea of a hash

[Music]

table if uh if you say that again with

the

[Music]

null oh a good question so if there's so

much spareness there's all of these

empty cells in the array couldn't you

just go in and free them or delete them

or just kind of shrink the array and not

have a a AA and aab and a a only have

the prefixes two or three characters

that you need you absolutely could do

that but now what you you lose is the

arithmetic benefit of being able to map

each letter to a number if you start uh

freeing up unused space you don't know

that Zacharias is necessarily at

location 25 Albus is still going to be

at location zero but if you've deleted

some of the elements in the middle

Zacharias could be at 24 if you've

deleted one 23 if you've deleted another

and so you don't have that arithmetic

immediacy that you need in order to

index into the array with constant time

and the same is going to be true if it's

uh two letters or three letters you need

to be able to trust that you can do some

quick math and jump to the right index

in constant time and that's again the

appeal of these arrays so when it comes

to the running time of a hash table

inserting values into it searching for

values therein at the end of the day it

technically is Big O of n because in the

craziest case you might have a huge

fancy hash table but everyone in the

universe has a name starting with h and

then it just evolves into a really long

link list just like a binary search tree

could do the same but if you choose a

smarter hash function maybe you mitigate

that and you don't rely only on the

first letter but on the second or the

third as well or some other combination

of that input and make your hash

function smarter odds are if you get a

good hash function you want it get it to

be more of like order of n divided by K

where K means constant mathematically

and so K is the number of buckets so

ideally you want like a uniform

distribution you want like this many

people here this many people here you

don't want there to be some or no people

you want a uniform statistical

distribution and maybe you get that from

Human names maybe you don't but that's

kind of the challenge of a hash function

of course Big O of n over K is not a

thing because we always throw away

constants like K so it's still Big O of

n but again the distinction today is

that okay yes academically you learned

in cs50 that sure it's Big O ofin but my

God it's 26 times faster if you do the

hash function well and you spread

everyone out over the hash table and

that's the appeal of these kinds of

structures

and we've got one more for you if I may

something now known as a try so it turns

out that a try is even cooler if you

like this kind of thing in that it does

not devolve into Big O of n it is truly

constant time but there's going to be a

price there's going to be a gotcha a try

is sort of a fancier tree and it's short

for retrieval but pronounced try for

weird historical reasons but a try is a

tree each of whose nodes is an array

right so this is all like crazy mashups

now people started inventing data

structures just by combining different

ones unfortunately a lot of the good

ideas are taken but you just have

benefits from certain aspects of those

data structures and combining them

ideally gives you the best of both

worlds so to speak so here might be the

root of a try it's literally a big node

a big rectangle but it's actually an

array so there's like 26 locations in

this picture here and here's how you use

a try for instance to store names just

like the hash table it you treat each of

the elements of that array in that node

as like a letter of the alphabet so a

through z or 0 through 25 and if you

want to store someone's name in here you

do so as follows if you want to store

like uh an H you index into the H

location and if you want to store the

second letter of someone's name like an

a well you add another node below it and

such one is connected to the other and

you then identify the a in that array

and then you go on and maybe put a g if

the goal is to store spoiler now Hagrid

in this data structure and then the r

and the I and then the D but when you

get to the D the end of the name you

have to somehow flag that this is the

end of a name that we've embedded into

this data structure so whereas all of

these are called out in white just to

make obvious what we're connecting to

what green has to be like a bull that's

true that just indicates like the buck

stops here like d is the last letter in

someone's actual name and what's kind of

cool now about a try is that we can

repeat this for other names as well so

for instance here is where we might put

Harry as well and notice they share a

common prefix ha for Hagrid ha for Harry

so we're reusing some of these nodes

some of these arrays we can even slip

Hermione in here too borrowing only the

H but she gets the H then the E then r r

m i o n e and so forth and we Mark at

the end of her name too that she's in

there now what's the takeaway here well

what is the running time of a try how

many steps does it take to find someone

in this data structure and let me zoom

out so that it sort of suddenly becomes

a massive data structure with even more

in it uh maybe it looks sorry no I'll

keep it on this one maybe it looks a

little something like this with just

these three names but how many steps

does it take to find Hagrid or Harry or

Hermione no matter how many names are in

this data structure there's three at the

moment but it takes what h a g r i d so

six steps to find Hagrid uh h a r r y

five steps to find Harry h e r m i o n e

eight steps to find Hermione but notice

that those steps are only dependent on

what the lengths of the human's names

and let's assume that no one's going to

have a infinitely long name it's going

to max out at what like eight no maybe

eight 18 maybe 20 30 there's actually

some pretty long human names out there

but it's going to be finite you know

it's abounded whereas most contexts n

could grow forever so what's compelling

here is if you assume that the longest

name is I don't know 50 for the sake of

a theme here then you know that finding

anyone in this data structure will take

you no more than 50 steps 50 is thus a

constant which means you have big O of

one running time it doesn't matter if

there's a million people in this phone

book or a billion people in this phone

book that's going to definitely add more

nodes to it but it's still going to take

you h a r uh sorry h a g r i d six steps

to find Hagrid h a r r y five steps to

find Harry even if there's a billion

other people in that data structure so

now we actually do seem to have constant

time if you assume that there's going to

be a bound on the length of the name why

don't we use tries for everything then

what's the price we're paying for this

data structure even though we've

represented just three characters here

yeah it's a lot of memory yeah and you

can see it even with these three names

most of the squares on the screen are

empty like bites and bits that are there

and are allocated and they need to be

there because you need to be able to do

that arithmetic thing of this being zero

this being 25 so you can jump from boom

boom boom boom based on each of the

letters but it's a hugely sparse data

structure which means it takes up a

crazy amount of memory now maybe that's

tolerable especially for short names but

that's going to be the trade-off as well

and this is such a tension in Computing

almost any time you want to improve time

you want to speed up the efficiency the

speed of your algorithm you're going to

spend space if by contrast you want to

decrease the amount of space you might

very well have to increase the running

time it is indeed this seesaw back and

forth and you your colleagues your

company need to decide what resource is

the most precious heck it might be much

harder to code one of these data

structures than another you're a human

your time is valuable do you really want

to spend hours implementing a try when

you know hey in 30 minutes I can bang

out an array nowadays or a linked list

even there too development time is going

to be yet another resource and why

sometimes there's good code or bad code

it depends on what you're prioritizing

so what do each of these nodes look like

in a try well we can keep calling it a

node this is a very generic term for

just a container in these data

structures in this story though let me

claim that everyone has a number like a

phone number a string akhr star every

every node has 26 children or

technically an array of size 26 that can

point to more of these nodes notice that

I don't need to store the name of

someone in a try because it's implicit

in the path that you take to find them

so that's a minor optimization but it

saves us some space but this would be

just a different data structure we could

use to actually solve this problem as

well albeit at a very expensive cost and

what do we need our variable to be that

stores the try just like four we just

need a single pointer that hangs on to

the root of this structure that's null

if it's empty or non-null if it's

actually pointing at

something any questions then on

tries and if it's feeling like a lot the

fire the the fire hydrant it is we

started with arrays then link list then

tries but questions on how we've just

assembled from these basic building

blocks

[Music]

yeah a good question why is this not uh

size 26 uh it's just like with the try

just like with the link list before it

just tends to be en code convenient to

have a separate additional pointer

that's small that just points to the

beginning of the data structure because

that way it can be null thereby clearly

indicating there are no nodes the whole

structure is empty if you allocated one

of those nodes you absolutely could but

then you'd be just wasting space even if

it's empty and it creates an ambiguity

so just having a single pointer linked

to the beginnings of all of these things

is a good thing other questions now on

tries or trees or hash tables or

arrays so what problems might arise well

here's a counter example what names are

manifest in this try

here feel free to just call it

out what do you

see Danielle and Danielle so presumably

if these are two uh names here one of

which is a prefix of another notice that

the data structure still works and I

chose you know a friend's name and then

appended a couple of more characters to

it that's also a name because we have

here d a n i e l and the green

technically is implemented as a bull or

something like that that indicates a

word stops here but we don't want to

preclude storing Danielle as well who's

a super string if you will of Daniel and

so that's okay too so long as the

structure allows for the pointers to

keep going so even that works out okay

whereas it might not have otherwise and

in terms of the running time just to be

clear at the end of the day tries do

give you actual constant time for

insertions lookups deletions and the

like because it's dependent only on the

length of the input the key if you will

and not on how many other people are in

your phone or a dress book and now

thought we'd conclude with a visual if

you've gotten out into the square anyone

recognize this okay okay sweet green a

local salad place what are we looking at

here and what's its connection to today

um you're about to become all the

geekier in the real world cuz you will

start to see data structures

everywhere what is this or how does this

work maybe in salad form who's been to

Sweet

green okay either of you so how does

this

[Music]

work okay good so if you order a salad

for someone named L when it's ready they

put it in the l section here and so this

is kind of a set of key value pairs

right if L is the first letter of

someone's name the value hopefully is

the salad and so what you kind of have

here is a dictionary key value pairs

where it's not words and definitions

it's names and salads and you can think

of this too as kind of a hash table why

even though it actually doesn't fit on

one long shelf cu the store is only so

big this is really an array and

apparently a is missing or maybe it's

around around the corner but this array

just happens to wrap onto multiple lines

but it's still conceptually a single

Dimension but suppose two people have

the name L what do they do

typically yeah so maybe they they well

if they run put that much effort into it

they might look at the second letter and

then the third letter odds are this is

not that interesting a problem to solve

optimally in that way but they probably

do start stacking the salads on top of

each other maybe scooching it over just

a little bit and so what do you have

there well now you start to view the

lens through like cs50 glasses like okay

you have an array and then you have like

these link lists that are sort of

growing here but even then you run into

a problem why because it's not really a

link list because at some point you're

going to hit the boundary here so it's

kind of like an array of arrays because

you can only fit what like three or four

salads here and so long story short we

started today deliberately talking about

real world things like stacks and cues

and even though it did escalate quickly

into binary search trees and hash tables

and tries even those things are

everywhere even though they don't call

them as such these are just solutions to

problems and now with this final week of

SE under your belt you have all the more

of a technical toolkit via which to

implement these things and code next

week we'll be able to trust that someone

else solved all these problems we'll

introduce Python and lines of code like

this will finally become lines of code

like that so that's the promise ahead

and we'll see you next

[Applause]

[Music]

time

[Music]

[Music]

[Music]

yeah

[Applause]

[Music]

[Applause]

yeah

[Music]

all right this is cs50 and this is week

six wherein we finally transitioned from

scratch to see to now Python and indeed

this is going to be somewhat of a unique

experience in that just like a few weeks

past perhaps for the first time and now

uh today you're going to learn a new

language but the goal isn't just to

throw another fire hose of content and

syntax and whatnot at you but rather to

really equip you all to actually teach

yourself new languages in the future and

so indeed what we'll do today what we'll

do this coming week is sort of prepare

you to stand on your own and once python

is p a and the world has moved on to

some other language in some number of

years you'll be well equipped to figure

out how to wrap your mind around some

new syntax some new language and solve

problems as well now you recall in week

zero this is where we started just

saying hello to the world and that

quickly escalated just a week later in C

to see be something much much more

cryptic and if you've still sort of

struggled with some of the syntax find

yourself checking your notes or your

previous code like that's totally normal

and that's one of the reasons why there

are languages besides C out there among

them this language called python humans

over the decades have realized gee that

wasn't necessarily the best designed

decision or humans have realized wow you

know what now that computers have gotten

faster with more memory and more faster

CPUs we can actually do more with our

programming languages so just as human

languages evolve so do actual

programming languages and even within a

programming language there's typically

different versions We for instance have

been using version C11 of C which was

updated in 2011 um but python itself

continues to evolve and it's now up to

version three plus and so there too

these things will evolve in the coming

days thankfully what you're about to see

is hello world for the third time but

it's going to be literally this none of

the crazy syntax above or below fewer

semicolons if any fewer curly braces and

really a lot of the distractions get out

of the way so to get there let's

consider exactly how we've been

programming up until now so you write a

program in C and you've got hopefully no

syntax error so you're ready to build it

that is compil it and so you've run make

and then you've run the program like/

hello or if you think back to week two

where we took a peak underneath the hood

of what make is doing it's really

running the actual compiler something

called clang maybe with some command

line arguments creating a program called

hello and then you could do do/ hello so

today you're going to start doing

something similar in spirit but fewer

steps no longer will you have to compile

your code and then run it and then maybe

fix or change it and then compile your

code and run it and then repeat repeat

the process of running your code is

going to be distilled into just a single

step and the way to think of this for

now is that where a c is frequently used

as indeed a compiled language whereby

you convert it first to zeros and ones

Python's going to let you speed things

up whereby you the human programmer

don't have to compile it you're just

going to run what's called an

interpreter which by Design is named the

exact same thing as the language itself

and by running this program installed in

VSS code or eventually on your own Max

or PCS this is just going to tell your

computer to interpret this code and

figure out how to get down to that lower

level of zeros and ones but you don't

have to compile the code yourself

anymore so with that said let's consider

what the code is going to look like side

by side in fact let's look back at some

scratch blocks just like we did with C

in week one and do some side by sides

because even though some of the syntax

this week and Beyond's going to be

different like the ideas are truly going

to be the same there's not all that much

intellectually new just yet so whereas

in week zero we might have said hello to

the world with this purple puzzle piece

today of course uh or rather in week one

it looked like this in C but today

moving forward it's going to quite

simply look like this instead and if we

go back and forth for just a moment here

again is the version in C noticing the

very seike characteristics and just at a

glance here in Python I claim it's now

this what do you apparently need not

worry about

anymore what's gone so semicolon is gone

and indeed you don't need those to

finish most of your thoughts anymore

anything else so the back sln is absent

and that's kind of curious because we're

still going to get a new line but we'll

see that it's become the default and

this one's a little more subtle but now

the function is called print instead of

print F so it's a little more familiar

in that sense all right so when it comes

to using libraries that is code that

other people have written in the past

we've done things like hash include

cs50.h to use cs50's own header file or

standard IO or standard lib or string or

any number of other header files you

have all used well moving forward we're

going to give you for this first week a

similar cs-50 Library just very

short-term uh training wheels that will

quickly take off because in reality it's

a lot easier to do things in python as

we'll see but the Syntax for this now is

going to be to import the cs50 library

in this way and when we have now this

ability we can actually start writing

some code right away in fact let me

switch over to vs code here and just as

in the past I'll create a new file but

instead of creating something called C

I'm going to go ahead and create my

first program called hello.py using Code

space hello.py that of course gives me

this new tab and let me actually quite

simply do what I proposed print quote

unquote hello world without the back

slash without the semicolon without the

F in print and now let me go down to my

terminal window and I don't have to

compile it I don't have to do dot slash

i instead run a program called python

whose purpose in life is now to

interpret my code top to bottom left to

right and if I run python of hello.py

crossing my fingers as always voila now

I have printed out hello world so we

seem to have gotten the new line for

free in this sense where it's

automatically happening the dollar sign

isn't weirdly on the same line like it

want was in week one but that's just a a

minor detail here if we switch back to

now some other capabilities well indeed

with the cs50 library you can also not

just import the library itself but

specific functions and you'll see that

temporarily we're going to give you a

helper function called get string just

like in C that just makes it work

exactly the same way as in C and we'll

see a couple of other functions that

will just make life easier initially but

quickly will we take those training

wheels off so that nothing is indeed

cs50 specific all right well how about

functions more generally in Python let's

do a whirlwind tour if you will much

like we did in that first week of C

comparing one to the other so back in

our world of scratch one of the first

programs we wrote was this one here

whereby we asked the human their name we

then used the return value that was sort

of automatically stored in this answer

variable as a second argument to join so

that we could say hello David or hello

Carter so this was back in week one uh

week zero in week one we converted it to

this and here's a perfect example of

things like escalating quickly and again

this is why we start in scratch there's

just so much distraction here to achieve

the same idea but even today we're going

to chip away at some of that syntax so

in C we had to declare the argument as a

we had to declare the variable as a

string here we of course had the

semicolon and more well in Python the

comparable code now is going to look

more simply like this so semicolon is

again gone on both lines for that matter

so that's good what else appears to have

changed or disappeared yeah type of

variable yeah so I didn't have to

specifically say that answer is now a

string and indeed python is is

dynamically typed and in fact it will

infer from Context exactly what it is

you are storing in that variable other

details that seem a little bit

different little bit different what else

jumps out at you here I'll go back this

was the C version and maybe Focus now on

the second line because we've rather

exhausted the first here's now the

python version what's different here

yeah yeah there's no percent s anymore

there's no second argument at the moment

per se to print now it is still a little

weird it's as though I've like deployed

some addition here arithmetically but

that's not the case some of you have

programmed before and plus some of you

might know means what in this

context so to combine or more

technically anyone know the buzz word

yeah

to concatenate so to concatenate is like

the fancy way of what scratch calls

joining which is to take one string on

the left one string on the right and to

join them together to glue them together

if you will so this is not addition it

would be if it were numbers involved

instead but because we've got a string

hello comma and another string

implicitly in this variable based on

what the human typed in in response to

this get string function that's going to

concatenate hello comma space and then

David or Carter or whatever the human

has typed in but turns out there's going

to be different ways to do this in

Python and we'll show you a few

different ones and here too try not to

get too hung up on or frustrated by like

all of the different ways you can solve

problems odds are you're going to be

picking up tips and techniques for years

to come if you continue programming so

let's just give you a few of the

possible ways so here's a second way you

could print out hello comma David or

hello comma Carter but what has changed

in the previous version I used

concatenation explicitly and the space

here is important grammatically just so

we get that in the final phrase

now I'm proposing to get rid of that

space to add a comma outside of the

double quotes as well but if you think

back to C this probably just means that

print similar in spirit to print F can

take not just one argument but even two

and in fact because of this comma in the

middle that's outside of the double

quotes it's hello comma and then it will

be automatically concatenated with even

without using the plus to whatever the

value of answer is and by default just

for grammatical prettiness the print fun

function always gives you a space for

free in between each of the multiple

arguments you pass in we'll see how you

can override that down the line but for

now that's just another way to do it now

perhaps the better if slightly cryptic

way to do this or just the increasingly

common way is probably this third

version which looks a little weird too

and probably the weirdness jumps out

we've automatic we've suddenly

introduced these like Curly braces which

I promised were mostly gone and they are

but inside of this string here I've done

a curly brace which might mean what just

intuitively and here is sort of an

example of how you learn a new language

just kind of infer from Context how

python probably

works what might this mean yeah able to

tell that this is not one of the actual

stat

inside yeah so this is an indication

because the curly braces because this is

the way python was designed that we want

to plug in the value of answer not

literally a ANS w r and the fancy word

here is that the answer variable will be

interpolated that is substituted with

its actual value but but but and this is

actually weird looking and this was

introduced a few years ago to python

what else did I have to change to make

these curly braces work

apparently

yeah yeah there's this weird F and so

it's sort of like part of print F but

now it's inside the curly it's inside

the parenthesis there this is just the

way python designed this so a few years

ago when they introduced what are called

format strings or F strings you

literally prefix your quoted string with

the uh letter F and then you can use

trickery like this like putting curly

braces so that the value will be

substituted automatically if you forget

the F you're going to literally see

hello comma curly brace answer close

curly brace if you add the F it's indeed

interpolated the value is plugged in all

right questions on how we can just say

hello to the world via python in this

case

yeah if you do this without the without

the F if you omit the F you will

literally see c h e l l o comma curly

brace a NS w r close curly brace so in

fact let's do this let me go back to vs

code here quickly I've still got my file

called hello.py open and let me go ahead

and change this ever so slightly so I'm

going to go ahead and uh let's say from

cs50 import get string and that's just

the new syntax I propose using to import

a function from someone else's Library

I'm going to now go ahead and ask the

question uh let's go ahead and use get

string storing the result and answer so

get string quote unquote what's your

name question mark and then on this line

I'm going to deliberately make a mistake

here exactly to your question let me

just say hello comma answer and just

this now even though answer is a

variable Python's not going to be so

presumptuous as to just plug in the

value of a variable called answer what

it's going to do of course is if I type

in my name whoops I typed too fast let

me go ahead and rerun that again if I

run python of hello.py type in my name

and hit enter I get hello comma answer

well let me do one better let me apply

these curly braces as before let me

rerun python of hello.py What's Your

Name daav ID and here's again the answer

to your question now we get literally

the curly braces so the fix here

ultimately is just going to be to add

the F there rerun my program again with

daav ID and now hello comma David so

this is admittedly a little more cryptic

than the ones with the Plus the comma

but this is just increasingly common why

because you can read it left to right

it's nice and convenient it's less

cryptic than the percent s's so it's

sort of a new and improved version if

you will of printf in C based on Decades

of experience of programmers doing

things like this questions on printing

in this

way we're now on our way to programming

in

Python anything all right well what more

can we do with this language here well

let me propose that we consider that we

have for

instance a few other features that we

can add to the mix as well

namely let's say some data types as well

so let me flip over here to um back to

the slides and there's different data

types in python as we'll soon see but

they're not as explicit as we already

saw by using a string from get string

you don't have to explicitly State what

it is but you solve recall and see all

of these various data types and then in

python kind of nicely enough this list

is about to get shorter and so here is

our list in C here is an abbreviated

list in Python so we're still going to

have strings but they're going to be

more succinctly called stirs now St Str

we're still going to have in for

integers we're still going to have

floats for floating Point values we're

even going to have bulls for true and

false but what's missing now from the

list is long and floats and why is that

or rather long and double well recall

that in C those used more bits well in

Python the smaller data types previously

int and Float themselves just use more

bits for you and so you don't need to

distinguish between small and large you

just use one data type and the language

gives you a bigger range than before it

turns out though there's going to be

some other features as well of python

these data types one of which will be

called range another of which will be

list So Gone will be arrays we'll

actually use something literally called

a list tle sort of like XY pairs for

coordinates and things like that uh dict

for dictionaries so we have built-in

capabilities for storing keys and values

we'll see and even a set sort of

mathematically a set is like a

collection of values but it

automatically gets rid of duplicates for

you so all of these things we could

absolutely Implement in C if we wanted

and indeed in problem set five you've

been implementing your very own spell

checker using some form of hashtable

well it turns out that in Python you can

solve those same problems but perhaps a

little more readily in fact let me go

back over here to vs code and let me

propose that I do the following let me

go ahead and create a file called

dictionary. let me propose that I try to

implement say problem set five our spell

checker in Python instead of c and

Achieve ultimately the same kind of

behavior uh whereby I'll be able to

spell check a whole bunch of words so

this is jumping the gun a little bit

because you're about to see syntax will

revisit over the course of today but for

now I've got a new file called

dictionary. py and let me begin to

create uh some placehold ERS for

functions we'll see in just a bit that

in Python you can define a function

called check and that check function can

take a word as it's input and I'll come

back to this in just a moment in Python

I can define a second function like load

which itself will take a whole

dictionary just like in problem set five

and I'll go ahead and come back to the

implementation of this meanwhile we

might similarly Implement a function

called size which takes no arguments but

ultimately is going to return the size

of my dictionary of words and then last

lastly for consistency with problem set

five we might Define an unload function

whose purpose in life is to free any

memory that you've been using just to

give it back to the computer now odds

are whether you're still working on

speller or half finished speller like

you wrote a decent amount of lines of

code and indeed it's been by design a

challenge but one of the reasons for

these higher level languages like python

is that you can stand on the shoulders

of programmers before you and solve very

common problems much more quickly so

that you can f us on building your new

app or your web application or your own

project to solve problems of interest to

you so at the risk of crushing some

Spirits let me propose that in Python if

you want a dictionary for something like

a spell checker well that's fine go

ahead and give yourself a variable like

words to store all of those words and

just assign it equal to a dictionary or

dict for short in Python that will give

you a hashtable now it turns out in

speller recall you don't need to worry

about words and definition conditions

it's just about spellchecking the words

so strictly speaking we don't need keys

and values we just need keys so I'm

going to save myself a few more key

strokes by just saying that technically

in Python using a set suffices again a

set is just a collection of values with

no duplicates but they don't necessarily

have uh keys and values it's just one or

the other but now that I have on line

one I claim the equivalent in Python of

a hash table I can actually do something

like this here's how I might implement

the check function in Python if the word

passed into this function is in my

variable called words well return true

else go ahead and return false done wait

you're thinking if anything at all maybe

we want to handle lowercase instead of

just uppercase and lowercase well you

know what in Python if you want to force

a whole word to lowercase you don't have

to iterate over it with a loop you don't

have to use any of that ctype functions

or anything just say word. lower and

that will convert the whole thing to

lowercase for parody with the dictionary

all right how about something like the

load function in Python well in Python

you can open files just like in C for

instance in Python I might do open the

dictionary argument in read mode just

like fopen in Python I might do

something like this for each line in

that file let me go ahead and add to my

words variable that line and then let me

go ahead and close that file and I think

I'm done I'm just going to go ahead and

return true just because I think think

I'm already done now here too I could

nitpick a little bit technically if I'm

reading in every line from the file

every line in the dictionary ends with

technically a back sln but there's an

easy way to get rid of that uh just like

you might in see with an alternative

syntax what I'm actually going to do is

this let me grab from the current line

the current word by stripping off with

reverse strip R strip a function will

again see that just gets rid of the

trailing new line the back sln at the

end of that line and what I really want

to do then is add this word to that

dictionary meanwhile if I want to figure

out what the size is of my dictionary

well and see you're probably writing

code to iterate over all of those lines

and you're just going to uh count them

up using a variable not so in Python you

can just return the length of those

words and better still in Python you

don't have to manage your own memory no

more Malo no more free no more manual

thinking about memory the language just

deals with all of that for you so you

know what it suffices for me to just

return true and claim that unloading is

done for me and that's it again whether

you're in the middle of or already

finished this might perhaps suggest some

frustration but also Enlightenment in

this in that this is why higher level

languages exist you can build on top of

the same principles the same ideas with

which you've been dealing struggling

even this past week but you can now

express yourself all the more succinctly

like this one line implements a hash

table for you and all of this now now

just uses that hash table in a simpler

way any questions now on this keeping in

mind that the point nonetheless of

speller and P said 5 is to understand

what's really going on underneath the

hood and better still to notice this

this might seem all rather amazing but

let me go ahead and do this I've

actually got a couple of versions of

speller written here and I've got a

version written in C that I won't show

the source code for but I'm going to go

ahead and make that version of speller

in C and I'm going to go ahead here and

let's say split my window here for just

a moment and I'm going to go into a

python version of spell really that I

just wrote and on the left hand side

here let me go ahead and run speller the

version I compiled in C using a big text

like uh the Sherlock Holmes text which

is a whole lot of words in it and on the

right hand side let me run python of

spell. Pi which is a separate file I

wrote in advance just like we give you

speller.c and I'll similarly run this on

the Sherlock Holmes text and I'm going

to do my best to hit enter on the left

and the right of my screen at the same

time but we should see hopefully the

same list of misspelled words and the

timings thereof so here we go on the

right here we go on the

left all right sort of a race to see

which one wins here C is on the left

python is on the right okay

interesting hopefully Python's close

behind note that some of the is internet

delay and so it might not necessarily be

a crazy number of seconds but the system

is indeed using if we measure at a low

level how much time the CPU spent

executing my code C took a total of 1.64

seconds that was pretty fast even though

it took a moment more for all of the

btes to come over the Internet the

python version though took what 2.44

seconds so what might an inference be I

mean one maybe I'm just better at

programming in c than I am in Python

which is probably not true

but what else might you infer from this

example should we maybe give up on

python stick with

C no so where what might be going on

here like why is the python version that

I claim is correct and I think the

numbers all line up just not the

times where's the trade-off here well

here again is sort of this design

tradeoff

yeah yeah exactly in order to save the

human programmer time there's a lot more

features built into python more

functions more automatic management of

memory and so forth and you have to pay

a price like someone else's code is

doing all of that work for you but if

they've written some number of lines of

code those are just more lines of code

that need to be executed for you whereas

here the computer is at the risk of

oversimplifying only running my lines of

code so there's just less overhead and

so this is a Perpetual trade-off

typically when using a more userfriendly

and more modern language one of the

prices you might pay is performance now

there's a lot of smart computer

scientists in the world though trying to

push back on those same tradeoffs and so

these interpreters like the command I

wrote python technically can especially

if you run a program again and again

they can actually sort of secretly

behind the scenes compile your code for

you down to zeros and ones and then the

second the third the fourth time you run

that program it might very well be

faster so this is a bit of a head fake

here in that I'm running them once and

only once but we could get benefit over

time if we kept running the python

version again and again and perhaps

fine-tune the performance but in general

there's going to be this trade-off now

would you rather spend the 60 seconds I

wrote implementing a spell checker or

the 6 hours 16 hours you might be or

have spent implementing the same and C

you know probably not for productivity

sake this is why we have these

additional languages just for fun let me

flip over to another screen here and

open up a version of python that's

actually on on my in just a second on my

own uh Mac instead of the cloud so that

I can actually do something with

Graphics so here I just have a black and

white terminal window on my very own Mac

and I've pre-installed python just like

we've done so for VSS code in the cloud

for you uh notice that I've got this uh

photo of uh perhaps one of your favorite

TV shows here with the cast of The

Office notice all of the faces in this

image here and let me propose that we

try to find one face in the crowd sort

of CSI style whereby we want to find

perhaps the stranton Strangler so to

speak and so here is an example of this

this guy's face now how do we go about

finding this specific face in the crowd

well our human eyes obviously can pluck

him out especially if you're familiar

with the show but let me go ahead and do

this instead let me go ahead and propose

that we run code that I already wrote in

advance here this is a Python program

with more lines of code that we won't

dwell on for today but it's meant to

motivate what we can do from a a pillow

uh Library implying a python image

Library I want to import some type of

information called some type of some

feature called image so that I can

manipulate images not unlike our own

problem set 4 and this is kind of

powerful you in Python you can just

import face recognition as a library

that someone else wrote from there I'm

going to create a variable called image

I'm going to use this face recognitions

libraries load image file function it's

a little verbose but it's similar in

Spirit to F open and I'm going to open

office. jpeg I'm going to then declare a

second variable called face locations

plural because what I'm expecting to get

back per the documentation for this

library is a list of all of the faces

locations that are detected all right

then I'm going to iterate over each of

those uh faces using a for Loop that

we'll see in more detail I'm going to

then infer what the top right bottom and

left Corners are of that face and then

what I'm going to do here is show that

face alone if I've detected the face in

question so let me go ahead here and run

detect. and we'll see not just the one

face we're looking for but if I run

python of detect. piy it's going to do

all of the analysis I'll see a big

opening here now of all of the faces

that were detected in this here program

okay some better than others I guess if

you zoom in on catching someone typical

Angela if you now want to Now find that

one face I think we need to train the

software a bit more so let me actually

open up a second program called

recognize that's got more going on but

let me with a wave of a hand point out

that I'm now loading not only the

office. JPEG but also toby. JPEG to sort

of train the algorithm to find that

specific face and so now if I run this

second version recognize. with python of

recognize. py hold my breath for just a

moment it's an analyzing presumably all

of the faces you see the same original

photo but do you see one such face

highlighted here this adversion of the

code found Toby highlighted him with

this green and voila we have face

recognition so for better for worse this

is what's happening increasingly

societally nowadays and honestly even

though I didn't write the code live

because it's a good dozen or more lines

of code it's not terribly many and

literally all the authorities all we

have to do is import face recognition

and voila you have access like these

techn IES are here already but let's

consider for just a moment how did we

find Toby like how might that Library

even though we're not going to look at

its implementation details how does it

find Toby and distinguish him from all

of these other faces in the crowd what

might it be doing

intuitively think back even to pet 4

like what you yourselves have access to

data wise

yeah you know pixels in one area area

and a lot

of

that it's a lot of a lot of

simar yeah exactly and to summarize for

for camera here we have trained the

software if you will by giving it a

photo of Toby's face by so by looking

for the same or really similar pixels

especially if it's a slightly different

image of Toby we can perhaps identify

him in the crowd and what really is a

human face well at the end of the day

the computer only knows it as a pattern

of bits or really at a higher level a

pattern of pixels so maybe a human face

is perhaps best defined in general as

like two eyes and a nose and a mouth

that even though all of us look similar

structurally odds are the measurement

between the eyes and the nose and the

width of the mouth the skin tone and all

of these other physical characteristics

are patterns that software could perhaps

detect and then look sort of

statistically through the image looking

for the closest possible match to these

various measurement shapes colors and

sizes and the like and indeed that might

might be the intuition but what's

powerful here again is just how easy and

readily available this technology now is

all right so with that said let's

propose to consider what more we can do

with python itself get back to the

fundamentals so that you yourselves can

start to implement something along those

same lines so besides having access to

things like a get string function um the

cs50 library provides a few other things

as well namely in C we had these but in

Python we're going to have fewer in

Python our library short term is going

to give you not only get string but also

get in and get float why it's actually

just kind of annoying as we'll student

see to get back an integer or a float

from a user and just make sure that it's

an INT and a float and not like a word

like cat or dog or some string that's

not actually a number well we can import

not just the specific function get

string but we can actually import all of

these functions one at a time like this

as we'll soon see or you can even in

Python import specific functions from a

file one of you asked a while back when

you import when you include something

like cs50.h or standard i.h you're

actually getting all of the code in that

file which potentially can add bulk to

your own program or time in this case

when you import specific functions from

python you can be a little more narrowly

uh precise as to what it is you want to

have access to all right so with that

said let's go ahead and see what

conditionals look like in Python so in

the left hand side again here we'll see

scrp

and here for instance was just kind of a

contrived example asking if x is less

than y then say x is less than y in C it

looked like this in Python now it's

going to look like this instead and

here's before in C and here's after and

just to call out a few of the obvious

differences what has changed in Python

for conditionals it would

seem sort of what's the difference

yeah yeah so there's no more curly

braces and indeed you don't use those

what appears to be taking their place if

you might

infer what seems to have taken their

place what do you think so the colon at

the start of this line here but also

even more important now is this

indentation below it so some of you and

we know this from Office hours have a

habit of like uh indenting everything on

the left right and it's just kind of

this crazy mess to look at frustrating

for you surely but C and clang is pretty

tolerant when it comes to things like

Whit space in a program python uh-uh

they realized years ago that let's help

humans help themselves and just require

standard indentation so four spaces

would be the norm here but because it's

indented below that colon that indeed

indicates that this now is part of that

condition something else has gone

missing versus C in this conditional

what else is a little

simplified yeah so no more parentheses

you can still use them especially when

you need to logically to do order of

operations like in math but in this case

if you just want to ask a simple

question like if x less than y you can

just do it like that how about when you

have an if else well this is almost the

same here with these same changes in C

this looked like this and it's starting

to get a bit bulky at least if we use

our curly braces in this way in Python

we can tighten things up further even

though strictly speaking in C you don't

always need the curly braces but here

gone are the parentheses again gone are

the curly braces indentation is

consistent and we've just added another

keyword else with a colon but no more

semicolons as well how about something

larger like this and if else if else

this one's a little curious but in C it

looked like this if else if else in

Python it now looks like this and

there's perhaps one curiosity here that

honestly all these years later I still

can't remember how to spell it half the

time what's weird about

this what do you spot as

different uh yeah over

here yeah instead of else if it's l if

why apparently El space if was just too

many keystrokes for humans to type so

they condensed it into this way probably

means it's a little more distinguishable

too for the computer between the if and

the else to but just something to

remember now it's indeed L if and not

else if all right so what about

variables in Python I've used a couple

of them already but let's let's um

distill exactly how you define uh and

declare these things as well so in

scratch if we wanted to create a

variable called counter and set it equal

initially to zero we would do something

like this specify that it's an INT use

the assignment op operator and the

thought with a semicolon in Python it's

just simpler you name the variable you

use the assignment operator as before

you set it equal to some value and

that's it you don't mention the type you

don't mention the semicolon or anything

more what if you want to change a

variable like the counter by one that is

incremented by one you have a few

different ways here in C we saw syntax

like this where you can say counter

equals counter plus one which again

feels like illogical how can counter

equal counter plus one but again we read

this code really right to left updating

its value by one um in Python it's

almost the same you just get rid of the

semicolon so that logic is there but

recall in C we could do something

slightly different that we can also do

in Python in Python you can also more

succinctly do this plus equals and then

whatever number you want to add or you

can even change it to subtract if you

prefer sadly gone is something you've

probably typed a whole lot what was the

other way you can add

one Plus+ is no more sadly in Python

just too many ways to do the same thing

so they got rid of it in favor of just

this syntax here so keep that in mind as

well what about loops when you want to

do something in Python again and again

well in scratch in week zero here's how

we meowed three times specifically in C

we had a couple of ways of doing this

this was like the more mechanical

approach where you create a variable

called I you set it equal to zero you

then do while I is less than three the

following and then you yourself

increment I again and again mechanical

in the sense that like you have to

implement all of these gears and make

them turn yourself but this was a

correct way to do that in Python we can

still achieve the same idea but we don't

need the int keyword we don't need any

of the semicolons we don't need the

parentheses we don't need the curly

braces we can't use the Plus+ so maybe

that's a minor step backwards if you're

a fan but otherwise the code the logic

is exactly the same but there's other

ways to achieve this same idea recall

that in C we could also do this you

could use a for Loop which it does

exactly the same thing both are correct

both are arguably welld designed it's

kind of to each their own when it comes

to choosing between these in Python

though we're going to have to think

through how to do this so you don't do

the same for loop as in C the closest I

could come up with is this where you say

four I or whatever variable you want to

do the counting in literally the

preposition and then you use square

brackets here and we've used square

brackets before in the context of like

arrays and things like that and the 012

looks like an array in some sense even

though we've also seen arrays with curly

braces but these square brackets for now

denote a list python does not have

arrays an array is that contiguous chunk

of memory back to back to back that you

have to resize somehow by moving things

around in memory as per two weeks ago in

Python though you can just create a list

like this using square brackets and

better still as we'll see you can add or

even remove things from that list down

the road um this though is not going to

be very welld designed this will work

this will iterate in Python three

times but what might rub you the wrong

way about this design even if you've

never seen python before how does this

example not end well

yeah yeah like if you're making a large

list you have to type out each one of

these numbers like comma three comma

four comma 5 comma dot dot do 50 comma

dot dot dot 500 like surely that's not

the best solution to have all of these

numbers on the code on the screen

wrapping endlessly on the screen so in

Python another way to do this would be

to use a function called range which

technically is a data type unto itself

and this returns to you as many values

as you ask for it range takes some other

arguments as well but the simplest use

case here is if you want back the

numbers 0 1 and two a total of three

values you say hey python please give me

a range of three values and by default

they start at zero on up but this is

more efficient than it would be to

hardcode the entire list at once and the

best metaphor I could up with is

something like this like here for

instance is a deck of cards this is sort

of normal human size and there's

presumably 52 cards here so writing out

0 through 51 on code would be a little

ridiculous for the reasons you know it

would just be very unwieldy and ugly and

wrapping and all of that it would be the

phys it would be the virtual equivalent

of me like handing you all of these

cards at once to just deal with and

right you know they're not that big but

like it's a lot of cards to hold on to

it requires a lot of memory or physical

storage if you will what range does

metaphorically is if you ask me for

three cards I hand you them one at a

time like this so that at any point in

time you only have one number in the

computer's memory until you're handed

the next the alternative the previous

version would be to hand me all three

cards at once or all 52 cards at once

but in this case range is just way more

efficient you can do range of a thousand

that's not going to give you a list of a

thousand values all at once it's going

to give you a thousand values one at a

time reducing memory sign signicantly in

the computer itself all right so besides

this what about doing something forever

in scratch well we could do this

literally with a forever block which

didn't quite exist in C in C we had had

to hack it together by saying while true

because true is by definition TR always

true so this just in uh deliberately

induces an infinite Loop for us in

Python the logic's going to be almost

the same and infinite Loops in Python

tend to actually be even more common

because you can always break out of them

as you couldn't see in Python it looks

like this and this is slightly more

subtle but gone are the curly braces

gone or the parentheses but ever so

slight difference

toal capital T for true and it's going

to be a capital f for false stupid

little differences eventually you're

going to mistype one or the other but

these are the kinds of things to keep an

eye out and to start recognizing in your

mind's eye when you read code questions

now on any of these building

blocks

yeah

I in the for Loop was I re uh it was set

to zero on the first iteration then one

on the next then two on the third and

the same thing for range it just doesn't

use up as much memory all at once other

questions now on any of these building

blocks of

python no all right well let's go ahead

and build something a little more than

hello let me propose that over here we

Implement maybe the the simplest of

calculators here so let me go back to vs

code here open my terminal uh window and

open up say a file called

calculator. and in calculator. we'll

have an opportunity to explore some of

these building blocks but we'll allow

things to escalate pretty quickly to

more interesting examples so that we can

do the same thing ultimately as well and

in fact let me go ahead and do this

moreover I've brought some code with me

in advance uh for instance something

called calculator 0. C from the first

week of C

and let me go ahead and split my window

here in fact so that I can now do

something like uh this let me move this

over here here calculator. Pi so now I

have on the left of my screen

calculator. C or calculator z.c because

that's the first version I made and

calculator. Pi on the right let me go

ahead and Implement really the same idea

here so on the right hand side the

analog of including cs50.h would be from

cs50 import get int if I want to indeed

use this function now I'm going to go

ahead and give myself a variable X

without defining its type I'm going to

use this get int function and I'm going

to prompt the user for X just like in C

I'm then going to go ahead and prompt

the user for another int like y here

just like in C and at the very end I'm

going to go ahead and do print X Plus Y

and that's it now granted I have some

comments in my C version of the code

just to remind you of what each line is

doing but I've still distilled this into

like six lines or really four if I get

rid of the blank line so it's already

perhaps a bit tighter here here but

there's also it's tighter because

something really important historically

is missing what did I seem to Omit

altogether that we haven't really

highlighted yet

yeah yeah the main function is gone and

in fact maybe you took for granted that

it just worked a moment ago when I wrote

hello but I didn't have a main function

in hello either and this too is a

feature of python and a lot of other

languages as well instead of having to

adhere to these long-standing Traditions

if you just want to write code and get

something done fine just write code and

get something done without necessarily

all of the same boiler plate so whatever

is in your python file left indented if

you will by default is just going to be

the code that The Interpreter runs top

to bottom left to right well let me go

ahead now and run code like this let me

go ahead and open that back up my

terminal window run python of

calculator. piy and I'll do X is one y

is 2 and as you might expect it gives me

three slight aesthetic bug I put my

space in the wrong place here so that's

a new mistake let me fix that

aesthetically let me rerun python of

calculator. Pi type in one type in two

and voila there is now my same version

again but let me propose now that we get

rid of this training wheel we don't want

to keep taking one step forward and then

two steps back by adding these training

wheels so let me instead do this in my

version of calculator. suppose that we

take away already the training wheel

that is the cs50 library here and let me

instead then use just Python's built-in

function called input which literally

does just that it gets input from the

user and it stores it as before in X and

Y so this is not cs50 specific this is

real world Python Programming well let

me go ahead and run again python of

calculator. Pi and of course if x is one

and Y is 2 X + Y should of course still

be three

hm it's apparently 12 according to

python until cs50's Library gets

involved but does anyone want to

infer what's just went wrong

yeah exactly the input function by

Design always returns a string of text

after all that's what the human typed in

and even though yes I type the number

keys on the keyboard it's still coming

back is all text now maybe we should use

like a get in function well that doesn't

exist in Python all you can do is get

textual input a string from the user but

we can convert one to the other and so a

fix for this so that we don't accident

Al concatenate that is join X+ y

together would be to do something like

this let me go back to my python code

here and whereas in C we could

previously do type casting we could

convert one type to another that

generally wasn't the case when you were

doing something complex like a string to

an INT you could do a Char to an INT and

vice versa but for a string recall there

was a special function in the ctype

library called a to I like ask e to

integer that's uh that closest analog

here and in fact the way to do this in

Python would be to use a function called

int which indeed is the name of the data

type 2 even though I have not yet had to

type it and I can convert the output of

the input function automatically from a

string immediately to an INT and now if

I go back to my terminal window rerun

python of calculator. piy with one and

two for X and Y now I'm back in business

so that then is for instance what the

cs50 library does if temporarily this

week is it just deals with the

conversion for you and in fact bad

things could happen if I type the wrong

thing like dog or cat instead of a

number but we'll cross that bridge in

just a moment as well all right what if

we do something slightly different now

with our calculator instead of just

addition let me go ahead and do how

about uh div instead of addition let's

do division instead so Z equals x / y

thereby giving me a third variable Z let

me go ahead and run python of

calculator. piy again I'll type in one

I'll type in three this time and what

prog what problem do you think we're

about to

see or is it gone what happened when I

did this in C albeit with some slightly

more cryptic syntax when I divided one

number like one one divided by

three anyone recall

yeah yeah so it would round down to the

nearest integer whereby you experience

trunc so if you take an integer like one

you divide it by another integer like

three that technically should be

0.33333 infinitely long but in uh C

recall you truncate the value if you

divide an INT by an INT you get back an

INT which means you get only the integer

part which was the zero now python

actually handles this for us and avoids

the truncation but it leaves us still

with one other problem here which is

going to be for instance not necessarily

visible at a GL

this looks correct this has solved the

problem in C so truncation does not

happen the integers are automatically

converted to a float a floating point

value but what other problem did we trip

over back

in week uh

one what else got a little dicey when

dealing with simple

arithmetic anyone recall well the syntax

in Python's a little different but let

me go ahead and do this it turns out in

Python if you want to see more

significant digits than what I'm seeing

here by the default which is a dozen or

so let me go ahead and print out Z as

follows let me first print out a format

string because I want to format Z in an

interesting way and notice this would

have no effect on the difference this is

just a format string that for no

compelling reason at the moment is

interpolating z in those curly braces

using an F string or format string if I

run this again with one and three we'll

see indeed the exact same thing but when

you use an F string you indeed have the

ability to format that string more

precisely just like with percent F in

Python you could start to fine-tune how

many significant digits you see in uh p

in C rather in Python you can do the

same but the syntax is a little

different if you want the computer to

interpolate Z and show you 50

significant digits that is 50 numbers

after the decimal point syntax is

similar to C but it's a little different

you literally put a colon after the

variable's name 50 means show me the

decimal point and then 50 digits to the

right and the F just indicates please

treat this as a floating point value so

now if I rerun python of calculator. Pi

divide 1 by 3 unfortunately python has

not solved all of the world's problems

for us this again was an example of

floating point in Precision so that

problem is still latent so just because

the world has advanced doesn't

necessarily mean that all of our

problems from C have gone away there are

solutions using third-party libraries

for scientific calculations and the like

but out of the box floating point in

Precision is still an issue meanwhile

there was one other problem in C that we

ran into involving numbers and that was

this integer overflow recall that an

integer in C only took up what like 32

bits typically which meant you could

count as high as four billion or maybe

if you're doing positive and negatives

as high as two billion after which weird

things would happen the number would go

to zero or negative or just it would

overflow or wrap back around well

wonderfully in Python they did it at

least address this whereby you can count

as high as you want and python will just

use more and more and more and more bits

and bites to store really big numbers so

integer overflow is not a thing with

that said python is limited to how many

digits it will show you on the screen at

once as a string but mathematically your

math will be correct now so we've taken

a couple steps forward One Step sideways

but indeed we've solved some of our

problems here all right questions now

now on any of these examples thus

far

question all right well how about uh how

about another problem that we

encountered in C let's revisit it here

in python as well so let me go ahead and

on the left hand side here let me open

up a file called say compare let's see

uh how about a file called compare 3. C

on the left and let me go ahead and

create a new file on the right called

compare. Pi because recall that bad

things happened when we needed to

compare two values in C so on the left

here is a reminder of what we once did

in C whereby if we want to compare

values we can get an INT in C stored in

x a get int in C stored in y we then

have our familiar conditional logic here

just printing out if x is less than y or

not well we can certainly do the same

thing ultimately in Python by using some

fairly familiar syntax and let's just

demonstrate this one quickly let me go

over here too I'll do from cs50 import

uh get int even though I could do this

instead with the input function itself x

equals get int and I'll prompt the user

for that y equals get int and I'll

prompt the user for that after that

recall that I can say without

parentheses if x is less than y then

print out without the F uh X is less

than y then I can go ahead and say else

if x is greater than y i can print out

uh quote unquote X is greater than y if

you'd like to interject now what did I

screw

up anyone yeah L if right so L if L if x

is greater than y else this part's the

same print X is equal to Y so there's

not all that much new there's no New

Logic going on here but at least

syntactically it's a little cleaner

indeed this program is only 11 lines

long albeit without any comments let me

go ahead and run python of compare. Pi

let's see is 1 less than two indeed

let's run it again is 2 less than 1 no

it's greater than and let's lastly type

in one and one twice X is equal to Y so

we've got a pretty side by side onetoone

conversion here let's do something a

little more interesting then and see how

about I open instead something where we

actually compared for a purpose so if I

open up from last uh from earlier in the

course how about uh agree do C which

prompt the user to agree to something or

not and let me code up a new version

here called agree. pi and I'll do this

on the right hand side with agree. pi

but on gree do c on the left notice that

this is how we did this sort of yes no

thing in C we compared c a character

equal to single quotes y or equal to

single quotes little Y and then the same

thing for n now in Python this one's

actually going to be a little bit

different here let me go ahead and in

the python version of this let me do

something like this uh we'll use get

string uh actually no we'll just use

input in this case so let's do uh s

equals input and we'll ask the user the

same thing do you agree question mark

then let's go ahead and say if s equals

equals how about uh

y huh how do I do this well a few things

turns out I'm going to do this s equals

equals little y then I'm going to go

ahead and print out a agreed and L if s

equals equals capital N or S equals

equals lowercase n I'm going to go ahead

and print out not agreed and I claimed

for the moment that this is identical

now to the program on the right the

program on the left in C but what's

different so we're still doing the same

kind of logic these equal equals for

comparing for equality but notice that

nicely enough python got rid of the two

vertical bars and it's just literally

the word or if you recall seeing

Ampersand Ampersand to express a logical

and in C you can just write literally

the word and and so here's a hint of why

python tends to be pretty popular people

just like that it's a little closer to

English there's a little less of the

cryptic syntax here now this is correct

as this code will now work but I've also

used double quotes instead of single

quotes and I also omitted a few minutes

ago for my list of data types in Python

the word Char in Python there are no

chars there are no individual characters

if you want to manipulate individual

character you use a string that is to

say a stir of size one now in Python you

can use single quotes or double quotes

I'm deliberately using double quotes

everywhere just for consistency with how

we treat strings in C it's pretty common

though to use single quotes instead if

only because on most keyboards you don't

have to hold the shift key anymore I

mean humans have really started to

optimize just how quickly they want to

be able to code so using a single quote

tends to be pretty popular in Python and

other languages as well they are

fundamentally the same uh single or

double unlike in C where they have

meaning so this is correct I claim and

in fact let me run this real quick I'll

open up my terminal window here let me

get rid of the version and see run

python of agree. piy and I'll type in y

okay I'll run it again and type in

little Y and I'll stipulate it's going

to work for no as well but this isn't

necessarily the only way we can do this

there are other ways to implement the

same idea and in fact I can go about

doing this this instead let me go back

up to my code here and we saw a hint of

this earlier we know that lists exist in

Python and you can create them just by

using square brackets so what if I

simplify the code a little bit and just

say if s is in the following list of

values capital y or lowercase y it's not

all that different logically but it's a

little tighter it's a little more

compact so L ifs is in capital N or

lowercase n i can express that same idea

too so here again it's just getting a

little more pleasant to write code

there's less like hitting of the

keyboard you can express yourself a

little more succinctly and using the

keyword in Python will figure out how to

search the entire list for whatever the

value of s is and if it finds it it will

return true automatically else it will

return false so if I run agree. Pi again

and type in capital y or lowercase y

that still now works well I can typen

tighten this up further if I want to add

more features well what if I want to

support not just why Big Y and little y

but how about yes or yes or in case the

user's yelling or you know someone who

doesn't really isn't good with caps lock

types in yes wait a minute but it could

be weird like do we want to support this

or this I mean this this just gets

really tedious quickly combinatorically

if you consider all of these possible

permutations what would be smarter than

doing something like this if you want to

just be able to tolerate yes in any form

of capitalization like logically what

would be nice maybe whatever theut is

you justf over to all lower and then

exactly super common Paradigm why don't

we just force the users's input to all

lowercase or all uppercase doesn't

matter so long as we're self-consistent

and just compare against all uppercase

or all lowercase and that will get rid

of all of the possible permutations

otherwise now in C we might have done

something like this we might have

simplified this whole list and just said

let's say uh we'll do how about

lowercase so y or yes and we'll just

leave it at that but we need to force

now s to lowercase well in C we would

have used the ctype library we would

have done like two lower and called that

function passing it in although not

really cuz in C type those operate on

individual characters or chars not whole

strings we have actually didn't see a

function that could convert the whole

string in C to lowercase but in Python

we're going to benefit from some other

feature as well it turns out that python

supports what's called objectoriented

programming and we're only going to

scratch the surface of this in cs50 but

if you take a higher level course in

programming or CS you explore this as a

different Paradigm up until now in C

we've been focusing on what's called

really procedural programming you write

procedures you write functions top to

bottom uh left to right and when you

want to change some value we were in the

habit of using a procedure that is a

function you would pass something like a

variable into a function like to Upper

or to lower and it would do its thing

and hand you back a value well it turns

out that it would be nicer programming

wise if some data types just had

built-in functionality like why do we

have our variables over here and all of

our helper functions like two upper and

two lower over here such that we

constantly have to pass one into the

other it would be nice to sort of bake

into our data types buil-in

functionality so that you can change

variables using their own default be uh

buil-in functionality and so

object-oriented programming otherwise

known as oop is a technique whereby

certain types of values like a string

AKA stir not only have properties inside

of them attributes just like a struct in

C your data can also have functions

built into them as well so whereas in C

which is not objectoriented you have

strs and strs can only store data like a

name and a number when implementing a

person in Python you can for instance

have not just a structure otherwise

known as a class storing a name and a

number you can have a function like call

that person or email that person or

actual verbs or actions associated with

that piece of data now in the context of

strings it turns out that strings come

with a lot of useful functionality and

in fact this URL here which is in docs.

python.org which is the official

documentation for python you'll see a

whole list of methods that is functions

that come with strings that you can

actually use to modify their values and

what I mean by this is the following if

we go through the documentation poke

around it turns out that strings come

with a function called Lower and if you

want to use that function you just have

to use slightly different syntax than in

C you do not do to lower and you do not

say as I just did lower because this

function is built into s itself and just

like in C when you want to go inside of

a variable like a structure and access a

piece of data inside of it like name or

number when you also have functions

built into data types AKA methods a

method is just a function that is built

into a piece of data you can do s do

lower open pen closed pen in this case

and I can do this down here as well if

if s do lower in quote unquote uh n or

no the whole thing I can force this

whole thing to lowercase so the only

difference here now is in

object-oriented programming instead of

constantly passing a value into a

function you just access a function

that's inside of the value it just works

because of how the language itself is

defined and the only way you know that

these functions exist is the

documentation a Class A book a website

or the like questions now on this

technique all right I claim this is

correct now even though you've never

programmed most of you in Python before

not super well-designed there's an

subtle inefficiency now on lines three

and five together what's dumb about how

I've used lower might you

think

yeah yeah if you're going to use the

same function twice and ask the same

question expecting the same answer why

are you calling the function itself

twice maybe we should just store the

result in a variable so we could do this

in a couple of different ways we for

instance could go up here and create

another variable called T and set that

equal to s. lower and then we could just

change this to be T here but honestly I

don't think we technically need another

variable Al together here I could just

do something like this let's change the

value of s to be the lowercase version

thereof and so now I can quite simply

refer to S again and again like this

this reusing that same value now to be

sure I have now just lost the user's

original input and if I care about that

if they typed in all caps I have no idea

anymore so maybe I do want to use a

separate variable altogether but a

takeaway here too is that strings in

Python are technically what we'll call

immutable that is they cannot be changed

this was not true in C once we gave you

arrays in week two or memory in week

four you could go to town on a string

and change any of the characters you

want upper casing lower casing changing

it shortening it and so forth but in

this case uh this returns a copy of s

forced to lowercase it doesn't change

the original string that is the memory

the bytes in the computer's memory when

you assign it back to S you're

essentially forgetting about the old

version of s but because python does

memory management for you there's no

maloc there's no free python

automatically frees up the original

bytes like yees and hands them back to

the operating system for you all right

questions now on this

technique questions on

this in general I'll call out the python

documentation will start to be your

friend because in class we'll only

scratch the surface with some of these

things but in docs. python.org for

instance there's a whole reference of

all of the built-in functions that come

with the language as well as for

instance those with the string all right

well let me go ahead and before we take

a break let's go ahead and create

something a little familiar to based on

our week here in C let me propose that

we revisit those examples in involving

some meow so for instance when we had

our cat meow back in the first week and

then second in C we did something that

was a little stupid at first whereby we

created a file as I'll do here this time

called meow. p and if I want a cat to

meow three times I could run it once

like this little copy paste and now

python of meow. py and I'm done now

we've visited this example like two

times at least now in scratch it and see

it's correct I'll stipulate but what's

obviously poorly

designed what's the fault here

yeah it should just be a loop right like

why type it three times literally

copying and pasting is almost always a

bad thing except in C when you have the

function prototypes that you need to

borrow but in this case this is just

inefficient so what could we do better

here in Python well in Python we could

probably change this in a few different

ways we could borrow some of the syntax

we proposed in slide form earlier like

give me a variable called I set it to

zero no semicolon while I is less than

three if I want to do this three times I

can go ahead and print out meow and then

I can do i+ equals one and I think this

would do the trick python of meow. and

we're back in business already well if I

wanted to change this to a for Loop well

in Python it would be a little tighter

but this would not be the best approach

so for I in uh

012 I could just do print meow like this

and that too would get the job done but

to my to our discussion earlier this

would get stupid pretty quickly if you

had to keep enumerating all of these

values like what did we introduce

instead the the range function exactly

so that hands me back way more

efficiently just the values I want

indeed one at a time so even this if I

run it a third a third or fourth time

we've got the same result but now let's

transition to where we went with this

back in the day how can we start to

modularize this like just like it would

be nice I claimed if MIT had given us a

meow function wouldn't it be nice if

like python had given us a meow function

maybe less compelling in Python but how

can I build my own function well I did

this briefly with the spell checker

earlier but let me go ahead and propose

that we could Implement now our own

version of this in python as follows let

me go ahead and start fresh here and use

the keyword defa so this did not exist

in C you had the return value the

function name the arguments in Python

you literally say defa to define a

function you give it a name like meow

and now now I'm going to go ahead and in

this function just print out meow and

this lets me change it to anything else

I want in the future but for now it's an

abstraction and in fact I can uh move it

out of sight out of mind just going to

hit enter a bunch of times to pretend

like now it exists but I don't care how

it is implemented and up here now I can

do something like this 4 I in range of

three let me go ahead and not print meow

anymore let me just call meow and

tightening up my code further but I

think let's see python of meow. py this

is I think going to be the first time it

does not work

correctly okay so here we have sadly our

first python error and let's see the

syntax is going to be different from C

or clangs output traceback is like the

term of art here this is like a trace

back of all of the lines of code that

were just executed or really functions

you called the file name is

uninteresting this is like my codes

space specifically but the file name is

important here meow. py uh line two is

is the issue okay I didn't get very far

before I screwed up and then there's a

name error and you'll see in Python

there's typically these capitalized uh

keywords that hint at what the issue is

it's something related to names of

variables meow is not defined all right

you're programming python for the first

time you've screwed up you're following

some online tutorial you're seeing this

reason through it like why might meow

not be defined what can weer infer about

python how to troubleshoot

logically is it me ised after maybe is

it because meow is defined after you

know as smart as python seems to be Visa

VC they have some similar design

characteristics so let's try that so let

me scroll all the way back down to where

I move this earlier let me uh get rid of

it way down there I'll copy it to my

clipboard and let me just kind of hack

something together let me just put it up

here and let's see if this works so now

let me clear my terminal run python of

meow. okay we're back in business so

that was actually really good intuition

good debugging technique to sort of

reason through it now this is kind of

contradicting what I claimed back in

week one which was that you know the

main part of your program ideally should

just be at the top of the file like

don't make me look for it it's not a

huge deal with like a four-line program

but if you've got 40 lines 400 lines you

don't want like the juice juicy part of

your program to be way down here and all

of these functions way up here so it

would be nice maybe if we actually have

a main function and so it actually turns

out to be a convention in Python to

define a main function it's not a

special function that's automatically

called like in C but humans realized you

know what that was a pretty useful

feature let me Define a function called

main let me indent these lines

underneath it let me practice what I'm

preaching which is put the main code at

the top of the file and wonderfully in

Python now you do not need prototypes

there's none of that hackish copying and

pasting of the return type the name and

the arguments to a function like we

needed in C this is now

okay instead except for one Minor Detail

let me go ahead and run python of

meow. hopefully now I've solve this

problem by having a main function but

now nothing has happened all right even

if you've never programmed in Python

before What might explain this

behavior and how do I

fix again when you're off in the real

world learning some new language all you

have is deductive logic to debug yeah I

remember

right so the solution to be clear in C

was that we had to put the Prototype up

here otherwise we'd get an error message

in this case I'm actually not getting an

error message and indeed I'll claim that

you don't need the prototypes in Python

just not necessary because that was

annoying if nothing

else but what else might explain yeah

I'm

back yeah maybe you have to call Main

itself if main is not some some special

status in Python maybe just because it

exists isn't enough and indeed if you

want to call Maine the new convention is

actually going to be as the very last

line of your program typically to

literally call Main it's a little stupid

stupid looking but you know they made a

design decision and this is how now we

work around it python of meowy now we're

back in business but now logically why

does this work the way it does well in

this case top to bottom line one is

telling python to define a fun fun

called Main and then Define it as

follows lines two and three but it's not

calling main yet line six is telling

python how to define a function called

meow but it's not calling these lines

yet now line 10 you're telling python

call Main and at that point python has

been trained if you will to know what

main is on line one to know what meow is

on line six and so it's now perfectly

okay for Maine to be above meow because

you never called them yet you defined

defined and then you called and that's

the logic behind this any questions now

on the structure of this technique

here now let's do one more then recall

that the last thing we did in scratch

and in Python uh scratch and in C was to

actually

parameterize uh these same function so

suppose that you don't want Maine to be

responsible for the loop here you

instead want to very simply do something

like meow three times and be done with

it well in Python it's going to be

similar in spirit toy but again we don't

need to keep mentioning data types if

you want me now to take some argument

like a number n you can just specify n

as the name of that argument or you

could call it anything else of course

that you want you don't have to specify

int or anything else in your code now

inside of meow you can do something like

for I in let's say I definitely now

can't do this because like that would be

weird to start the list and end it with

n so if I can come back over here what's

the solution how can I do something n

times yeah using range so range is nice

cuz I can pass in now this variable n

and now I can meow whoops now I can

print out quote unquote meow so it's

almost the same as in scratch almost the

same as in C but it's a little simpler

and if now I run meow. P I'll have the

ability now to do this here as well all

right questions on any of this right now

we're sort of like taking the stroll

through week one we're going to

momentarily escalate things to look not

only at some of these basic but also

other features like we saw with face

recognition with the speller or the like

um because of how many of us are here we

have a huge amount of candy out in the

lobby so why don't we go ahead and take

a 10-minute break and we come back we'll

do even fancier more powerful things

with python in 10 all right so we are

back among our goals now are to

introduce a few more building blocks so

that we can solve more interesting

problems at the end much like those that

we began with you recall from a few

weeks ago we played with this sort of

two-dimensional Super Mario World and we

tried to print a vertical column of like

three or more bricks well let me propose

that we use this as an opportunity to

now Tinker with some of Python's more uh

useful more userfriendly functionality

as well so let me code a file called

mario. py and let's just print out like

that the equivalent of that vertical

column so it's of height three each one

is a hash so let's do for I in range of

three initially and let's just print out

a single hash and I think now python of

mario. py voila we're in business

printing out just that same pyramid

there or just that same column there

what if though we want to print a column

of like some variable height where the

user tells us how tall they want it to

be well let me go up here for instance

and instead how about we'll use um let's

do this how about uh from cs50

import how about the get in function as

before so it will deal with making sure

the user gives us an integer and now in

the past whenever we wanted to get a

number from a user we've actually

followed a certain Paradigm in fact if I

open up here for instance uh how about

Mario in how about Mario 1. C from a

while back you might recall that we had

code like this and we specifically use

the do while loop and see whenever we

want to like get something from the user

maybe again and again and again until

they cooperate at which point we finally

break out of the loop so it turns out

python does have while Loops does have

four Loops does not have do while loops

and yet pretty much anytime you've

gotten user input you've probably used

this Paradigm so it turns out that the

python equivalent of this is to do

similar in spirit but using only a while

loop and a common Paradigm in python as

I alluded earlier is to actually

deliberately induce an infinite Loop

while true capital T and then do what

you want to do like get an INT from the

user and prompt them for the height for

instance in question and then if you're

sure that the user has given you what

you want like n is greater than zero

which is what I want in this case cuz I

want a positive integer otherwise

there's nothing to print you literally

just break out of the loop and so we

could actually use this technique in C

it's just not really done in C you could

absolutely in C have done a while true

loop with the parentheses lowercase true

you could break out of it and so forth

but in Python this is like the python

way and this is actually a term of art

this way in Python is pythonic like this

is the way everyone does it quote

unquote doesn't mean you have to but

that's sort of the way like the cool

python programmers would Implement an

idea like this trying to do something

again and again and again until the user

actually cooperates but all we've done

is take away the do while loop but still

logically we can implement the same idea

now below this let me go ahead and just

print out for I in range of n this time

because I want it to be variable and not

three I can go ahead and print out the

hash let me go ahead and get rid of the

C version here open my terminal window

and I'll run again python of mario. py

I'll type in three and I get back those

three hashes but if I instead type in

four I now get four hashes instead so

the takeaway here is quite simply that

this would be the way for instance to

actually get back a value in Python that

is consistent with some parameter like

greater than zero how about this let's

actually uh practice what we preached a

moment ago with our meowing examples and

kind of factoring all this out let me go

ahead and Define a main function as

before let me go ahead and assume for

the moment that a get height function

exists which is not a thing in Python

I'm going to invent it in just a moment

and now I'm going to go ahead and do

something like this for I in the range

of that height well let's go ahead and

print out those hashes so I'm assuming

that get height exists let me go ahead

and Implement that abstraction so Define

a function now called get height it's

not going to take any arguments in this

design while true I can go ahead and do

the same thing as before assign a

variable n the return value of get int

prompting the user for that height and

then if n is greater than zero I can go

ahead and break but if I break here I

logically just like can see end up

executing below the loop in question but

there's nothing there but if I want get

height to return the height what should

I type here on line 14

logically what do I want to return to be

clear yeah so I actually want to return

n and here's another curiosity of python

Visa BC there doesn't seem to be an

issue of scope anymore right in C it was

super important to not only declare your

variables with the data types you also

had to be mindful of like where they

exist inside of those curly braces in

Python it turns out you can be a little

looser with things for better for worse

and so on line 11 if I create a variable

called n it exists on line 11 12 and

even 13

outside of the while loop so to be clear

in C with a while loop we would have

ordinarily had not the colon we would

have had the curly brace like here and

over here and a week ago I would have

claimed that in C N does not exist

outside of the while loop by nature of

those curly braces even though the curly

braces are gone python actually allows

you to use a variable anytime after you

have assigned it a value so slightly

more powerful as such how however I can

tighten this up a little bit logically

and this is true in C I don't really

need to break out of the loop by using

break recall that or know that I can

actually once I'm ready to go I can just

return the value I care about even

inside of the loop and that will have

the side effect of breaking me out of

the loop and also breaking me out of and

returning from the entire function so

nothing too new here in terms of C

versus python except for this issue of

scope and I indeed returned n at the

bottom there just to make clear that n

would still exist so either of those are

correct now I just have a Python program

that I think is going to allow me to

implement this same Mario idea so let's

run python of mario. and okay so nothing

happened uh python of mario.

py what did I do wrong yeah I have to

call Main so at the bottom of my code I

have to call Main here and this is a

stylistic detail that's been subtle um

generally speaking when when you are

writing in Python um there's not a cs50

style guide per se there's actually a

python style guide that most people

adhere to um it's and in this case

double blank lines between functions is

the norm I'm doing that deliberately

although uh it might otherwise not be

obvious but now that I've called main on

line 16 let's run mario. once more aha

now we get there now we see it type in

three and I'm back in business printing

out the values there

yeah

sure why do I need the if condition at

all why can't I just return n here as by

doing return n or if I really want to be

succinct I could technically just do

this the only reason I added the if

condition is because if the user types

in negative 1 -2 I wanted to prompt them

again and again that's all but that

would be totally acceptable too if you

were okay with that result instead well

let me do one other thing here to point

out why we are using get in so

frequently this new training wheel all

be it temporarily so let me go back to

the way it was a moment ago and let me

propose now to take away get int I

claimed earlier that if you're not using

get int you can just use the input

function itself from

python but that always returns a string

or a stir and so recall that you have to

pass the output of the input function to

an INT either on the same line or if you

prefer on another line instead but it

turns out what I didn't do was show you

what happens if you uh don't cooperate

with the user uh with the program so if

I run python of mario. now works great

even without the get int function and I

can do it with four still works great

but let me clear my terminal and be

difficult now as the user and type in C

for the height instead enter now we see

one of those tracebacks again this one

is different this isn't a name error but

apparently a value error and if I kind

of ignore the stuff I don't understand I

can see invalid literal for INT with

base 10 cat that's a super cryptic way

of saying that c a is not a number in

decimal notation and so I would seem to

have to somehow handle this case and if

you want to be more Curious you'll see

that this is indeed a trace back and um

C tends to do this too or the debugger

would do this for you too you can see

all of the functions that have been

called to get you to this point so

apparently my problem is initially in

line 14 but line 14 if I keep scrolling

is uninteresting it's main but line 14

leads me to execute line two which is

indeed in main that leads me to execute

line nine which is in get height and

okay here's the issue so the closest

line number to the error message is the

one that probably reveals the most line

nine is where my issue is so I can't

just blindly ask the user for input and

then convert it to an INT if they're not

going to give me an INT now how do we

deal with this well back in problem set

two you might recall validating that the

user typed in a number and using a for

Loop and the like well it turns out

there's a better way to do this in

Python and the are kind of there if you

want to try to convert something for a

number to a number that might not

actually be a number turns out Python

and certain other languages literally

have a keyword called try and if only

this existed for the past few weeks I

know but like you can try to do the

following with your code what do I want

to try to do well I want to try to

execute those few lines except if

there's an error so I can say except if

there's a value error specifically the

one I screwed up and created a moment

ago and if there is a value error I can

print out an informative message to the

user like not an integer or anything

else and what's happening here now is

literally this operative word try the

pro python is going to try to get input

and try to convert it to an in and it's

going to try to check if it's greater

than zero and then try to return it all

why all of three of those lines are

inside of indented underneath the tri

block except if something goes wrong

specifically a value error happens then

it prints this but it doesn't return

anything and because I'm in a loop that

means it's going to do it again and

again and again until the human actually

cooperates and gives me an actual number

and so this too is what the world would

call pythonic in Python you don't

necessarily rigorously try to validate

the users's input make sure they haven't

screwed up you honestly take a more

laxidasical approach and just try to do

something but catch an error if it

happens so catch is also a term of art

even though it's not a keyword here

except if something happens you handle

it so you try and you handle it it's

sort of best effort programming if you

will but this is baked into the mindset

of the Python uh programming community

so now if I do python of mario. py and I

cooperate works great as before try and

succeed three Works four works if though

I try and fail by typing in cat it

doesn't crash per se it doesn't show me

an error it shows me something more user

friendly like not an integer and then I

can try again with dog not an integer I

can try again with five and now it works

so we won't generally have you write

much in the way of these try except

blocks only because they get a little

sophisticated quickly but that is to

reveal what the get int function is

doing this is why we give you the

training wheels so that when you want to

get an INT you don't have to jump

through all these annoying Hoops to do

so but that's all the library is really

doing for you is just try and accept you

won't be left with any training wheels

ultimately questions now on getting

inputs and trying in this way

anything at all

yeah Tri block it say that oh you could

you put the condition outside of the tri

block short answer yes and in fact I

struggled with this last night when

tweaking this example to show the

simplest version I will disclaim that

really I should only be trying literally

to do the The Fragile part and then down

here I should be really doing what

you're proposing which is do the

condition out here the problem is though

that logically this gets messy quickly

right because except if there's a value

error I want to print out not an integer

I can't compare n against zero then

because n doesn't exist because there

was an error so it turns out and I'll

show you this this is now the advanced

version of python there's actually an

else keyword you can use in Python that

does not accompany if or L if it

accompanies try and accept which I think

is weirdly confusing a different word

would have been better but if you really

prefer I could have done this instead

dead and this is one of these design

things where like reasonable people will

disagree generally speaking you should

only try to do the one line that might

very well fail but honestly this looks

kind of stupid now it's just

unnecessarily complicated and so my own

preference was actually the original

which was yeah I'm trying a few extra

lines that really aren't going to fail

mathematically but it's just tighter

it's cleaner this way and here's again

the sort of like you know arguments

you'll start to make yourself as you get

more comfortable with programming you'll

have an opinion you'll disagree with

someone and so long as you can back your

argument up pretty

probably all right so how about we now

take away some piece of magic that's

been here for a while let me go ahead

and uh Delete all of this here and let

me propose that we revisit uh not that

vertical column and the exceptions that

might result from getting input but

these like horizontal question marks

that we saw a while ago so I want all of

those question marks on the same line

and yet I worry we're about to see a

challenge here because print up until

now has been putting new lines

everywhere automatically even without

those backslash NS well let me propose

that we do this for I in the range of

four if I want four question marks let

me just print four question marks

unfortunately I don't think this is

correct yet let me run python of mario.

and of course this gives me a column

instead of the row of question marks

that I want so how do we do this well it

turns out if you read the documentation

for the print function it turns out that

print not surprisingly perhaps takes a

lot of different arguments as well and

in fact if you go to the documentation

for it you'll see that it takes not just

positional arguments that is from left

to right separated by commas turns out

python has supports a fancier feature

with arguments where you can pass the

names of arguments to functions too so

what do I mean by this if I go back to

vs code here and I've read the

documentation it turns out that yes as

before you can pass multiple arguments

to python like this like hello comma

David comma me that will just

automatically concatenate all three of

those positional arguments together

they're positional in the sense that

they literally flow from left to right

separated by commas but if you don't

want to just pass in values like that

you want to actually print out as I did

before a question mark but you want to

override the default behavior of print

by changing the line ending you can

actually do this you can use the name of

an argument that you know exists from

the documentation set it equal to some

alternative value and in fact even

though this looks cryptic this is how I

would override the end of each line to

be quote unquote that is nothing because

if you read the documentation the

default value for this end argument does

someone want to guess is is back sln so

if you read the documentation you'll see

that back sln is the implied default for

this end argument and so if you want to

change it you just say end equals

something else and so here I can change

change it to nothing and now rerun

python of mario. and now they're all on

the same line now looks a little stupid

cuz I made that sort of week one mistake

where I still need to move the cursor to

the next line that's just a different

problem I'm just going to go over here

and print nothing I don't even need to

print back sln because if print

automatically gives you a backslash n

just call print with nothing and you'll

get that for free so let me rerun python

of mario. Pi and now it looks a little

prettier at the prompt and to be super

clear as to what's going on suppose I

want to sort of make an exclamation here

I could change the back sln default to

like an exclamation point Just for kicks

and if I run python of mario. py again

now I get this sort of you know

exclamation with question marks and

exclamation points as well so that's all

that's going on here and this is what's

called a named argument it literally has

a name that you can specify when calling

it in and it's different from positional

in that you're literally using the

name let me propose something else

though and this is why people kind of

like python there's just kind of cool

ways to do things that's kind of a you

know ver it's a three line verbose way

of printing out four question marks you

know I could certainly take the you know

shortcut and just do this but that's not

really that interesting for anyone

especially if I want to do it a variable

number of times but python does let you

do this if you want to uh multiply a

character some number of times not only

can you use Plus for concatenation you

can use star or an asterisk for

multiplication if you will that is

concatenation again and again and again

so if I just print out quote unquote

question mark Time 4 that's actually

going to be the tightest way the most

distinct way I can print four question

marks instead and if I don't use four I

use n where I get n from the user bang

like now I've gotten rid of the four

Loop entirely and I'm using the the star

operator to manipulate it instead and to

be super clear here in so far as python

does not have milock or free or memory

management that you have to do guess

what python also doesn't

have anything on your minds the past

couple of

weeks doesn't have pointers yes so

python does not have pointers which just

means that all of that happens for you

automatically underneath the hood Again

by way of code that someone else wrote

how about one more throwback with Mario

we've talked about in week one this sort

of two-dimensional structure where it's

like I claim like 3x3 a grid of bricks

if you will well how can we do this in

Python we can do this in a couple of

ways now let me go back to my mario. py

and let me do something like for I in

range of we'll just do three even though

I know now I could use get int or I

could use input and int and if I want to

do something two-dimensionally just like

in C you can Nest your for Loop so maybe

I could do 4J in range of three and then

in here I could print out a uh hash

symbol and then let's see if that gives

me nine total so if I've got a nested

Loop like this python of mario.

hopefully gives me a grid no it gave me

a column of

nine why

logically even though I've got my row

and my

columns

yeah yeah the line ending so in my row I

can't let print just keep adding new

line adding new lines so I just have to

override this here and let me not screw

up like before let me print one at the

end of the whole row just to move the

cursor down and I think now together now

we've got our 3x3 of course we could

tighten this up further like if I don't

like the nested loop I probably could go

in here and just print out for instance

a uh a brick times three or I could

change the three to a variable if I've

gotten it from the user so I can tighten

this up further so again just different

ways to solve the same problem and again

sort of evidence of why a lot of people

like python there's just some more

pleasant ways to solve problems without

getting into the weeds constantly of

doing things like like with um uh for

loops and wild Loops endlessly all right

well how about some other building

blocks lists are going to be so

incredibly useful in Python just as

arrays were in C but arrays are annoying

because you have to manage the memory

yourself you have to know in advance how

big they are or you have to use pointers

and malok or realloc to resize them like

oh my God like the past two weeks have

been painful in that sense but python

does this all for free for you in fact

there's a whole bunch of functions that

come with python that involve lists and

they'll ow us ultimately um to do things

again and again and again uh with uh

within the same data structure and for

instance we'll be able to get the length

of a list you don't have to remember it

yourself in a variable you can just ask

python how many elements are in this

list and with this I think we can solve

some some old problems too so let me go

back here to vs code let me close Mario

and give us a new program called scores.

piy and rather than show the C and the

python now let's just focus on Python

and in scores. C way back when we just

averaged like three test scores or

something like that 72 73 and 33 a few

weeks ago so if I want to create a list

in this python version of 72 73 33 I

just use my square bracket notation C

let you use curly braces if you know the

values in advance but Python's just this

and now if I want to compute the average

in pi in C recall I did something with a

loop I added all the values together I

then divided by the total number values

just like you would in grade school and

that gave me the average well python

comes with a lot of super handy

functions not just length but others as

well and so in fact if you want to

compute the average you can take the sum

of all of those scores and divide it by

the length of all of those scores so

python comes with length comes with sum

you can just pass in a whole list of any

size and let it deal with that problem

for you so if I want to now print out

this average I can print out average

colon and then I'll plug in my average

uh string for variable for interpolation

let me make this an F string so that it

gets formatted and let me just run

python of scores. Pi and there's my

average it's sort of rounding weird

because we're still vulnerable to some

floating point in Precision but at least

I didn't need loops and I didn't have to

write all this darn code just to do

something that you know Excel and Google

spreadsheets can just do like that well

python is closer to those kinds of tools

but more powerful and that you can

manipulate the data yourself how about

though if I want to um get a bunch of

scores manually from the user and then

sum them together well let's combine a

few ideas here how about this first let

me go ahead and uh import um the cs50 LI

get in function from the cs50 library

just so we don't have to deal with try

and accept or all of that and let me go

ahead and give myself an empty list and

this is powerful in py in C there's

really there's no point to an empty

array because if you create an empty

array with square bracket notation like

it's not useful for anything but in

Python you can create it empty because

python will grow and shrink the list for

you automatically as you add things to

it so if I want to get three scores from

the user I could do something like this

for I in range of three and then I can

grab a variable called score or anything

I could call get int prompt the human

for the score that they want to type in

and then once they do I can do this

thinking back to our objectoriented

programming capability now I could do

scores dot a pen

and I Canen that score to it and you

would only know this from having read

the documentation heard it in class in a

book or whatnot but it turns out that

just like strings have functions like

lower built into them lists have

functions like append built into them

that just literally appends to the end

of the list for you and python will grow

or Shrink it as needed no more Malo or C

or Realo or the like so this just

appends to the scores array the scores

list that score and then again and again

and again so the array starts at

sorry the list starts at size zero then

grows to one then two then three without

you having to do anything else and so

now down here I can compute an average

with the sum of those scores divided by

the length of the total number of scores

and to be clear length is the total

number of elements in the list doesn't

matter how big the values themselves are

now I can go ahead and print out an FST

string uh with something like average

colon average and curly braces and and

if I run python of scores. piy I'll type

in just for the sake of discussion the

three values I still get the same answer

but that would have been painful to do

in see unless you committ it in advance

to a fixed size array which we already

decided weeks ago was annoying or uh you

uh grew it dynamically using malok or

realloc or the like all right what else

can I do well there's some nice things

you might as well know exist um instead

of scores. aen you can do slight

fanciness like this like if you want to

append something to a list you can

actually do plus equals and then put

that thing in a a temporary list of its

own and just use what is essentially

concatenation but not concatenation of

strings but concatination of lists so

this new line six appends to the scores

list this tiny little list I'm

temporarily creating with just the

current new score so just another piece

of syntax that's worth seeing that

allows you to do something like that as

well all right well how about we go back

to strings for a moment and all these

examples as always are on the course's

website afterward suppose we want to do

something like converting characters to

uppercase well to be clear I could do

something like this let me create a

program called uppercase dop let me

prompt the user for a before string as

by using the input function or get

string which is almost the same and I'll

prompt the user for a string

beforeand then let me go ahead and print

out uh how about the keyword after

and then end the new line with nothing

just so that I can see before on one

line and after on the next line and then

let me do this and here's where python

gets pleasant too with loops for C in

before print c. uper n equals quote

unquote and then I'll print this here

all right that was fast but let's try to

infer what's going on so line one just

gets input from the user stores it in a

variable called before line two

literally just prints after but doesn't

move the new line to uh the cursor to

the next line what it then does is this

and in C this was a little more annoying

you needed a for loop with I you needed

array in uh notation with the square

brackets but python if you say four

variable in string so for c for

character in string Python's going to

automatically assign C to the first word

letter that the user types in then on

the next iteration the second letter the

third letter and the fourth so you don't

need any square bracket notation you

just you see and python will do it for

you and just hand you back one at a time

each of the letters that the user has

typed in so if I go back over here and I

run for instance python of uppercase

dopy and I'll type in how about uh David

in all lowercase and hit enter you'll

now see that it's all uppercase instead

by iterating over it indeed one

character at a time but we already know

thanks to object-oriented programming

strings themselves have the

functionality built in to not just

uppercase Single Character

but the whole string so honestly this

was a bit of a silly exercise I don't

need to use a loop anymore like in see

and so some of the habits you've only

just developed in recent weeks it's time

to start breaking them when they're not

necessary I can create a variable called

after set it equal to before do uper

which indeed exists just like lower

exists and then what I can go ahead and

print out is for instance uh let's get

rid of this print line here and do it at

the end after and print the value of

that variable so now if I rerun

uppercase Pi type in David and all

lowercase I can just uppercase the whole

thing all at once because again in C in

Python you don't have to operate on

characters individually questions on any

of these tricks up until

now now all right how about a few other

techniques that we saw and C that we'll

bring back now in Python so it turns out

in Python there are other librar you can

use two that unlock even more

functionality so in C if you wanted

command line arguments you just change

the proo the signature for main to be

void instead of void to be int argc

comma string argv Open brackets for an

array or Char star eventually well it

turns out in Python that if you want to

access command line arguments it's a

little simpler but they're tucked away

in a library otherwise known as a module

called CIS the CIS or system module now

this is similar in spirit to the cs50

library and that it's got a bunch of

functionality built in but this one

comes with python itself so if I want to

create a program like greet py in VSS

code here let me go ahead and do this

from the CIS Library let's import argv

and that's just a thing that exists it's

not built into main because there is no

main per se anymore so it's tucked away

in that library and now I can do

something like this if the length of

argv equals equals 2 well let's go ahead

and print out something friendly like

hello comma AR V bracket 1 and then

close quotes else if the length of RV is

not equal to two let's just go ahead and

print out hello world now at a glance

this might look a little cryptic but

it's identical to what we did a few

weeks ago when I run this python of

greet with no arguments it just says

hello world but if I instead add a

command line argument like my first name

and hit enter now the length of arv is

no longer one it's going to be two and

so it prints out hello David instead so

the takeaway here is that whereas in C

argv technically contained the name of

your program like hello or dog greet and

then everything the human typed Python's

a little different in that because we're

using The Interpreter in this way

Technically when you run python of greet

py the length of arv is only one it

contains only greet so the name of the

file it does not unnecessarily contain

python itself because what's the point

of that being there omnes it does

contain the number of words that the

human typed after python itself so argv

is length one here argv is length two

here and that's why when it did equal to

I saw hello David instead of the default

hello world so same ability to access

command line arguments add these kinds

of inputs to your functions but you have

to unlock it by way of using Arvy uh

instead in this way if you want to see

all of the words you could do something

like this uh just as if we combine ideas

here for I in range of how about length

of

arv then I can do this print argv

bracket I all right a little cryptic but

line three is just a for Loop iterating

over the range of length of argv so if

the human types in two words the length

of argv will be two so this is just a

way of saying iterate over all of the

words in arv printing them one at a time

so python of greet dopy enter just

prints out the name of the program

python of greet dopy with David prints

out greet dopy and then David I can keep

running it though with more words and

they'll each get printed one at a time

but what's nice too about Python and

this is the point of this exercise

honestly this looks pretty cryptic this

is not very pleasant to look at if you

just want to iterate over every word in

a list which argv is watch what I can do

I can do for ARG or any variable name in

ARG V let me just now print out that

argument I could keep calling it I but I

seems weird when it's not a number so

I'm changing to AR as a word instead if

I now do python of greet py it does this

if I do python of greet David it does

that again David ma it does that again

so this is again why Python's just very

appealing you want to do something this

many times iterate over a list just say

it and it reads a little more like

English and there's even other fanciness

too if I may it's a little stupid that I

keep seeing the name of the program

greet dopy so I it'd be nice if I could

remove that python also supports what

are called slices of arrays sorry slices

of lists even though I get the

terminology confused if Arvy is a list

then it's going to print out everything

in it but if I want a slice of it that

starts at location one all the way to

the end you can use this funky syntax in

between the square brackets which we've

not seen yet that's going to start at

item one and go all the way to the end

and so this is a nice clever way of

slicing off if you will the very first

element because now when I run greet doy

David men I should only see David and

men if I only want one element I could

do one to two if I want all of them I

could do zero onward I could give myself

just two of one of them in this way so

you can play with the start value and

the end value in this way to sort of

slice and dice these lists in different

ways that would have been a pain in see

just because we didn't really have the

built-in support for manipulating arrays

as cleanly as this all right just so

you've seen it too though this one is

less uh exciting to see live if I go

ahead and create a quick program here it

turns out there's something else in the

CIS Library the ability to exit programs

either exiting with status code one or

zero as we've been doing anytime

something goes right or wrong so for

instance Let Me Whip up a quick program

that just says if the length of cy.

argv uh does not equal two then let's y

at the user and say you're missing a

command line argument otherwise command

line argument and let's then return cy.

exit one else let's go ahead and

logically just say print a formatted

string that says hello as before cy.

arv1 now things look different all of a

sudden but I'm doing something

deliberately first let's see what this

does so on line one I'm importing not

argv specifically I'm importing the

whole CIS library and we'll see why in

second well it turns out that this Arvy

the CIS library has not only the Arvy

list it also has a function called exit

which I'd like to be able to use as well

so it turns out that if you import a

whole library in this way that's fine

but you have to refer to the things

inside of it by using that same

library's name and a DOT to sort of

namespace it so to speak so here I'm

just saying if the user types in does

not type in two words yell at them with

missing command line argument and then

exit with one just like in C when you do

exit one just means something went wrong

otherwise print out hello to this and

this is starting to look cryptic but

it's just a combination of ideas the

curly braces means interpolate this

value plug it in here cy. Arvy is just

the verbose way of saying go into the

CIS library and get the argv variable

therein and bracket one of course just

like arrays and C is just the second

element at the prompt so when I run this

version now python of exit. py with no

arguments I get yelled at in this way if

however I type in two arguments total

the name of the file and my own name now

I get greeted with hello David and it's

the same idea before this was a very

low-level technique but same thing here

if you do Echo dollar sign question mark

enter you'll see the exit code of your

program so if I do this incorrectly

again let me rerun it without my name

enter I get yelled at but if I do Echo

dollar sign question mark there's the

secret one that's returned again just to

show You parody with C in this case

questions now on any of these techniques

here all right how about something

that's a little more powerful too we

spent so much time in week zero and one

doing searching and then eventually

sorting in week three well it turns out

python can help with some of this too

let me go ahead and create a program

called names. py that's just going to be

an opportunity to maybe search over a

whole bunch of names let me go ahead and

import CIS and then just so I have

access to exit and let me go ahead and

create a variable called names that's

going to be a list with a whole bunch of

names uh how about Here Charlie and Fred

and George and Jenny and Percy and

lastly Ron so a whole bunch of names

here and you know it'd be a little

Annoying to implement code that iterates

over that from left to right and see

searching for one of those names in fact

what name well let's go ahead and ask

the user to input the name that they

want to search for so that we can tell

them if the name is there or not and we

could do this similar to C in Python

doing something like this so for n in

names where n is just a variable to

iterate over each name if how about the

name I'm looking for equals the current

name in the list AKA n let's print out

something friendly like found and then

let's do cy. Exit 0 to indicate that we

found whoever that is otherwise if we

get all the way to the bottom here

outside of this Loop let's just print

not found because if we haven't exited

yet and then let's just exit with one

just to be clear I can continue

importing all of CIS or I could do from

CIS import exit and then I could get rid

of CIS dot everywhere else but you know

sometimes it's helpful to know exactly

where functions came from so this two is

just a matter of style in this case all

right so let's go ahead and run this

python of names. piy and let's look for

like Ron all the way at the end you know

all right he's found and let's search

for someone outside of the family here

like herion not found okay so it seems

to be working in this way but I've

essentially implemented what

algorithm what algorithm would this seem

to be per lines seven and eight and nine

and

10 yeah so it's just linear search it's

a loop even though the syntax is a

little more succinct today and it's just

iterating over the whole thing well

honestly we've seen an even more tur way

to do this in Python and this again is

what makes it a more pleasant language

sometimes why don't I just do this

instead in of iterating one at a time

why don't I just say this let me go

ahead and change my condition to just be

How about if the name we're looking for

is in the names list we're done we found

it use the in preposition that we've

seen a couple of times now that itself

asks the question is something in

something else and python will take care

of linear search for us and it's going

to work exactly the same if I do python

of names. search for run it's still

going to find him and it's still going

to do it linearly in this case but I

don't have to write all of the lower

level code myself in this

case questions now on any of this the

code's just getting shorter and

shorter now what about uh let's see what

else might we have here how about this

it turns out let's go ahead and

Implement that phone book that we

started metaphorically with in the

beginning of the course let's code up a

program called phonebook dopy and in

this case let's go ahead and let's

create a dictionary this time recall

that a dictionary is a little something

that implements something like this like

a two column table that's got keys and

values words and definitions names and

numbers and let's focus on the last of

those names and numbers in this case

well I claimed earlier that python has

built-in support for dictionaries dict

objects that you can create with one

line I didn't need it for speller

because a set is sufficient when you

only want one of the keys or the values

not both but now I want some names and

numbers so it turns out in pyth python

you can create an empty dictionary by

saying dict open parenthesis close and

that just gives you essentially a chart

that looks like this with nothing in it

or there's more succinct syntax you can

alternatively do uh this with two curly

braces instead and in fact I've been

using a shortcut all this time when I

had a list earlier where my

variable uh was called scores and I did

this that was actually the shorthand

version of this hey python give me an

empty list so there's different Syntax

for achieving the same goal in this case

if I want a dictionary for people I can

either do this or more commonly just two

curly braces like that all right well

what do I want to put in this well let

me actually put some things in this and

I'm going to just move my Clos curly

brace to a new line if I want to

implement this idea of keys and values

the way you do this in Python is key

colon value comma key colon value so

you'd implement it more in code so for

instance if I want Carter to be the

first key in my phone book and I want

his number to be+ one 617495 1000 I can

put that as the corresponding value the

colon is in between both are strings or

stirs so I've quoted both deliberately

if I want to add myself I can put a

comma and then just to keep things

pretty I'm moving the cursor to the next

line but that's not strictly required

aesthetically it's just good style and

here I might do+ 1 949 468 uh

2750 and now I have a dictionary that

essentially has two rows here David uh

Carter and his number and David and his

number as well and if I kept adding to

this this call this chart would just get

longer and longer suppose I want to

search for one of our numbers well let's

prompt the user for the name for whose

number you want to search by getting

string or you know what we don't need

the cs50 library let's just use input

and prompt the user for a name and now

we can use this super syntax and just

say if name in people print the format

added string number colon and here we

can do this people bracket name okay so

this is getting kind of cool kind of

quickly kind of confusingly so let me

run this python of phone

book. let's type in Carter and indeed I

see his number let's run it again with

David and I see my number here so what's

going on well it turns out that a

dictionary is very similar in spirit to

a list it's actually very similar in

spirit to an array in C but instead of

being limited to keys that are numbers

like bracket 0o bracket 1 bracket two

you can actually use words and that's

all I'm doing here on line eight if I

want to check for the name Carter which

is currently in this variable called

name I can index into my people

dictionary using not a number but using

literally a string the name Carter or

David or anything else to make this

clear too notice that I'm at the moment

you using this format string which is

adding some undue complexity but I could

clarify this perhaps further as this I

could give myself another variable

called number set it equal to the people

dictionary indexing into it using the

current name and now I can shorten this

to make it clear that all I'm doing is

printing the value of that and in fact I

can do this even more cryptically if I

this would be weird to do but if I only

ever want to show David's phone number

and never Carter's I can literally quote

unquote index into the people dictionary

because now when I run this even if I

type Carter I'm going to get back my

number instead but that's all that's

happening if I undo that because that's

now a bug but I index into it using the

value of name dictionaries are just so

wonderfully convenient because now you

can associate anything with anything

else but not using numbers but entire

keywords instead so here's how if in

spell we gave you not just words but

hundreds of thousands of definitions as

well you could essentially store them as

this and then when the human wants to

look up a definition in a proper

dictionary not just for spellchecking

you could index into the dictionary

using square brackets and get back the

definition in English as well

questions on this

yeah

location a really good question so how

to summarize how is python finding that

name within that dictionary this is

where honestly speller in pet 5 is what

Python's all about so you have struggled

are struggling with implementing your

own spell checker and implementing your

own hash table and recall that per last

week the goal of a hash table is to

ideally get constant time access not

something linear which is slow and even

better than something uh uh logarithmic

like log base 2 of n so Python and the

really smart people who invented it they

have written the code that does its best

to give you constant time searches of

dictionaries and they're not always

going to succeed just as you and your

own problem set probably going to have

some collisions once in a while and

start to have chains of length lists of

words but this is where again you defer

to someone else someone smarter than you

someone with more time than you to solve

these problems for you and if you read

Python's documentation you'll see that

it doesn't guarantee constant time but

it's going to ideally optimize the data

structure for you to get as fast as

possible and of all of the data

structures um like a dictionary a

hashtable is really like the Swiss army

knife of computing because it just lets

you associate something with something

else and even though we keep focusing on

names and numbers that's a really

powerful thing because it's more

powerful than lists and arrays which are

only numbers and something else now you

can have any sorts of relationships

instead all right let me show a few

other examples before we culminate with

some more powerful techniques in pythons

thanks to libraries how about this

problem we encountered in week four

which was this let me code up a program

called again compare. Pi here but this

time compare to Strings and not numbers

so let me for instance do uh get one

string from the user called s just for

the sake of discussion let me get

another string from the user uh called T

so that we can actually do some

comparison here and if s equals equals T

let's go ahead and print out that

they're the same else let's go ahead and

print out that they're different so this

is very similar to what we did in week

four but in week four recall we did this

spef specifically because we had

encountered a problem for instance if I

run whoops

uh if I

run what's going on uh input

T come

on oh the okay wow okay long day all

right if I run the proper command python

of compare. py then let's go ahead and

type in something like cat in all

lowercase cats in all lowercase and

they're the same uh if though I do this

again with dog and dog they're the same

and of course cat and dog they're

different but does anyone recall from

two weeks ago when I typed in my name

twice both identically capitalized what

did it say that they were in

fact different and why was that like why

were two strings in C different even

though I typed literally the same

thing two different places in memory so

each string might look the same

aesthetically but of course was stored

elsewhere in memory and yet python

appears to be using the equality

operator equals equals like you and I

would expect as humans actually

comparing for us Char by Char in each of

those strings for actual equality so

this is a feature of python in that it's

just easier to do and why well this

derives from the reality that in Python

there are no pointers anymore there's no

underlying memory management it's not up

to you now to worry about those lower

level details the language itself takes

care of that for you and so similarly if

I do this and don't ask the user for two

strings but just one and then I do

something like this how about give

myself a second variable T set it equal

to s.

capitalize which note is not the same as

upper capitalize by Design per Python's

documentation will only capitalize the

first letter for you I can now print out

say two F strings here what the value of

s is and then let me print out with

another F string what the value of t is

and recall that in C this was a problem

because if you capitalize s and store it

in t we accidentally capitalized both s

and t but in this case in Python when I

actually run this and type in cat in all

lowercase the original s is unchanged

because when I use capitalize on line

three this is indeed capitalizing s but

it's returning a copy of the result it

cannot change S itself because again for

that technical term s is IM mutable

strings once they exist cannot be

changed themselves but you can return

copies and modified mutated copies of

those same strings so in short all of

those headaches we encountered in week

four are now solved really in the way

you might expect and here's another one

that we dwell on in week four with the

colored uh liquid in glasses let me code

up a program called swap. pi and in

swap. pi let me set x equal to 1 y equal

to 2 and then let me just print out an F

string here so how about X X is this

comma Y is that and then let me do that

twice just for the sake of demonstration

and in here recall that we had to create

a swap function but then we had to pass

it in by reference with the Amper sand

and like oh my God like that was kind of

peak complexity and see well if you want

to swap X and Y in Python you could do X

comma yal y comma X and now python of

swap whoops python of

swap. and there we go all of that's

handled for you sort of like a shell

game without even a temporary variable

in mind so what more can we do here how

about a few final building blocks and

these related now to files from that

week four suppose that I want to save

some names and numbers in like a a CSV

file comma separated values which is

like a very lightweight spreadsheet well

first let me create a uh phonebook CSV

file that just has name comma number as

like the first row there but after after

that I'm going to go ahead now and code

up a phonebook dopy program that

actually allows me to add things to this

phone book so let me split my screen

here so that we can see the old and the

new and down here in my code for phone

book. in this new and improved version

I'm going to actually import a whole

other Library this one called CSV and

here too especially for people in data

science and the like really like being

in a manipulate files and data that

might very well be stored in

spreadsheets or csvs comma separated

values which we we saw briefly in week

four in phonebook then it suffices to

just import CSV after reading the

documentation therefore because this is

going to give me functionality in code

related to CSV files so here's how I

might open a file in Python I literally

call open it's not fop now it's just

open and I open this file called

phonebook.com

going to open it in a pend mode not

right where it would change the whole

thing I want to append new line at a

time uh after this I want to get maybe

how about a name from the user so let's

prompt the user for some input for their

name and then let's prompt the user for

a number as well using input prompting

for number all right and now this is a

little cryptic and you'd only know this

from the documentation but if you want

to write line rows to a CSV file that

you can then view in Excel or the like

you can do this give me a variable

called writer but I could call it

anything I want let me use a CSV do

writer function that comes with this CSV

Library passing in the file this is like

saying hey python treat this open file

as a CSV file so that things are

separated with commas and nicely

formatted in rows and columns now I'm

going to do this use that writer to

write a row well what do I want to write

I want to write a short list namely the

current name and the current number to

that file but I don't want to use FR

printf and percent s and all of that

stuff that we might have had in the past

and now I just want to close the file

let me reopen my ter teral let me run

python of phone book. and let me type in

how about David and then +1

949468 2750 and hold crossing my fingers

watching the actual CSV at top left my

code has just added me to the file and

if I were to run it again for instance

with Carter in plus1 617495 1000

crossing my fingers again we've updated

the file and it turns out there's code

now via which I can even read that file

but I can first tighten this up just so

you've seen it it turns out into python

it's so common to open files and close

them you know humans make mistakes and

they often forget to close files which

might then end up using more memory than

you intend so you can alternatively do

this in Python so that you don't have to

worry about closing files you can use

this keyword instead you can say with

the opening of this file as a variable

called file do all of the following

underneath so I'm indenting most of my

code I'm using this new python specific

keyword called wi and this is just a

matter of saying with the following

opening of the file do those next four

lines of code and then automatically

close it for me at the end of the

indentation it's a minor optimization

but this again is sort of the pythonic

way to do things instead how else might

I do this too well it turns out that the

code I've written here on line n

especially is a little fragile right if

any human opens the spreadsheet the CSV

file in Excel Google spreadsheets Apple

numbers and maybe like moves the columns

around just because maybe they're

fussing they SA it and they don't

realize they've now changed my

assumptions I don't want to necessarily

write name and number always in that

order CU what if someone screws up and

flips those two columns by literally

dragging and dropping so it turns out

that instead of using a list here we can

use another feature of this Library as

follows instead of using a writer

there's something called a dictionary

writer or dict writer that takes the

same argument as input the file that's

opened but now the one difference here

is that you need to tell this dictionary

writer

that your field names are name and

number and let me close the CSV here

name and number are the names of the

fields The Columns in this CSV file and

when it comes time to write a new row

the syntax here is going to be a little

uglier but it's just a dictionary the

name I want to write to the dictionary

is going to be whatever name the human

typed in the number that I want to write

to the the CSV file is going to be

whatever the number the human typed in

but what's different next about this

code is by simply using a dictionary

writer here instead of the generic

writer now the columns can be in this

order or this order or any order and the

dictionary writer is going to figure out

based on the first line of text in that

CSV where to put name where to put

number so if you flip them no big deal

it's going to notice oh wait the columns

changed and it's going to insert the

columns correctly so just again another

more powerful feature that lets you

focus

on lets you focus on real work as

opposed to actually uh getting tied up

in the weeds of writing code like this

otherwise questions on this one as well

but what we will do now is come full

circle to some of the more uh

sophisticated examples with which we

began and I'm going to go back over to

my own Mac laptop here where I've got my

own terminal window up and running and I

was just going to introduce a couple of

final libraries that really speak to

just how powerful python can can be and

how quickly you can get up and running

to be fair can't necessarily do all of

these things in the cloud like in code

spaces because you need access to your

own speakers or microphone or the like

so that's why I'm doing it on my own Mac

here but let me go ahead and open up a

program called

speech. and I'm not using VSS code here

I'm using a program called VI that's

entirely terminal window based but it's

going to allow me for instance to import

the python text to speech version three

Library I'm going to give myself a

variable called engine that's going to

be set equal to the python text to

speech 3 libraries init method which is

just going to initialize this library

that relates to text to speech I'm going

to then use the engines say function to

say something like how about hello comma

world and then as my last line I'm going

to say engine. runand wait capitalized

as such to tell my program now to run

that speech and wait until it's done all

right I'm going to save this file I'm

going to run python of speech. py and

I'm going to cross my fingers as always

and hello world all right so now I have

a program that's actually synthesizing

speech using a library like this how can

I now modify this to be a little more

interesting well how about this let me

go ahead and prompt the user for their

name like we've done several times here

using Python's built-in name function

and now let me go ahead and use a format

string in conjunction with this Library

interpolating the value of name there

and at least if my name is somewhat

phonetically pronounceable let's go

ahead and run python ofp

speech. type in my name and hello David

okay it's a sort of weird choice of

inflection but we're starting to

synthesize voice not unlike Siri or

Google assistant or Alexa or the like

now we can maybe do something a little

more advanced too in addition to

synthesizing speech in this way we could

synthesize for instance uh an actual

graphic let me go ahead now and do

something like this let me create a

program called

qr. I'm going to go ahead and import a

library called OS which gives you access

to operating system related

functionality in Python I'm going to

import a library I've pre-installed

called QR code which is a

two-dimensional barcode that you might

have seen in the real world I'm going to

go ahead and create an image variable

using this QR code library's make

function which per its documentation

takes a URL like one of cs50's own

videos so we'll do this with YouTube uh

/xv f

zj5

p g g0 so hopefully that's the right

lecture and now we've got image. saave

which is going to allow me to create a

file called qr. ping think back now on

problem set 4 and how painful it was to

save files we'll just use the save

function now in Python and save this as

a ping file portable Network graphic and

then lastly let's just go ahead and open

uh the with the command open qr. ping on

my Mac so that hopefully this just

automatically opens all right I'm going

to go ahead and just double check my

syntax here so that I haven't made any

mistakes I'm going to go ahead and run

python of qr. enter that opens up this

let me go ahead and zoom in if you've

got a phone handy and you'd like to scan

this code

here whether in person or

online I apologize you won't appreciate

it amazing okay and lastly let me go

back into our speech example here uh

create a final ending here on our final

moments and how about we just say

something like this was cs50 like this

let's go ahead here fix my

capitalization just for tidiness get rid

of the name and now with our final

flourish and your introduction to python

equipped here we go this was cs50 all

right we'll see you next

[Applause]

time

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[Music]

[Music]

[Music]

all right this is cs50 and this is

already week seven and this is the week

where we'll continue where we left off

with python introducing you to a bit

more syntax and capabilities of the

language so you can solve like

interesting problems but a lot of those

problems increasingly are now going to

involve data in some form after all if

you think of most any website or mobile

app uh or uh process nowadays that

involves solving problems it almost

always involves some amount of data and

often data at scale lots and lots of

data and so what we're going to see

first today is that yes you can use

Python to solve all the problems past

that we've seen and also some data

specific ones but sometimes it's just

going to be annoying it's going to be a

little painful it's just going to be

more work than you might like to just

get to some answer and so today we'll

too introduce you to a new language

called SQL structured query language and

this is a language that wur is actually

much smaller relatively speaking than C

and python it sort of does less but it

does it really well and it's a language

for querying databases storing data in

it updating it inserting it deleting it

and so much more and it's the kind of

Technology that's used nowadays in

indeed web apps and mobile apps data

science analytics and and so much more

it's really good at storing lots and

lots of data now this is yet another

language and believe it or not next week

we'll introduce you to three more

languages HTML and CSS which are not

technically programming languages

they're all about Aesthetics and markup

of information but also JavaScript which

is in fact a programming language but

the goals here in cs50 really are going

to be to empower you to program more

generally and indeed when you're out

there in the real world some years from

now invariably there's going to be some

new other popular language out there and

hopefully in this week and next week and

Beyond among the goals is not just to

teach you these languages specifically

but again like how to teach yourself the

future languages that we've not even

heard about just yet so with that said

let's begin with a survey of sorts if

you go to this URL on your phone or

laptop cs50. l/f favorites a very simple

Google form awaits you that's just going

to ask you a couple of multiple choice

questions so go to cs50.

l/f favorites and that should lead you

to a Google form that looks a little

something like this asking you first as

of now in week seven what is your

favorite language among those options

here and then further down one more

question if you think back on problem

sets 0 through six what was if any your

favorite uh problem set problem be it in

scratch or C or python so answer those

two

questions and in a moment I'll flip over

to my screen here where you'll see and

anyone who's used Google forms knows the

spreadsheet that's collecting now this

data um Microsoft Office 365 can do the

same if you use one of those forms and

what you see here now is a spreadsheet

in Google Sheets enumerating all of the

audience's questions language is in

column B problem is in column C and each

row represents one student who has

responded uh a few of you were super

eager for class today at 8:33 a.m.

eastern time 10:32 11:10 okay so now

we're getting into the actual class time

here and if I scroll down we'll probably

see few dozen couple hundred Answers by

now and yeah so we're getting a whole

lot of answers here and I'm seeing some

patterns emerge but but it's not

necessarily obvious to the human eyes

what those patterns are now of course

you can use Google spreadsheets you can

like highlight the data and you can

create charts magically out of it but

you can only do what Google lets you do

with the data and same thing for

Microsoft Excel or apple numbers but

wouldn't it be nice to just be able to

manipulate the raw data relatively

simple though it is to just answer

questions about the data Maybe long-term

create your own charts customize it just

the way you want rather than be holding

to like software that's off the shelf

like this well how could we go about

doing this well let me propose that we

treat this data set now as what we're

going to call for now a flat file

database we'll see today that there's

fancier databases but the simplest

database in the world is really just

like a CSV file and we saw that a couple

of weeks ago in C we wrote a bit of C

code that used F printf to write data to

a file using commas as the separator we

didn't really do much more with csvs at

the time though because it's really

annoying painful timec consuming not fun

to use see for something like that

because of malok and memory and all that

stuff but with python it's going to be

much easier and so anytime you have

access to some data set where you can

just like download it to your own Mac or

PC or your Cloud environment it's sort

of a candidate for now writing code to

do something with the data Maybe analyze

it right away if it's been human inputed

manually maybe you have to clean it up

by doing a lot of find and replace but

not with your keyboard but rather with

code and so let me go ahead and do this

let me go back to my uh Google sheet

here that has all of the data that's

come in now and let me go ahead and

download this via uh the file menu

here and let's see download and you can

see a whole bunch of options of most

formats might be familiar but today

we'll focus just on this one comma

separated values or CSV that's going to

go ahead and download it on my Mac here

into my own downloads folder and now I'm

going to go ahead and do this let me go

ahead and pull up VSS code in the cloud

here and if you've never done this

before there's a couple of ways to do it

but the simplest way to upload a file to

your codes space so to speak is just a

sort of drag and drop that's going to

magically upload it to the server there

and we'll see that one it has a very

long file name which I'm I'm actually

going to clean this up because that's

going to be very tedious to type in my

code so I could either rightclick of of

course up here but I'm going to use my

Linux command so let's move this file

called cs50 2022 something or other and

let's just name it more simply

favorites. CSV so all lowercase no

spaces sort of good Basics and let me go

ahead now and open up this file with

code favorites. CSV I'll close my file

explorer and we'll see exactly the same

data as before but not quite as pretty

as Google Sheets makes it be rather we

see here that I still have three columns

timestamp language problem and then all

of the values down below including the

timestamps and the answers therefore but

it doesn't have proper columns it just

has commas separating them now we could

very easily write python code just like

we wrote code to manipulate files like

this either to write or read but in

instead let's do something that's a

little more pleasant which is indeed in

the form of python so python actually

comes with Native support for csvs it

has indeed a package called CSV that

just lets you read and write and do a

whole bunch of useful stuff when it

comes to CSV files so let's go ahead and

do something with this file let me go

back here to VSS code I'm going to close

favorites. CSV for now but just remember

in your mind that timestamp was the

first column language was the second

column and problem was the third and

notice because we're using commas they

don't again line up perfectly but that's

not a problem there are two commas in

every line presumably and I'm going to

go ahead and now create a file called

how about uh favorites. piy so that I

can start writing some code to

manipulate this data and let's do

something simple let's just write a

simple program in Python that opens this

file reads it and prints something out

just as like a safety check that I know

what I'm doing even though it's not

going to be useful so in Python if you

want CSV support you import CSV and that

gives you access to all the magical

capabilities thereof let me now go ahead

and use this technique to open a file in

Python which is similar in C but with

python I'm going to do this the keyword

with I'm going to open a file called

favorites. CSV which was the shorter

name I gave it this is optional but just

for explicitness I'm going to open it in

read mode explicitly just like f open

took a second argument as well and I'm

going to name this file once open quite

simply file though I could call it

anything I want and now it's just an

open file so far as python knows at this

moment it's just text or better yet it's

just zeros and ones if you want this

python package called CSV to actually do

something useful with it you have to

load this file now into the library and

the simplest way to do this is to give

myself like a variable called reader

because I want to read this file though

this too I could call anything else I'm

going to then set that equal to the

return value of a function called CSV do

reader and I pass to that per the

documentation the open file so step one

I open the file and this just gives me

me access to the bytes there in step two

now with csvreader tells the python

package called CSV to do something

useful with it and start analyzing the

commas and uh allow me to uh parse it

further so let's go ahead and do this

let me go ahead now and within this Loop

let's say this with sorry within this

open file let's do this for every row if

you will or line in the file AKA reader

let's go ahead and print out now just

how about Row Bracket one now what's

going on here well it turns out if you

read the documentation for the CSV

reader function what it hands you back

is essentially this special object so to

speak that allows you to treat it as

though it's just a really big list of

lines from the file AKA reader so by

saying for Row in reader this is a way

more succinct way of saying uh give me

the first line in the file plus plus

give me the second line in the file Plus

plus and so forth that we would have

done what much more mechanically in C

this is just much more pythonic and

English friendly if you will so in every

iteration of this Loop row is going to

contain all of the data from the current

row but better yet what the reader

function does for me is it hands me each

row not just as a big string or Stir of

text in Python it gives me what

apparently based on the syntax on line

six any Instinct yeah it's giving me

back indeed a list and I'm I presume the

visual clue for you was the fact that

we're using square brackets here and

indeed Row Bracket one is going to be

not the first but the second element in

that list and so just take a guess when

I run this code in a moment What's going

to get printed the timestamp the

language or the

problem the yeah oh the language the

language because it's the second column

that is in the file delimited by those

those commas so let me go ahead and do

this let me clear my terminal down here

here let me run python of favorites.i

and enter and there's everything it was

super fast but there's a really long

list here and in fact if I increase the

size of my terminal and start scrolling

up you'll just see all of the raw data

now this isn't that useful yet I could

have just glanced at the CSV but clearly

now I have the ability to open the file

parse it so to speak that is break it up

into its constituent parts and do

something with specific Parts therein

all right so if I want to do this a

little more pleasantly though let me at

least least make this semantically a

little cleaner and you know what just

for clarity let me just give myself a

variable it's not strictly necessary but

I know that this is the favorite uh for

instance uh language so let's just call

it favorite set it equal to Row Bracket

1 and now just to be more explicit in my

code even though again we don't need the

variable per se this codee's of course

going to do the same thing it's just

using an additional variable called

favorite if I go down here scroll up run

the program again I get back the exact

same data but this is a stepping stone

to something that's even more powerful

about python support for CSV files is

that you don't have to just treat the

return value as a list with zero and one

and two so just thinking intuitively

here why is this maybe not the best

design to hand you the programmer back

the data in a list that's numerically

indexed with

012 it clearly works but critique this

what could go wrong what's a little

poorly designed yeah you have to always

remember what

are

yeah exactly so if yeah so it's up to

you to repeat it's up to you to remember

like what column the data is actually in

and you know God forbid you're you're

collaborating with someone else on the

spreadsheet you know you've if you've

used Google spreadsheets you can move

columns around maybe just because you

want to visually reorganize things and

if you do this and then someone else

downloads that same data all of their

code is going to break so that's just

really bad design it's fragile just

because it's you're sort of on the honor

System that one means the data that you

want so wouldn't it be nice if we could

be a little more explicit well recall

that the very first line in this file is

actually this and I paused the output

this time so that we can see more

optionally I just reran favorites St pi

and notice one of these things is not

like the other every output was either

scratch or C or python except for this

first one why am I seeing the word

language

here where did language come from you

didn't have the ability to manual input

oh no where did it come from

yeah yeah the header the very first row

in the file which by human convention

generally just defines what the columns

represent so that there's some human

useful information there now that's not

really intended to be part of my output

at the moment so there is a way to skip

this if you want to skip the first row

you can actually do something like this

you can say next row and that will just

ignore that row so that I'm starting

really with the every row thereafter but

there's a better way to handle this than

that that will get rid of the row in the

output but let me go ahead and use a

different feature of the CSV package

that's just going to make this a little

cleaner Al together so let me clear my

terminal window here let me undo this

next thing that I just added and instead

of using a reader let me go ahead and

use a dictionary reader abbreviated dict

reader that's going to now return me the

equivalent of all of the rows one at a

time so I can still call it Reader just

as before but as the name implies what

this reader is going to return is not a

list after list after list but a

dictionary a dictionary a dictionary and

remember a dictionary is just a

collection of key value pairs so what

does that mean what are the keys what

are the values well now that I'm using a

dictionary reader I can actually do this

instead of sort of on the honor System

remembering that I want column one I can

treat row now not as a list but as a

dictionary and that means I can go in

here and say quote unquote language and

we saw that back in week six python

allows you to index into dictionaries

using square bracket notation in strings

or stirs on the inside just like lists

allow for numbers but this now I think

is going to be a little more robust if I

run this again python of favorites. py

all of that worked out fine and let me

pause the output to by using this

program called more now I don't even see

the header so now whoever works it uh

with python wrote the code for this

package to just analyze that first line

of code use the header as you just

called it as the keys and then every

time you iterate through this Loop it

updates the values the values the values

but the keys stay the same any questions

then on this

technique spiced to say this would be

painful in C

yes

Associated exactly so the keys are

always going to be quote unquote

timestamp uh Pro language and problem

but on each iteration of this Loop here

uh the row is going to contain a

different row of values different row

values different row values so you're

going to get back one dictionary for

every student who submitted the Google

form if you will while iterating through

it there all right so once we have this

ability here why don't we go ahead and

transition

into how about not just using that

dictionary reader which makes the code a

little more robust because now if you

move the columns around no big deal it

doesn't matter if the numeric indices

change you can still use those keywords

instead well let's actually analyze the

data now I'm just spitting it out which

is not solving any problems for anyone

so let's go ahead and and count the

popularity of scratch C and Python and

see what everyone's uh been thinking

here all right so how might I do this

well let me go ahead and do this up here

before I start iterating let me give

myself let's say three variables and to

keep things simple I'll say one variable

called scratch set it equal to zero for

zero students so far C is going to equal

zero and python is going to equal zero

there's a slightly prettier way of doing

this just because this is like three

lines of code to do something very

simple you could alternatively in Python

but not C do scratch comma C comma

python equals 0 comma 0 so kind of

slightly more elegant just to fit it all

into one line but now let's just do

something more interesting on line seven

I'm still going to figure out what the

current favorite language is and now I'm

just going to do some conditional checks

How about if that favorite equals equals

quote unquote scratch let's go ahead and

increment Scratch by one we can't do

plus plus in Python but we can do plus

equals 1 how about L if favorite equals

equals c then let's do C+ equal 1 L and

not we could do else this is actually a

good design question should I do else

should I do L if any instincts here

yeah yeah really good instincts just in

case someone goes and adds another

language to the form next week because

we're obviously going to introduce

another language today you don't want

your code to now artificially inflate

the scores for python just because

you're conflating multiple languages

together so the more defensive sort of

better way to write this code I agree

would be L if favorite equals equals

python then let's go ahead and increment

python plus equals 1 and if there's a

new language next week we're obviously

going to have to update the code but at

least we're not miscounting we're just

missing the new language so I think

that's slightly more robust all right

now at the very bottom of this program

and outside of the loop when I'm all

done counting let me go ahead and print

out using some f strings how about the

total number of people who uh whose

favorite is scratch so this is just uh

week six F string syntax let me go ahead

and print out another F string for C and

I'm of course putting the variables in

curly braces all lowercase but the

English words I'm doing capitalization

for Let's do an final one with f uh

python colon and then in curly braces

python close quote and I think I'm done

so let me just hide my terminal for a

second here's the total program same

stuff as before open favorites. CSV open

it further with the dictionary reader to

do that processing for us initialize

three variables to zero just so we have

something to count with and then iterate

over the file row by row and this is

just some sort of week one style

conditional logic albe it in Python

counting things all right so how can we

now execute this let me go back to my

terminal python of favorites.i and here

we go uh as of today everyone who's

reporting in live via the Google form

their favorite languages are

interesting that's pretty interesting

too after just one week of python no

less so but scratch is a healthy

Contender there a lot of C so a pretty

good mix here so is this going to be the

best way to write this program long term

well as you noted if there's a new

language next week this week we're going

to have to constantly update this and

here's where you should let your mind

wander to like the future like if we

have a fourth language fifth language

6th sth eth which aspect here might kind

of have some code smell to it like this

probably isn't the best design to set us

up for the

future what might be better than this

yeah add language line yeah we have to

keep adding a language to line five and

okay not a big deal we could add like

SQL today and maybe JavaScript next week

but you know anytime a a line of code a

line of logic is just going to kind of

grow out of control and we've had this

chat a couple of times with different

syntax there's probably a better way

than that so let's do that instead of

using these individual variables we

could make maybe use a list but a list

would be a little confusing because like

what does braet zero mean what is

bracket 1 bracket two but a dictionary

recall is like this Swiss army knife of

data structures whereby you can

associate anything with anything else

keys with values so I dare say a cleaner

way to solve this problem that sets us

up for Less work or confusion later

would be to create like a new variable

called counts if that's what we're doing

counting things up and just set it equal

to an empty dictionary and you can

literally say adct with the open

parenthesis closed parenthesis nothing

or the more pythonic just use open and

Clos curly braces with nothing inside

that gives me an empty dictionary just

like square brackets gives me a list now

my logic down here has to change a

little bit but what's nice is I don't

need one conditional for every language

because again if we have a fourth a

fifth a sixth that chunk of code is also

going to grow a bit out of control too

so I can get rid of this here and what I

think I'm going to do is say this

whatever the current favorite is from

the current Row in the file why don't we

go into our counts variable at that key

and again favorite is a variable it's

not quote unquote favorite it's going to

be scratch or C or Python and then why

don't we go ahead and just increment

whatever the value of that count is at

that key now this is technically bugy

we're really close but there is a

bug does anyone want to conjecture what

the bug is

yeah a good question that answers my

question in uh nonetheless so no like

the magic you describe will not happen

and to repeat the the hypothesis will

this automatically create a key for

every uh language that we try plugging

into those square brackets short answer

no odds are this is going to create a

key error one of those traceback error

messages that you've probably seen by

now either in class or in problem sets

whereby if scratch hasn't appeared in

the dictionary before or C or python

like then the dictionary has no clue

what you're talking about so I think we

actually still need some conditional

logic but not that's going to grow

longer and longer with each language

what I think we probably want to do is

this if the current favorite is in the

counts dictionary and this is the

pythonic way of just saying is this key

in this dictionary then go ahead and

safely do count favorite plus equals 1

else to your conjecture now else what do

I want to do counts

favorite

equals yeah one so initialize a brand

new key to a brand new value of one

because I'm obviously just seen this

language otherwise increment again and

again and now down here I just need to

tweak my syntax a little bit I don't

need to print out all of these things

one at a time manually I can actually

get away I think with another loop at

the very bottom here so how about I do

this for each favorite in those counts

and this is again the pythonic way to

iterate over all of the keys in a

dictionary go ahead and print out using

an F string whatever the current

favorite is scratch or C or Python and

then a colon and then figure out what

its count is and you can do that by

going into the counts dictionary looking

at the favorite key and get back its

value so close my curly braces I close

my quotes and even though this looks

ugly at the moment now this is much more

dynamic because if we go and add SQL to

the CSV file tomorrow where we add

JavaScript next week this will just work

it will keep working now automatically

all I change is the Google form not my

actual code all right let's try python

of favorites.i cross my fingers as

always and there now is the data as of

now questions on this code here

yeah really good good question what if

you wanted to print it in a particular

order uh well I could give you a couple

of solutions like if you want to print

it out in it's already coincidentally in

alphabetical order so you got that for

free although that's just by chance here

but there is a way to do this and let me

propose that we go down here to my Loop

and I explicitly use a function you

might not have seen in Python yet but

it's literally called sorted which is

going to take either a list or in this

case a dictionary and by default sort it

by key alphabetically now if my

intuition is correct this is not going

to change the output because it's

already alphabetical but if you read the

documentation for the sorted function it

takes multiple parameters potentially

some of which are named parameters and

so you can actually do this if you want

to sort the counts but you want to

reverse the order for whatever reason

here so that it's reverse alphabetical

order now let me go ahead and rerun this

and I'll keep the previous output on the

screen enter and now it's backwards uh

alphabetically if you will other

questions on this here

here no how about then how about then we

transition

to changing sorting by value and let me

this is going to escalate a little

quickly briefly but then we'll we'll

tone it down again notice that right now

this is indeed sorting by key what if

especially if I have lots of data it'd

be nice to make like a top 10 list or in

this case a top three list and actually

see in order of the counts the values uh

what these popular ones are so it's not

C python scratch it should ideally be

python then C then scratch because of

the values and the magnitude thereof so

how can I do this well it turns out

there's another key another uh parameter

that you can pass to the sorted function

that is typically implemented as a

function itself and so I'm going to go

ahead and do this I'm going to

temporarily Define a function called get

value just to make my life easier and

this get value function is going to take

I'll say a language par parameter and

then all I'm going to do is return

whatever the count is of that language

so out of context this is just a super

simple function that you hand it a

language like scratch or C or python

it's just going to tell you what the

count is thereof in that uh in that

dictionary called counts but what I can

do now down here in my newly introduced

call to sorted is I can tell it what to

use as its key instead of using

literally the key scratch C python I can

sort of override that behavior and say

you know what to figure out what to sort

by go ahead and call this function

called get value notice that I have not

put parentheses after get value because

I don't want to call get value right

then and there I want to pass the get

value function as itself in argument to

the sorted function so that the sorted

function written years ago by the people

at python can call my version of get

value again and again and again when

they try to sort sort this actual data

so now if I add that and I leave reverse

equals true let's see what happens

python of favorites. py enter and now I

get my top 10 or in this case top three

list and if I had more sophisticated

data with like more columns Al together

that I actually care about I could even

sort this more uh powerfully as well but

let me clean this up a little bit just

so you've seen it even though we won't

use these that often in cs50 until the

end of the class will they come up again

technically this is a little bit H this

isn't necessarily the best design to

spend all this time implementing a

function and then only use it in one

place in general we've argued that H you

don't necessarily need a variable if

you're only going to use it in one place

you don't really need a function if

you're only going to use it in one place

and here we kind of have a good

candidate for that and so it turns out

in Python if you don't want to bother

creating a function just to use it once

you can create what's called an

anonymous function AKA a Lambda function

like the lamb Lambda symbol familiar and

a Lambda function the syntax is a little

strange looking but you say this you

literally say Lambda you literally then

say the name of the argument that you

want this Anonymous function with no

name to take then you have a colon and

then quite simply you write what you

want the return value of this function

to be you don't even say return

literally these Lambda functions are

meant to be used super turly so that you

can in one line Express something like

this and I admit this looks more cryptic

I think than the previous version but as

you get more comfortable with python or

other languages that support this

feature it allows you to not bother with

lines of code like that and just tighten

up your code a little bit so this line

here lamba language colon counts

language is the oneline version of this

and you don't even need to bother

picking a name for it Lambda tells

python I didn't waste any time thinking

of a name for this

function so questions then on this

technique technique of using python to

analyze data like

this any

questions we're almost done with python

questions no okay so why don't we make

things a little more interesting because

we had a much juicier data set with the

problems that we've assigned over the

past several weeks why don't we go ahead

and quite simply you know I think we

wrote pretty darn good code here so I

think we can pretty much just change a

bit of it to say let's see if I don't

want language I want problem and if I

want to sort by not language but problem

I think that's it I think if I didn't

Overlook something here just by changing

what column I'm reading the data from

and then just to be consistent renaming

my variables just so I know what I'm

looking at what will this program now do

after those minor

changes what will I see when I run

this

what would be the first thing I see when

I run

this tough crowd today yes problem yeah

the problem this the top problem so the

most popular problem which I'm a little

worried it might be hello or just

scratch but let's go ahead and see so

let me go ahead and open my terminal

window I'll even maximize my terminal

window so we can see a lot let me go

ahead and run python of favorites. py

I'm going to go ahead now and cross my

fingers that I didn't mess up and hit

enter and okay great we peaked early so

scratch was the most popular program

according to the data at the time I

downloaded it I'm sure other votes have

come in since filter uh in week four was

tied then with tan as well Mario is a

close third there and so forth so this

is helpful for us on staff that uh not

so much love down here at the bottom of

the list so it was a bunch of code to

write but now that we've written it in

this very versatile Dynamic way it's

pretty good for just like crunching data

and doing some analytics but it's still

a decent number of lines to have had to

write manually and this is where

sometimes it isn't necessarily the right

tool for the job but rather the job that

uh but rather a candidate for using some

other language alog together especially

when it's not just a one-time program

that you run and you want to see the

answer what if you want to take input

from the user and ask uh answer

questions dynamically like a mobile app

would like a website would like

Microsoft Excel or apple numbers or

Google Sheets would for for you well

let's make one final change for now to

this version of the program and actually

take in some user input so besides just

loading all of the data into memory

let's go ahead and down below here not

just print out the top 10 list if you

will but prompt the user for their

favorite so I'm going to use Python's

input function and I'm just going to

prompt them with favorite quote unquote

like tell me what your favorite problem

what problem uh rather uh you interested

in and now let me go ahead and say if

that favorite is in the counts variable

so you didn't type in something random

that we didn't actually assign as a

problem then let me go ahead and print

with a format string whatever that

favorite is of yours and show you the

actual popularity thereof by indexing

into counts using that favorite as the

key and printing this so now it's a

dynamic program it doesn't dump all of

the data and all of the summations

rather it's going to allow me to see

what my choice of favorite is and I'm

going to go ahead and say uh let's see

I'm a fan of Mario here so enter and

indeed we see the same value we saw a

moment ago but just for Mario but the

point now is that one all of this is

possible two it's way easier and more

pleasant than this would have been in C

this is still only 15 lines of code and

in C again there's the memory management

there's the iterating over the strings

trying to find the commas there's just a

lot more work but honestly even when you

just want to answer a question like this

in Excel in Apple Numbers Google sheets

you know generally you can just

highlight things you can click a button

and boom you get your answer for

summation or Max or Min or any of those

sort of Basics wouldn't it be nice if we

weren't taking a step backwards as

programmers and being sort of more

powerful and yet we now have to do more

of the work so maybe sometimes Python's

not or any language is not the best tool

for the job and that's going to now

allow us to introduce more generally

something called a relational database

graduating from Mere flat file databases

like text files or binary files in which

all of your data is stored to something

more proper but first

questions really good question to

reiterate if I were to is this case

sensitive so if I were to type in Mario

in all lowercase and hit enter I

actually get no such response now that

might be acceptable because the problem

technically what is a capital M but

that's a little uh ridiculous to be that

pedantic about the input so how could we

solve

this any tips for how we can make this a

little more robust

yeah okay

yeah yeah so we could use a few

different functions one of which is

called title which will change it to

title case where it capitalizes like in

most English sentences the first letter

of that sentence we could use capitalize

we could use upper we could use lower

but indeed we could just decide how we

want to standardize the capitalization

either uppercase lowercase or some

combination thereof and just make sure

that you change the counts themselves

make sure that you do the same to

favorite and make sure that maybe you

keep a backup of the data if you want to

show the original version that came from

the CSV without presuming to just

capitalize everything for the user but

indeed that would be the most common

scenario you just make things case

insensitive when doing those matches

other questions now on python before we

leave it behind for the coming

week all right well then let's introduce

these relational databases so relational

database is what like every is a super

popular way of storing lots of data like

this is what the Twitter's of the world

the Googles of the world the metas of

the world use to store some of their

data at scale there are alternatives to

relational databases um indeed today

we'll talk about a language called SQL

there's so a movement if you will or an

alternative generally called No SQL

which is just the opposite you don't use

SQL there are things called

objectoriented databases and the like

but if you've ever heard of MySQL or

postgress SQL or uh Microsoft SQL server

or Oracle or Maria DB or a bunch of

other products both free and Commercial

this is what they're talking about

databases that are designed to store

lots of data and what's nice about

relational databases is that they're

really similar to the spreadsheets with

which you were presumably familiar long

before today's class so a relational

database is going to store as you'll see

all of the data in rows and columns now

the terminology will thereafter be a

little different instead of having

sheets you're going to have tables but

those tables are still going to have

rows and columns and you're going to

have even more control over the

performance of your data when you start

to access it using this structured query

language or SQL this is a language you

can use for web apps mobile apps uh a

lot of analysts would sit down and their

Mac or PC and actually ask questions of

data to get back the answer and

wonderfully even though there will be

some new syntax today SQL really just

does four basic things crud is the sort

of crude acronym here crud is a way of

remembering that a relational database

supports ultimately creating data

reading data updating data and deleting

data so even if you're feeling like wow

this is a lot of new syntax which it

isn't relative to our past languages the

only things you're doing really are

creating dat data reading data updating

and deleting the same now a little

confusingly in SQL the corresponding

functions or commands that exist that

map to crud are actually this so it's

still create but there's another one

called insert uh it's not read which is

more of the computer scientist way of

saying it but select which is a little

more explicit like select data you care

about update is still update delete is

still delete but there's another command

called drop which lets you drop that is

delete entire tables as well so you can

create tables using syntax that's

generally going to look like this you'll

say create table you'll give the name of

the table uh which you can call almost

anything you want but generally all

lowercase no spaces is best then in

parentheses you can specify a comma

separated list of the columns that you

might want in this table so this is the

code equivalent in the SQL language of

like manually opening Google Sheets or

Excel or numbers and like clicking in

the top left cell and like typing

timestamp and then in the next typing

language and then in the third typing

problem this is the way to sort of

Define what your headers are if you will

in a spreadsheet but now it's called a

table now we won't use this command

manually first let's do something a

little simpler we're going to start off

by just importing this data ourselves

and I'm going to go ahead and do this

let me go back to vs code here I'm going

to leave behind favorites. py for now

because now we're going to transition to

this other language called SQL and to do

this I am going to create a new database

file and I'm going to do so using a

command called SQL light 3 which is just

the third version thereof and I'm going

to give the database a name of

favorites. DB there's different

conventions but this is one of the most

common when I hit enter this is going to

create for me a new empty database just

like opening an Untitled spreadsheet in

Excel Google Sheets or Apple Numbers I'm

being prompted do I want to create

favorites. DB I'll hit y for yes okay

we're up and running now you're going to

notice a different prompt I'm not in my

Linux prompt per se which is always the

dollar sign I'm now inside of the

program called SQL light and we're going

to use SQL light SQL light 3 as just an

interactive way for now of playing with

SQL code at the end of today we'll show

you how you can use SQL in Python code

so that you still write python code to

do whatever you want but you can talk to

databases using Python and this is

exactly how web apps mobile apps work

for instance on iOS uh and an i phone an

iPad or the like if you want to store

data it's very often stored in a SQL

database as we're about to do um but you

might use a language called Swift or

objective c and same exists in the world

of Android using Java or cotlin or

something else to query the database so

we're going to see SQL in isolation for

now like an analyst might just use at

their Mac or PC but we're going to tie

it together by Day end so at this

terminal uh SQL light let me go ahead

and execute uh this command first I'm

going to first put SQL light into CSV

mode because I'm going to cut some

Corners initially and I'm just going to

automatically import all of the data

that was submitted via that Google form

which I exported as a CSV and uploaded

to my code space and I'm just going to

automatically say turn this CSV file

into a SQL database for me just so I

don't have to figure out what those

create table commands are so to do this

I'm going to say mode CSV so that SQL

light knows that this is the command uh

knows that this is a CSV file it's

literally mode so the dot comes before

the keyword there and now I'm going to

say do import and then the name of the

file I want to import which is

favorites. CSV and now the name of the

table that I want to create with that

data and just for consistency I'm going

to call it favorites I could change

these things to be anything I want but

I'm going to do that and voila nothing

seems to have happened but just like in

C and in Python in Linux when nothing

seems to happen that's usually a good

thing it means I didn't mess up so if I

want to see what just happened there's

this other command and these commands

that start with dots these are SQL light

specific which is indeed a lightweight

version of SQL they're not SQL per se so

if you're using Oracle or something else

like that you're not going to use these

exact commands you'll see the ones we

use in just a moment and here's the

first when I type schema the schema of a

database is the design of the database

what are the tables what are the columns

and all of that so when I type schema

this actually in this case shows me that

create table command that was

automatically run for me by just doing

this import line once I get more

comfortable with SQL I could literally

type this out myself or use some program

to generate that as well but what it's

creating for me is this create table if

it doesn't exist even though it's more

tur than that I want to create a table

called favorites and then the columns

for that table are going to be timestamp

which is going to be text comma language

which is also going to be text comma

problem which is also going to be text

that was just inferred very trivially by

the do import command to just figure out

that yes just give me a three column

database table based on the Google

form okay questions on this these are

commands you run once to get up and

running you don't run these commands

frequently but we have them on the slide

just for reference all right so now

let's do something a little more

interesting I'm going to clear my SQL

light terminal here but I'm still in SQL

light I'm going to now use some of my

first SQL commands which recall were uh

were among them uh select so crud c r UD

D the r was select this is maybe the

most common the most useful the most

powerful thing to use with a SQL

database selecting data to answer

questions akin to the ones we were

trying to answer with python this is the

general syntax anytime you want to

select data from a SQL database you

literally say select you then specify

the column or columns that you want to

select data from you literally write the

word from and then you specify the name

of the table you want to get that data

from semicolon in this case everything

that's in capitals here is a sequel

keyword strictly speaking you don't have

to capitalize things but we would

encourage you to do so stylistically and

especially as you're learning and even

as you're writing it it just helps to

distinguish SQL from like words you

chose like the names of the columns and

the data there're in so uh do adopt

early on this convention so let me go

back now to my code space here I'm

running my terminal window with SQL

light 3 inside of it suppose that I just

want to get all of the data from the

favorites table which was automatically

imported let's do this select I want

everything well I can do timestamp comma

language comma problem but you know what

here's a uh convenience already if you

want everything there's what's called a

wild card character in SQL which is just

a star in asterisk which means give me

every column without my knowing even

what they're called let me go ahead now

and say from favorite it's semicolon and

this is the SQL way of opening the

database iterating over every row

they're in printing out every row

therein done so those three steps which

was like nine lines of python code give

or take earlier is now one line of SQL I

hit enter there is all of the data so I

see now all of the data just outputed as

a CSV here but it's not the CSV file

it's now actually the table and in fact

just for good measure let me do this CU

you'll see the behavior a little

different the next time we open the file

I've just exited out of SQL light 3 I'm

going to rerun it but I'm not going to

reimport the data or do anything like

that because my file now exists in fact

let me take one step back if I type LS

at my Linux prompt there's my favorites.

py from before there's my favorites. CSV

from before and here's a third file that

I did create a moment ago when I first

ran SQL light 3 so the data is

persistent it's not using Ram or memory

anything I do now is save there so let's

go ahead and rerun SQL light 3 with the

same file but I'm not going to I don't

have to reort everything because the

file already exists let me now do that

same thing again select star from

favorites to get all of the data and

what you'll see now is the same data but

it's a little prettier now because I

reran it I effectively disabled CSV mode

this time and what I'm now seeing is the

entire contents of this database table

called favorit now there's nothing new

here but you're just seeing now like an

asky or Unicode version of all of the

same data from that database well

suppose I want to get a subset of the

data well let me clear my screen and

just like in Linux I can hit contrl L

just to clean things up aesthetically

suppose I want to get just the languages

so I could do select language from

favorites and this will now select not

all three columns AKA star this will

only select the language column and all

of the data they're in if I hit enter

voila now I just see those there no time

Stamps no problems it's just a slice of

the table if you will all right not that

interesting still still because it's

just a big column of data but now things

get more interesting it turns out in SQL

that there are functions that come with

this language just like C just like

python in SQL some of the more useful

ones some of the simpler ones are these

here average count distinct lower Max

Min upper which pretty much uh do what

they say and count is a particularly

useful one let's start with that you

know it's a reasonable question to be

asked uh how many people submitted the

Google form by the time I actually

downloaded the CSV well why don't we go

ahead and do this let me go back to VSS

code here in my terminal window let me

select not star but the count of star so

give me the count of the rows that are

being returned from the database called

uh the database table called favorites

now when I hit enter I'm not going to

get all the data I'm just going to get

simply a number 430 rows came back so

that's pretty good I now know how much

data is in there well what languages

were in there well I could do select

language from favorit just as before but

that's not that useful especially if I'm

inheriting the data like I'm the analyst

who's been handed a data set by my boss

and they want me to like crunch some

numbers okay I could like load this into

Excel I could sort it but you can use

SQL now to answer pretty basic questions

too if you want to select the distinct

languages in the data set because you

didn't you weren't privy to the Google

form let me go ahead and select only the

distinct languages from the favorites

table and now I hit enter and I get back

a much more succinct answer just the

three languages in question not really

that useful since I created the Google

form but certainly if you're inheriting

data from someone else you've just

downloaded a data set at least now I'm

arguably wrapping my mind around what's

going on now this is not necessary for

such a small data set but I can combine

these things select the count of the

distinct languages in this data set

called favorites and now I should get

back what

answer so hopefully indeed an answer

called three and what you're getting

back notice aesthetically too is like a

mini temporary table when I asked for uh

just the distinct languages what SQL

hands me back is this temporary table in

memory that has one column called

language and then two row uh three rows

now this is not saved anywhere it's just

executed ephemerally like this but

that's why it's depicted in this way

what you're getting is subsets of your

data smaller tables containing some of

your data and same thing down here this

is like a crazy long

uh column name you can rename it if you

really want uh but uh that's all we're

seeing there and in fact if that's a

little ugly we can actually Alias These

Things N is a common uh name for a

variable a number in any programming

language so I can actually alas this to

be a column called n hit enter and now

I'm getting a tiny tiny table whose

column is called n that just has the one

value

there all right questions on

these application of these functions

here no questions

yeah say a little louder as oh as as

literally in English so name this column

rename this column as this technically

it creates an alias for the column so

that's all

yeah exactly distinct will operate on

whatever you handed in parentheses and

get rid of all of the duplicates giving

you back just the unique

correct other questions here

yeah good question when you define an

alias like n which I just did does it

become like a variable you can reuse

short answer no in this case but you can

reuse it within your same query even

though these these queries are getting a

little longer admittedly statements that

they are uh you can actually reuse n in

even longer queries so later in your

query and we'll see a few that are start

to going to start to grow in length so

it's a a nice way of nicknaming things

just to be a little more tur in your

query so we can transition to some of

these more sophisticated queries because

it turns out there's some other uh

techniques we can introduce as well here

are some other keywords in SQL and again

even though like this is another list of

things there's only four things

fundamentally we're doing creating

reading updating and deleting data these

are just allowing us to like fine-tune

how we do it exactly so where is going

to allow us to filter data as we'll do

in just a moment like select data where

this conditional is true uh like is

going to be an alternative to an equal

sign so instead of looking for exactly

scratch or exactly python or exactly C

you can look for something like dot dot

dot and it can be a little bit of a

fuzzier match if you will uh with other

characters as well order by is going to

deal with sorting limit is going to just

let me limit the total number of rows

that come back to one or 10 or finite if

I don't want to see all 400 plus rows

all at once because I'm just trying to

wrap my mind around it and group by is

best shown by example so let's play with

just a couple of these as well let me go

back to vs code here I'll clear my

screen I'm still in the same SQL light

instance and let's count how many of you

likeed C without writing python code as

before so let me go ahead and select the

count of the rows from favorites where

the language in each row equals c and

the convention in SQL light is to use

single quotes anytime you're surrounding

a string that's meant to represent a

literal piece of text uh as opposed to C

which was double quotes or python which

was either so this is selecting the

count of rows from favorites table where

the language in question is C enter and

this gives me 98 notice though if I omit

that predicate like we did before you'll

get back the total number of rows that

were in the table so where is what's

called a predicate that just allows me

to filter things just like an if

condition or the like in a language that

we've seen before you can be a little

more specific like how many people

really liked C and the Mario program uh

problem specifically well let's do this

uh let's go ahead and do select the

number of rows from the favorites table

where the language is C and so it's uh

still literally the word ands and or

just like in Python but not like in C uh

and

equals Mario so let's see if there's any

fans of both C and the Mario problem and

three of us really like those two things

together in this case all right what

else can we do well more compelling

might be to see kind of like in Python

for each language what was the

popularity thereof and at the moment we

don't really have a way of doing that

except in Python where we had the loop

and we had those variables with the

dictionary that did all that counting

for us you know totally doable but

tedious especially if your job is to

analyze data my God like even writing 15

lines of code to answer simple questions

is kind of ridiculous SQL can do better

for us so let me go ahead and do this

let me go ahead and select every

language and the count thereof from the

favorites table but this time Group by

language so this was another one of the

keywords that we can use in this

abbreviated list of extra features of

SQL and this one's a little takes a

moment to wrap your mind around but this

is going to give me a two column

temporary table where the First Column

is a language and the second column is

the count thereof from this data set and

group by language just means that only

show me scratch once only show me c once

only show me python once that is group

all of the identical values together but

keep track of how many of them there are

and so now if I go over to SQL light and

I hit enter now I have in SQL version

the exact same output that I had from

python that took me what 15 plus lines

before now we're down to just one

because SQL structured query language is

all about constructing queries like this

to answer questions and get back answers

quickly if we want to clean this up a

little bit you asked earlier about

sorting order well we can do that too

there's another uh key phrase we can use

here we can order by the count of those

rows and then run that query here so now

unfortunately they're from smallest to

biggest but we can reverse that it turns

out and my query is starting to wrap

here I'll I'll zoom out for a moment if

you want to order by count the default

is in ascending order abbreviated ASC if

you want to reverse the sort in SQL

instead of using reverse equals true

like we did in Python you say DC for

descending order and now we get almost

the same output but flipped in Reverse

so it's just a lot faster to answer

questions once of course you get some

muscle memory and some comfort with it

well what else can I do you know what if

I just care about the most popular

language I don't care about the second

place or the third place languages or

anything else well let me add one more

Clause here limit the answer to one and

no matter how many rows should come back

now I just get the number one language

as of the data set we collected with 27

uh 270 votes for

it

questions on

this any questions

here no well what if uh you know we're

starting to introduce SQL and it was

kind of too late to make it into the

this the Google form so it turns out

there Syntax for this too you can create

data of course not just the tables but

the data they're in and here's like the

typical Syntax for inserting data into a

SQL database you literally say insert

into the name of the table and then in

parentheses you specify one or more

columns for which you have values that

you want to insert this is to say you

don't have to give values for every

column in the given row if you only have

answers to some of those questions you

can enumerate them here like this but

the values you insert are going to be

these so you literally say after the

closed parenthesis values and then in a

second set of parentheses with the same

length comma separated list you specify

what values do you want to insert so

it's a little verbose and frankly longer

term you're going to use like python

code to automatically do these kinds of

insertions but let's go ahead and try

this right now if I do select

distinct uh language from favorites

again we see this just these three

candidates but we've now taught you a

bit of sequel so let's do insert into

favorites the column called

language uh and you know what let's I'm

going to give a problem here the values

for which and let me Zoom back out are

going to be quote unquote SQL and quote

unquote 50v you'll see soon see what

that's all about semicolon nothing seems

to happen but that's usually a good

thing and now if I scroll back up in my

my queries in SQL light three you can

scroll back and forth in time and uh to

avoid retyping things now I should see

indeed four candidate languages here now

suppose that you were never really a fan

of c and maybe you uh programmed a

little bit in high school or in the real

world and you liked C++ well there's a

whole lot of answers for C so select

star from favorites where language

equals quote unquote C so here's

everyone who submitted the answer for C

let's presume that no they didn't really

want C they wanted C++ which is not a

language we teach in the class but I

could also now do this you can use the

update command to set a column or

columns to different values where some

condition is met so if I do update table

name set column name equal to some value

filtering it perhaps by where some

condition is true so suppose I've

changed my mind or you know what let's

go ahead and do

update uh favorites set language equal

to maybe C++ where language equals c now

this is destructive so you generally

don't want to do this unless you have a

backup of your data too overriding what

people's answers are this seems to have

been successful because no error

messages and if I rerun the previous

select that gives me all of the

favorites where language equals c now

indeed I get none but if I search for

C++ now I get a lot and if I get rid of

that wear Clause altogether and just

look at the contents of my database now

you see that indeed C++ is comingled

with all the other data this is not what

you all intended of course so I can undo

this let me go ahead and undo what I

just did let me set my favorite language

to C where language equals C plus plus

but the predicate is important this I'm

not going to do what if I accidentally

omitted this predicate the wear Clause

how would that screw things up might you

think uh yeah I'm

back it would set Every Rose language to

indeed see and this is dangerous and if

you start googling around for like

sequel mistakes or the like people in

the real world have accidentally run

commands like this and without naming

names a former member of our teaching

staff at one point accidentally re ran a

command like this and changed every

student's name in our database to Bobby

I think it was the same name for every

row because they simply forgot a

predicate so here to two like there's

dangers in code and you should adopt The

Habit quite quickly of always one

backing up your data like with CP for

instance in Linux or any other technique

or just making sure before you hit enter

that yes this is indeed the query I want

to execute and generally speaking in the

real world there should be process

controls in place like the intern should

not have access to the datab the

production database the live database

and the like but you have a lot of power

now with these queries so just be all

the more careful cuz very easily can you

do bad things so let me undo this where

language equals quote unquote C++ and

I'll Zoom back out enter and now I think

we're back in business C is among the

answers

yeah is essentially doing what at the

end replace it's essentially find and

replace yes in like lay person's terms

this is find and replace implemented

with SQL and in fact the authors of

Microsoft Word or Google Docs might very

well be using language like this sequel

when you go to the nice graphical

userfriendly find and replace box this

may very well be what they're doing

underneath the hood or of course they

could be using some other language

altogether there's one last uh syntax

that's worth knowing delete which for

better for worse is even more

destructive whereby it allows you to

delete rows from tables it's distinct

from drop which lets you delete tables

themselves this focuses on rows so

suppose that you really really didn't

like let's say uh tedin was a little

challenging if you adopt if you tackle

that more comfortable problem so if you

really don't want to even think about t

in anymore so why don't we do uh delete

from favorites where problem equals and

I won't execute it for real tiamin this

would have the effect of deleting every

row including the language therein and

the time stamp where the student

answered tiamin worse than this would be

this why might this be bad

okay chuckling because like there's no

predicate there's no filter which means

literally this would delete all of the

data so again with great power here

comes great responsibility now this has

just been a data set of what 430 rows by

us dynamically created there's of course

some really juicy data sets in the real

world and one website you might have

heard or an app you might have used is

IMDb the internet movie database which

wonderfully makes some of their data

available for download as CSV files or

technically tsv files tab separated

values but what we did in advance of

class was download some of that data for

both TV shows in the real world and

movies in the real world and what's

wonderful about this data set is it's

not just dozens or hundreds or even

thousands of lines there are millions of

rows of Juicy data TV shows and movies

with which most folks are probably

familiar at least with the subset and

we'll see in just a little bit that this

data comes in the form of now six

different tables that we've given you

and the tables and question for today

are going to be the people in the TV

business the Stars they in the shows

that people are producing and the like

this is a picture we'll revisit to let

enable you to wrap your minds around

what the actual data is this feels like

a good opportunity though for a snack in

fact in just a moment we have a whole

lot of Rice Krispie treats out in the

lobby but if folks could perhaps

acknowledge uh this mini wedding cake

here cs50 own Carter zeni is getting

married this week so congratulations to

Carter

as

well

congrats all right there's

only okay there's only one piece of cake

in that box but a lot of Rice Krispy

Treats in the transip let's take 10

minutes and we'll be back with Internet

Movie Database in 10 all right we are

back so if you've never been like you

can actually go to I imdb.com right now

and play around or download the mobile

app and it's just big database of a lot

of TV show and movies and actors and the

like but what indeed is nice is you can

download some of that data and that's in

what I've done in advance and what we've

done is we wrote some python code to

convert some of the uh flat file

databases that they let you download and

we converted it into a SQL database with

six tables so not just one but six that

ultimately are these here and let me

just help you wrap your minds around

what this picture is which is a entity

relationship diagram which is just to

say each of these boxes on the screen

represents a table and each of the

arrows or edges represents some kind of

relationship across the tables because

up until now the only data we had were

those three columns in the favorites

table but what's gets really useful

about SQL databases just like a Google

spreadsheet or an Excel file is you can

have multiple sheets or in a database

multiple tables and so what we're about

to see is that in this IMDb database for

TV shows there's going to be a dedicated

table for all the people in the TV

business there's going to be a dedicated

table for all of the TV shows that are

in their database as of right now

there's going to be a dedicated table

for writers in that industry for the

ratings of uh shows for the genres to

which shows belong comedy and the like

and then lastly there's going to be this

table which somehow

Associates people with the TV shows that

they star in and vice versa and so let's

consider first what this looks like in

code and we'll see that it's going to

overwhelm intentionally at first but I'm

going to do this I'm going to go back to

my terminal window and during the break

I downloaded from the course's website a

file called shows. DB which we made in

advance for you and if I type LS I'll

see all of my favorites files from

before the CSV the DB and the python

file but now there's shows. DB so I'm

going to go ahead in my full screen

terminal window here I'm not using

actual tabs or code files now I'm going

to run SQL light3 on the file called

shows. DB and I'm just going to see this

version information here let me clear my

screen and run the one command I ran

earlier to show us the schema of the

favorites database now we'll see the

schema for the shows database and

there's a lot going on here but let me

scroll back up to the very top the

beginning and we see this here so when I

run. schema we see a dump really of all

of the SQL create table commands that

were run in order to create this

database for you and one of those tables

is called genres and another people

ratings shows stars and so forth and the

columns therein even though it's

formatted a little more prettily than

the automatically generated create table

statement for favorites whereby we have

one column per line of output here uh in

the for instance people table there's

going to be an ID column like unique

identifier like a Harvard ID a Yale ID

or the like uh a name column a birth

year and then some other stuff if I

scroll down to shows every show in the

world is going to have a unique ID as

well a title of course the year in which

it debuted and the total number of

episodes as of the time we downloaded

the data and then what else is there

some of these are a little less obvious

like ratings here so ratings don't have

an ID column but they have a show ID

column and a rating like a fivepoint

scale or 10o scale or the like and then

the total number of votes that were

collected to contribute to that rating

IMDb allows people to like up vote and

down vote uh shows and movies and the

like and then similarly is genre

structured there's a show ID and then

there's a genre which is going to be

like an English word like comedy or

drama or something else and then what

else let's go a little further at the

bottom here for stars and writers if we

go to the very bottom here stars and

writers are similarly structured too

they have a show ID and a person ID so

show and person and then this writer's

table has a show ID and a person ID and

there's a whole lot of other words that

we'll come to in just a moment but what

are the what is this code hinting at

well if I go back to the picture from

earlier here you'll see that this

picture captures the relationships among

these various tables so for instance if

we focus on shows for just a moment a

show again has a unique ID a title a

year in which it debuted and a total

number of episodes if you want to figure

out what genre or genr a show belongs to

cuz some shows are just comedies some

shows are just just dramas but you know

some shows are arguably comedies and

dramas depending on the episode or the

like so you can imagine wanting to

associate two or three or even more

genres with a show this line here in

this second table allows us to do that

every Row in the genres table we'll see

has uh two items a show ID which relates

to the ID of a show and that's why these

lines literally line up with that

specific column name and genre which is

going to be like quote unquote comedy

quote unquote drama or something else

now with that said design question why

have we deliberately not just gotten rid

of this genre's table and made our lives

simpler by just adding a genre column to

this shows table and again a table is

just like a sheet with rows and columns

at the moment shows only have four

columns ID title year episodes why not

just add a fifth column called genre and

put the show genre

there any intuition

here why not just keep things simple

like yeah and

back exactly if you add a fifth column

here and call it genre then you have to

pick a genre specifically you have to

put in that cell presumably comedy or

drama or music or something else now you

could write multiple words in the cell

but generally speaking that would be

sloppy bad design like every cell just

like in a spreadsheet should really have

one value it might have multiple words

but it shouldn't be like a weirdly comma

separated list of multiple things it

should just be in a different cell in

that case so if you instead were to

design this with just a single column

called genre you're imposing what a

computer scientist would call a on toone

relationship every show has one genre

and that's not necessarily a good thing

or strictly speaking it would be a many

to one because the same genre could

belong to multiple shows but each show

could only have one genre in that case

what a relational database allows you to

do and relational is indeed the

operative word it allows you to factor

out some of your information and then

have maybe one show here in one row but

then in this genres table you could have

one row for that one show genre or you

could have two rows in the genres table

for comedy and for drama or if it has a

third genre you could just add another

row here so you still have one row for

the show itself with all the juiciest

details but a variable number of rows by

having this relationship with another

table meanwhile ratings work the same

way at least in this case a show has ID

title year and episodes but if you want

to figure out its rating you have to

kind of Follow the arrow here so to

speak and look up the corresponding show

ID in this table find the rating of that

show and the total number of ratings so

that's been factored out Two For Better

or For Worse um now let's consider

people people have just three columns ID

name and birth but there's no mention of

the TV show in which people have starred

or the TV shows that a person has

written well why is that well if you

just had a fourth column here called

show well you would have to decide what

show is that person in and no one could

ever act again in another show because

there's no room to store the data but if

someone of course a popular actor can St

star in multiple shows well we could

have one ID for that person one name one

birth year obviously like there's only

one Steve Carell as an actor in the

world of people but Steve Carell in this

example could have his person ID

whatever his Harvard ID equivalent Yale

ID equivalent is appear in multiple rows

in this table so that it can be

associated with multiple shows and this

allows you to create what's called a one

to many relationship or technically it's

bidirectional it's a many to many

relationship why well one show can

certain certainly have multiple people

in it and multiple people writing for it

just in the real world but conversely

one uh person could certainly act in

multiple shows or write multiple shows

so this is what you get with relational

databases you put your sort of canonical

data for people in one place for for

shows in another place and then you use

these additional tables to relate one

thing to another so we won't dwell on

the pictures that's just if you sort of

uh can wrap your mind around the data

set better that way that's one way of

thinking about it but recall that the

code we just saw for the schema again

escalated quickly like there's a lot of

keywords I haven't mentioned yet but

some of these are perhaps familiar

they're capitalized differently here but

integer is on the list here null is on

the list albeit technically not null so

let's tease apart some of these key

words and consider what they're actually

doing for your database because now

we're in exploring features that do not

exist in the world of spreadsheets alone

so it turns out in a SQL database

specifically SQL light which is the

version of SQL we use in cs50 and which

is commonly used for things like mobile

applications nowadays it's like a

lightweight version of SQL it's when you

aren't trying to run Twitter and have

billions and billions of rows

necessarily you've got hundreds

thousands tens of thousands maybe even a

few million but not crazy uh numbers uh

crazy amounts of data in the world of

SQL light specifically there's these

five data types so just like in C we had

int and Char and the like in SQL we have

these uh BL which is kind of funny but

it just means binary large object so

it's like a binary data type zeros and

ones that aren't necessarily uh fitting

into the other categories integer which

of course is an integer as we know it

numeric which is kind of a catchall for

numbers that are formatted specially so

like a date uh would be like year year

year year-month month Dash day day um

and this is actually a wonderful thing

depending on the country you're from you

might think your date system in your

country is great or it's horrible the US

system is horrible because we have month

day and then year which is impossible to

sort it is the wrong way objectively to

store data and yet here we are using

this at scale other countries have

gotten this better numeric and SQL

itself standardizes that stuff so it

doesn't matter what country you're from

you're storing your data in this

particular way for instance times are

standardized and other types of numeric

data as well real is synonymous with

float so something with a decimal point

and some number of uh digits thereafter

and then text is just uh for strings and

the like with other even fancier

databases like MySQL postgress SQL

Oracle and other products you might have

heard of there's even more data types

where you have to make even finer grain

decisions but for SQL light it's indeed

pretty lightweight and you or we just

have to decide the data types for each

column in a table but there's these

additional constraints in the world of

SQL you can additionally say that cells

in this column may or may not be null so

if you want to protect yourself from

yourself so you don't screw up and

insert a null that is a blank value you

can explicitly design a table to have a

column that cannot be null and so in

fact someone came up during the break to

ask me about my having manually inserted

SQL quote unquote SQL into our favorites

database you might recall that I kind of

cheated I just inserted uh SQL quote

unquote and 50v the name of a new

problem quote unquote but what did I not

insert into the

database a Tim stamp and I could have I

could have put like the current day in

time a few minutes ago but I didn't and

that's fine if it's uh if it's

acceptable to you and the product you're

building but I could have prevented that

if we had defined the table to have a

timestamp column that isn't just text

but it's text that's not null SQL would

have complained and would not have let

me complete that insertion so there's

these kinds of built-in defenses that

you don't necessarily get with a

spreadsheet alone and unique means

exactly that if you want to make sure

that every Row in that column is unique

maybe for email addresses or in the US

Social Security numbers or anything that

you want to make sure you don't have two

versions of you can specify that the

column is unique and there's other such

constraints as well but again this is

just a list of features that you get

from a proper relational database but

perhaps the most intellectually

interesting one and the most powerful

one is what's called here a primary key

and a foreign key and let me go back now

to this output if we look at shows

you'll see that a show again has an ID a

title a year and a number of episodes

and now the data types might make sense

the ID it turns out just like a Harvard

ID a Yale ID is going to be an integer

so a simple number the title of course

is going to be text but not null like it

would be weird if a TV show had no name

like that can't be like every the whole

world would break or your TV gu and

whatnot so that makes sense there to say

not null uh year is numeric so it's a

standardized form episodes is an integer

like how many episodes have been

produced and then lastly notice this the

primary key of the show's table is

apparently the column called ID

mentioned a few lines earlier this just

means that the database will use the ID

column as the unique identifier so it's

similar to the unique keyword but

primary key just means the database is

going to treat it as special too and

make sure that it is uniquely

identifying your data but what's

interesting is this notice if I scroll

back up to people people were sort of

similarly structured but with different

attributes like up here we had a person

has an ID a name a birth year and a

primary key of ID so a ID is again

integer name is text but not null

because it'd be weird to have a human

with absolutely no name textually birth

is going to be numeric but the primary

key of people is ID as well so those are

the unique columns that the database

will just treat special why well we just

looked at shows we just looked at people

let's focus now on this one down here

Stars how do you determine who star in a

TV show well we had two columns the show

ID and the person ID this is the ex the

Incarnation of a many to- many

relationship one person could be in many

shows one show could certainly have many

people in it or writing for it but

notice this within this table of two

columns show ID and person ID there's

what's going to be called a foreign key

called show ID that references the shows

tables ID column and then another

foreign key called person ID though I

could call these things in parentheses

anything I want that references the

people tables ID column now you're not

going to often have to type commands

like this again you set the database up

once in the beginning typically maybe

with some help from a TF maybe with help

of Google or the like but once your

database is designed it's back to the

crud like create read update delete the

selects the inserts the deletions and

the like but what's this implying these

keywords like primary key and foreign

key are what are doing in code what this

picture was painting a moment ago these

lines here are drawn literally to line

up with the corresponding things

people's ID lines up with person ID

Show's ID lines up with show ID and so

you're just seeing graphical version

code version graphical code that creates

these relationships now given that let's

actually see what these things look like

so let me go back to vs code here let me

clear my screen I'm still within SQL

light with shows. DB let me go ahead and

do what I do with like any new database

if I ever download something or I'm

trying to wrap my mind around a problem

you know usually it doesn't come with a

pretty picture or uh you know a

three-hour lecture to explain what the

data set is rather you just have the

data set in your own knowledge of SQL so

let me play around so schema shows me

all the tables that might be a starting

point okay this is interesting I know PE

what people are let's go ahead and show

me all the people so select star from

people I'm just trying to wrap my mind

around what this data set looks like in

a more userfriendly way okay okay that's

already a lot of people as you see the

years flying by there's been a lot of

people in the TV

business so this was maybe not the best

query to run but this is indicative of

just how large this data set is from

IMDb okay when in doubt and when

whenever you lose control over your

computer control C is your friend to

interrupt what would have been better

because I don't think I need to know all

the million people in the world I could

do like limit me to 10 people all right

and that's enough now to get us sense of

like Fred a stair has an IDE of one

first person ever um birth year of 1899

Lauren ball and all of these other uh

people from yesterday year you see that

they are the first 10 people in the

database so there's an example of some

of the data now if I want to rep my mind

around what a show is you know I know it

technically I know it from the picture

but let's just look at some raw data so

instead of saying select star from

people let me go ahead and select star

from shows limit 10 and okay I've only

heard heard of or seen a couple of these

but these are older shows at that but I

see that every show has an ID a title a

year in which it debuted and a number of

episodes but perhaps most opaque is

going to be this select star from

Stars where this is the table that

Associates people with shows am I going

to see any names or show titles here not

according to the definition we saw

earlier oh I should have done my limit

let me interrupt that let me do that

again limit 10

no and this is where now you're

definitely in the programmer world

because like this would be the most

annoying spreadsheet to use on your Mac

or PC ever if you just had like a sheet

with all of these numbers that

Associates one thing with the other like

my God how do you figure out who this is

or what this is you have to like

manually control F or command F looking

for the data but a database doesn't care

once you know SQL you can sort of Stitch

these things back together so what

you're seeing here are foreign Keys

foreign Keys why because show ID

corresponds to the same numbers from

that other table called shows that has

an proper primary key called ID person

ID is a foreign key in this context

because it refers to numbers that belong

to really the people table and its ID

column so this is just a way of somehow

linking them and so if you think of I

always like think of um this in my

mind's eye is this if this is like the

people table this is the shows table and

there's this middle table in between the

Stars table there's some way of like

stitching those two together by lining

up the idas of one with the other and

getting back some more data so let's

actually play with some of this data how

about we start where we emphasized

earlier genres so let me go ahead and

take a quick look at all of the genres

in this database so select star from

genres star is usually going to be a

little overwhelming but it just gives me

a sense of what the data is but let's

actually look at um uh let's go look at

all of them there okay that's a lot

these are all official genres from IMDb

let me oh okay it went okay it wasn't

terribly long let me filter that down so

from genres where genre equals comedy uh

Capital C just based on the data I'm

seeing okay so what am I seeing now and

in fact let me limit this arbitrarily to

10 though I could limit it to anything I

want here are 10 comedies what are they

well who the heck knows like all I know

are the 10 show IDs now I could do

something like this as we've seen before

with SQL I could do all right well let's

figure out what this show idea is Select

star from shows where the IDE of the

show I'm looking for equals what 62 614

semicolon so I could like manually look

it up by cross referencing the other

table okay so that was the show in

question there the first comedy in the

data set let me look up the second one

so instead of that let's do 6 3881 enter

okay so that's that show and let's do

one more and suffice it to say this is

just getting tedious and vulnerable to

mistakes quickly this is not this surely

can't be the way to do this and indeed

SQL is going to let us do this a little

more powerfully instead let's do this

instead of getting this table

temporarily with all these show IDs and

all these genres let's refine the query

so let's just select the show ID from

the genres table where the genre equals

quote unquote comedy now I have a big b

list of show IDs all of which are comedy

how many well I can combine ideas from

earlier I can just count all of those

show IDs and or Star if I want to just

do that too but I can count all those

show IDs

4876 comedies and IMDB's database for TV

shows so feels like a lot but how can I

now use that information and get back

the titles of comedies in the database

without doing it manually well let's do

this I have a moment ago this query

select the show ID from genres where the

current genre is quote unquote

comedy um what if I kind of Nest these

queries kind of like grade school math

in parenthesis what if I combine this

whole thing in parenthesis and now let

me select what I really want let me go

ahead and select how about uh the title

of all shows where the idea of the show

is in this list of show

IDs so if you agree that the shows table

has an ID column which is its otherwise

known as its primary key the unique ID

that identifies it just like our Harvard

IDs our Yale IDs and you agree that per

a moment ago this shorter query will

give me back just the show IDs of all of

the comedies in the database you can

actually combine or Nest these queries

together uh it's going to respect SQL

light order of operations with

parentheses just like grade school math

so the thing in parentheses will be

executed first that gives it back a list

of IDs like what 48,000 IDs and then

this query the outer query is going to

get the title from all of the shows

where the ID of the show is in that big

list of 48,000 so if I now execute these

together I think the list is still going

to be a little long but let me execute

it together now I see this long list of

outputs a little overwhelming let's go

ahead and maybe limit it to just 10 as

before for discussion sake and now I see

10 comedies ordered arbitrarily from

however they're in the database that

happen to indeed be have comedy as their

genre if I want to do this a little more

cleanly I could do this let's see uh why

don't I order by title ascending order

which is alphabetically or the default

is also uh ascending limit 10 now I see

the top 10 I mean weirdly named things

with hash symbols presumably to get

their titles up to the beginning or

maybe these are hashtags uh here now we

have alphabetically the top first 10

shows that are

comedies any questions on these kinds of

queries it's kind of a lot but at the

same time it's just like composing the

smaller ideas from before into slightly

more useful

queries

yeah do for Keys have to set the

relationship when you create the table

the programmer or the database

administrator would create that

relationship by using those keywords

primary key and foreign key that create

that teaches the database what is

related to what per the picture so you

do that once and now I being the sort of

programmer who's familiar with the

database I am just using these foreign

keys in a way in a manner consistent

with their design but and this is where

it's useful at some point even if you no

one hands you a picture to make sure you

understand the database because that's

going to inform literally what you type

in SQL to get the data you care about

about well let's do something a little

more precise how about very reasonable

question and honestly this is exactly

what imdb.com in the app are for what if

you want to find all of the shows that

Steve Carell is in like kind of a

reasonable query like literally

something someone might type into Google

or more specifically IMDb it's not

really obvious at first glance how to do

that though because right like from my

database if these are my six tables well

I can pretty easily get Steve Carell

from here but I can really only get his

ID number whatever that is his name

which I know already and his birth year

okay interesting but has nothing to do

with the shows that he's in I can look

at shows over here but there's no

mention of Steve Carell right because

there's no person ID here where is that

relationship implemented well it's

implemented down here so how do we do

this well here's the perfect example of

a a lesson we've trying been trying to

emphasize for weeks of taking these baby

steps like break larger problems down

into smaller ones and let's do something

like this let's just get everything I

know about Steve Carell from the

database let's select star from people

where the name of the person is quote

unquote Steve Carell I just want to see

what data we've got and here's what we

have okay there's only one Steve Carell

born in 1962 and his unique ID is 136

797 according to IMDb this isn't some

like Global uh actor identifier per se

all right well how do I get now all of

the shows that Steve Carell is in well I

could do this

select star from Stars not to confuse

the two one's the symbol one's the table

name uh where person ID equals 13

6797 so I think this will now give me

everything from the Stars table that

relates to Steve Carell okay and you'll

see person ID is the same because I'm

literally searching for just Steve

Carell but there are what like 20 or so

shows that he's been in all right well

here's where things would get tedious

what are those shows well I could do

Select Title from shows where the ID of

the show equals and here's you know

whenever you copy paste you're probably

doing something wrong okay he's was in

The Dana Carvey Show familiar with that

let's do another one we'll copy paste

this uh where ID equals this over the

top another and if we keep digging we'll

probably find the office but my God like

that's going to take forever to do 20

queries manually it's not very Dynamic

but what if we just Nest these queries a

little more dynamically so let me start

from the beginning again what if we go

ahead and select everything we want we

know about uh people whose name

equals

Steve Carell that gave us earlier this

data I don't need all of that data I

know his name I don't care about his

birth year so let's change this to just

be give me the ID of Steve Carell and

that gives me back now this smaller

temporary data set all right can I now

use

this uh inside of another query well let

me wrap the whole thing with parentheses

and now let me say select star from the

Stars table where the person ID equals

this so I'm Del liely not using in

because I'm assuming there's indeed only

one Steve Carell in the world so I'm not

getting back a list of Steve Carell I'm

getting back the one and only in this

case so equal is fine in is when you

have multiple equal is when you have one

let me go ahead and hit enter now okay

that's more data than I need I don't

need like 20 copies of Steve Carell's

person ID so let me hit up let me go

back and let me just get show ID from

Steve Carell and now I have a list of

just the 20 or so show IDs that he has

been in all right how can I now use this

well let me hit up let me put the whole

thing in parentheses and now let me

select what I really want Select Title

from shows where and here's the final

flourish the shows table has an ID has a

title has a year and has an episode and

what I really want though is to check

which

shows have ID that is what anyone want

to finish the thought

I just want to

yeah exactly ID in this and this is

getting ugly and when you actually write

your queries in like a text file you can

format them nicely and indent them my

font is just getting I don't want to

make it too small to fit everything but

now we have three queries one is in

doubly nested parenthesis then there's

the middle one then there's the outer

one so this last query is going to get

me the title from shows where the ID of

the show is in this big list of 20 or so

show IDs that Steve Carell is in and I

knew that because I looked up his name

here and notice what I did not do this

time is I didn't manually hardcode his

ID number there's no need that would be

kind of a bad way to implement a website

uh if you're using a database underneath

the hood you want the IMDb for real to

search for whatever the human typed in

and no one's going to know Steve

Carell's person ID or anything else so

here we've done this all dynamically and

now if I hit enter I think I get all of

his shows let's go ahead and order this

by title just to make it tidy and you

probably will see at least one or more

shows that you know and probably the

most popular is dot dot dot the office

so this is literally the kind of query

that's being executed underneath the

hood when you go to uh websites or apps

like IMDb your textual query is probably

being plugged into a longer SQL query

like this where some programmer at IMDb

probably wrote this whole query in

advance weeks months years ago and

they're just somehow plugging in the

value that you the human type into the

search box or the like questions

now on finding this data or any

other no okay so where else could we go

with this well let's

consider how else we might combine data

suppose that the next question actually

perhaps appropriately would be focusing

in on not just like people and shows and

these stars but how do we kind of like

gather more information about the shows

themselves like the genres the ratings

or the like so indeed let's focus on

just these two tables here recall that

every show has an ID a title a year and

episodes but it also might have one or

more relationships with rose and this

other table is called genres and this is

so that a show can be a comedy can be a

drama can be any number of other things

one row per so you would see the same

show ID again and again and again with a

different genre written in English like

comma comedy drama or the like well how

do I kind of Recon subtitute that data

Well turns out there's a few different

ways to do this and let me propose that

we introduce this keyword here join and

this is really the most powerful of the

keywords in SQL itself it doesn't have

to be used we've seen with uh nested

queries that you can still select data

across multiple tables but here is

another way so let me do this let me go

back to my SQL light database and let me

select sort of in one uh breath exactly

the data I want select star from shows

and let's just limit this initially to

10 to see what it looks like all right

that's again the shows data select star

from genres let's limit that to 102 just

to wrap our minds around it and now this

is not that useful however the data in

the leftmost column here is the primary

key in the shows table these are just

unique IDs the data here in the genres

table recall show ID is the foreign key

so it's the same numbers but just copied

into another table so that we can have

this relationship across them how do I

kind of line up these numbers with these

numbers to get back like a wider table

that has title and year and episodes and

genre and heck ratings and all of that

too if we want well you can join these

Tables by just telling the database what

to join on what so let me do this select

star from shows join that table though

on the genres table well how do you want

to join those two tables and again the

two tables from the picture looked like

this how do you tell SQL

programmatically to sort of you know put

one of them right next to the other line

up all of the ID so that you just get

one larger data set well we can use

indeed this this query this uh syntax

called join so back to VSS code here and

let me join these two

tables sorry typo here join genres on

the shows tables ID column AKA it's

primary key equaling the genres tables

show ID column aka the foreign key so in

other words it looks a little cryptic

but I'm just telling SQL how to line up

these two tables and what column to

match with the other so that the numbers

line up and I get essentially a wider

table let me go ahead and hit uh

semicolon and enter and this is now

going to give me a lot of data we might

have to interrupt it but notice even at

a glance we're getting the ID the title

the year the number of episodes the ID

again redundantly but that's to be

expected if I'm joining them and the

genre all the way on the right let me

hit contrl C to interrupt let me just

limit this to the office so where title

equals quote unquote the office so we

can focus on just one sample uh datum

and here fun fact there's been more than

one office the one that you all probably

like is this one that started in 2005

with 188 episodes its ID in the shows

table is 3866 76 that's confirmed over

here too so again we've just joined the

two tables how by lining up those fields

but now that we can see that almost all

of the offices produced over the decades

are comedies except for this one there

was a version of the office produced in

2001 that was considered more of uh a

drama no unsure if it's related to the

other how can we Link in other data well

let's go ahead and Link in ratings too

or instead so instead of joining this

with genres let me go ahead and Rewind

here and join shows on ratings on shows.

ID equals ratings. show ID and let's

limit it to the office 2 for discussion

sake where uh title equals quote unquote

the office semicolon and now you can see

that among the various offices it looks

like the one that most of us probably

know and love is the highest rated also

with a 9.0 with like 585,000 people

having cast votes for whereas these

other shows seem to have been less

popular perhaps that's why indeed you

see fewer episodes for them as well so

even though we've put the data in

multiple places you can still kind of

reconstitute it by lining things up in

this way and rejoining the tables

questions now on

this like this is the heart of what SQL

does and what relational databases do

for

you

questions all right a few final features

like there's not all that much that uh

you know SQL takes practice like

anything else but in terms of syntax and

capabilities let's just introduce you to

a couple of final features here and how

and problems that arise and how we might

solve them let's do this uh as well so

let me go back into VSS code here and

let's just find out um Steve Carell's

information again last time we did it

with this nested nested query by getting

his ID and then the show IDs and then

the titles for those show IDs with join

you can do it a little differently and

any of these ways are fine one might

become EAS easier to you mentally than

another let's go ahead and select the

titles from what let's select the title

from uh the people table but and I'm

going to hit enter and when you're using

SQL light 3 interactively if you ever

find yourself with a prompt that says

dot dot dot angle bracket it means

you're continuing your thought onto the

next line if you didn't intend that you

can sometimes hit uh semicolon to just

end the thought and hit enter and just

even if it triggers an error but this is

one way of formatting my queries now a

little more nicely I'm just going to add

some wh space so that it's a little

easier to read what do I want to select

well I want to select the title of shows

from the people table joined with the

Stars table on the people tables ID

column equaling the Stars tables person

ID column so in other words if you think

back to what people are and what stars

are one has an ID one has a person ID

I'm just now connecting those two tables

I'm joining those two but I want to do

this as well with with another table let

me additionally join in so now I only

have two hands but now I'm putting a

third table joined in together here join

shows on stars. show ID equals shows. ID

so this is now linking three tables

together but I only care about this for

one person so where the name of the

person equals quote unquote Steve Carell

so more cryptic to be to be sure but

what we're doing with this query is just

taking all three tables that we care

about and we're joining them them all

together at once using this new join

syntax literally telling the database

what columns to line up with what and

then we filter at the very end just like

before to get back if I hit enter the

answer we want which in this case is a

little slower at the moment but that

same list of uh 20 or so shows that he's

been in there's one other way to do this

and again uh these are all in the slides

online so you can repeat them without

having to jot down everything and we'll

put them in the notes too but there's

another way to do this I could also use

an implicit join so that was an explicit

join because I literally typed the word

join multiple times at that but let me

go ahead and select the title from these

three tables people stars and shows and

this might just be nicer because if you

know what tables you want to select data

from just enumerate them separated by

commas which you might prefer in your

mind where the people ID equals the

Stars person

ID and the Stars show ID equals the

shows ID and the name of the person

equals Steve Carell so this is an

implicit joint and honestly I constantly

reference my notes for some of this

stuff too it's not the kind of thing

that's going to come like this to you

after just one day but it's just a

different way of expressing the same

thing I want to select data from three

different tables and hey SQL here is how

I want you to line those tables up so

that I can get like related data for

Steve Carell and this now will achieve

the same results ultimately let me hit

enter

H and there we go so a little slower and

performance might vary based on computer

based on uh implementation of SQL but I

think I still have the same answers now

suppose as I'm often do and I had to

look it up again last time suppose you

forget what uh how to spell Steve

Carell's name is it two RS two L's or

the like well I could also do something

like this select well let's just keep

this simple select star from people

where name equals I've been deliberately

getting it right so as to not embarrass

myself that's the Steve Carell I keep

querying if you forget well you could

try searching for just Steves but there

interestingly there's a bunch of Steves

we don't know when they were born uh but

that's probably not the Steve Carell we

want if we don't have his last name so I

could alternatively do well it's Steve

and then it's starts with a C I think

well it turns out there's another wild

card you can use in SQL we used the

asterisk to select all of the columns

you can in quotes use a percent sign to

say see something so there's some zero

or more characters after the letter c

and now this doesn't work cuz now I

would be literally looking for Steve

space c something but recall earlier I

mentioned that one other keyword which

is for fuzzier matching so to speak

where it's like not exactly what you're

looking for but it's like what you're

looking for if you instead say where his

name is like Steve space c something now

we'll get back a whole bunch of Steves

but I think now I could probably find

the one I'm actually looking for if I

don't remember his name you can use

multiple percent signs if you forget

what his first name is you could reverse

the order but that too is a uh very

powerful sequel feature at that

questions on these queries here

yeah

sorry what about

it oh yeah sure so the query I used here

there's a lot of Steves whose last name

starts with C oops too far uh the last

query I executed was this one here so

where the name is like quote unquote

Steve C percent sign so that's just

another tool for your toolkit here but

you'll you'll perhaps have noticed that

those two the prior to that query the

joins I did were sort of slow and

honestly this database isn't even that

big like yes it has tens of thousands of

rows in it but like in the real world

and most of the apps you and I use a lot

every day or websites like there's

Millions even billions of rows of data

and like if I had to wait on like my

computer here or my code space like a

second or two to get the data like

that's not going to work for millions of

users or customers certainly so how can

we actually improve things well it turns

out another upside of a proper

relational database is that it's not

just a spreadsheet where the onus is on

like you to like find the data you're

looking for you can also tell the

database to index the data for you an

index is an an efficient uh cheat sheet

for Finding data fast like books in the

real world often have indices at the end

of the at the end of the book where you

can look things up alphabetically and

then you can cross reference it for the

pages that that topic appears on same

idea in a database if you tell the

database in advance that you want to

search on a certain column frequently

you can tell it to build a fancy index

that will just allow you to search that

column Faster by default these columns

are going to be searched most likely via

linear search like not even binary

search because the data might not be

sorted because it came in in any order

but if you create an index you're

probably going to get something closer

to logar rmic than linear and that's

going to be a big plus overall so let me

do something simple here first let me

turn on a SQL light specific feature

that just is going to time all of my

queries by writing do timer on I just

want to keep track of uh how long each

of these commands takes this one is not

a slow command so this is just going to

be relative but let's just select

everything from the shows table where

the title thereof is the office let's

see how long this relatively simple

query takes all right not very long at

all in real terms like less than a

second .035 seconds so not slow by any

means but if you've got hundreds

thousands millions of users like every

one of those milliseconds could very

well add up so can we do better well we

can if I do this if I use syntax like

this once in the beginning of the design

of my database I create not a table but

an index with some name on a specific

table on one or more columns I can give

a clue a hint to the database in advance

saying please optimize with some Secret

Sauce searching or selecting on this

column in this table so that my searches

are faster so let me do this let me go

back to vs code here let me create an

index called how about title index I

could call it anything I want but I want

to search faster on titles so I'm going

to call this a title index where uh

rather uh title index on the table

called shows and then in parentheses is

the syntax the column called title so

again I've just borrowed this canonical

syntax and I've just translated into

something that's TV show specific all

right what is this going to do for me

once I hit enter this is going to create

in the computer's memory the database's

memory something called a b tree it's

not a binary tree a b tree is actually a

potentially more efficient data

structure that we didn't talk about a

few weeks back in week five but it looks

a little something like this which looks

similar to a binary tree but does anyone

notice what makes this not a binary tree

exactly binary tree by implying two has

no more than two children per node but

here's a perfect example one two three

and there could be four children five

children or more but the effect of that

if you have a very wide tree the upside

is that it's like very short like it

pulls the data higher up closer to the

node to the root node and recall that

the root node is where we began our

searches in the past whether it was a

BST a binary search tree even a try or

data structures we always began at the

top so the higher up you can pull the

data even if it makes the data structure

very wide you're going to be able to do

boom boom boom look up queries look up

data probably much faster certainly than

if it's just a very long list like a

column by default so with that said let

me go back to vs code I didn't create

the index yet let me go ahead and hit

enter and create it all right it took a

minute a moment it took like half a

second which obviously is not that slow

but with more data that could have been

even slower but it's a onetime operation

as of now and now let me hit up and let

me select the same data from shows where

title equals the office last time just a

moment ago it took 0.035 seconds no not

slow but also that's going to add up if

I have lots of users of IMDb let's go

ahead now and execute the same query

again how long did that

take

0.01 seconds now I mean practically

nothing and so that's the sort of

opportunity now when you've got lots of

data and you want to really speed up

these searches these indexes these

indices that just create for you these

magical data structures in the databases

memory it allows you to search on

columns that you are pretty sure you

want to search on more effectively Now

by contrast if you've ever used like

Google or Bing or some search engine

that has advanced search some of those

text boxes that you can search more

precisely in might very well be slower

why well probably you don't want to go

crazy and just index every column on

every table why well what might be the

intuition like if logically indexes

speed things up why not index everything

there's always going to be a tradeoff

here what might that

be

yeah yeah it's going to take a lot of

storage right this is just a slide on

the screen but like this has to go

somewhere like this needs space in the

computer's memory or on the hard drive

or the like and that's fine if you have

unlimited space but odds are like you

don't and that's going to get expensive

for different reasons so maybe you only

want to index certain columns and

certain tables and not all of them

because you know what even if a user

really wants to search maybe VI advanced

search on some other column or table

altogether fine if once in a while a

query is slow like we're probably

getting the bigger bang for our buck by

optimizing the common cases the more

popular queries that people actually

care about

too all right so let's come full circle

and bring this now back to uh how how we

actually began which was with some uh

python code so it turns out these are

not either or decisions it turns out in

the real world developers are constantly

using one two three languages at once in

fact next week I rattled off HTML CSS

and JavaScript one of which is a proper

programming language but those languages

are often used together totally normal

and common to use Python and SQL or Java

in SQL or Swift in SQL or any number of

different combinations with a database

language you might use use your

preferred programming language Java

python C++ to create the user interface

and the logic that builds the uh the

that implements the program itself but

for your data like SQL is a really good

candidates and indeed we've seen already

that SQL can just speed up certain

operations you can change you can uh

collapse 15 lines of code into just one

and you can use these things together so

let me come back to I'm going to quit

out of SQL light I'm going to minimize

my terminal window and here's where we

left off before with favorite .p with

favorites. everything was being stored

in uh favorites. CSV and recall that we

eventually imported that CSV file into

favorites. DB automatically with import

just so we could start playing around

with SQL but we can now tie these two

together and a way to do that is as

follows um cs50 has a library for python

you might recall having available uh get

string get int get float you don't

strictly need to use them in Python

because it's much easier to just use the

input function and then try accept and

convert things to int or float or the

like but it's a lot more work to use SQL

in Python without a third-party Library

a lot of the commercial options or

popular open source options are actually

just complicated to use so cs50 does

have a very useful function inside of

its library for python that you should

use and must use for the problem set

that just makes it easy to execute

python uh execute SQL inside of your

python code but it's built on top of a

very popular open source alternative so

you can use that too in the real world

so the documentation for that at this

URL here but I'll show you what we need

to know here by focusing back on

favorites.i so what I'm going to do here

as follows is this let me delete

everything from favorites. py except for

let's say uh this from cs50 import SQL

in all caps so that's importing a SQL

feature from cs50's library that's going

to allow me to open a DB file in code

how do I do that well let me create a

variable called DB for database though I

could call it anything I want let me

call this SQL function and pass in using

special syntax that's not cs50 specific

it's an industry thing SQL light colon

slash slash slash unlike every other URL

you type this one literally has three in

this context here and then the name of

the database which in this case is

favorites. DB so this is just a way of

telling this SQL library that we wrote

but that works exactly like third party

Alternatives openen favorites. DB using

the SQL light technology if you will all

right let's just ask the user a question

give me your favorite um uh problem so

we're going to use input instead of get

string but we could use get string but

they're pretty much the same for our

purposes let's ask the user for their

favorite and now in Python code let us

select from favorites. DB all of the

rows where students specify that problem

as their favorite so in SQL alone it

would be this select uh star from

favorites where problem equals and I'll

do um well whatever my favorite's going

to be like problem equals Mario for

instance so if I were just using SQL I

would literally write something like

that but I'm in a piy file now like I

have to use Python syntax but python

supports strings SQL is just text it's

just a string so I could certainly just

put my SQL code in a string perhaps and

then pass it to a python function and

here's the bridge between the two if you

just treat SQL as any old text we can

put it in a string and execute it so let

me actually do this let me go ahead and

create a variable called rows which is

eventually going to contain all the rows

from the database let me go ahead and uh

select db. execute this is the one

function you need to know about inside

of cs50's library and it literally

executes a SQL statement and then in

quotes you pass it literally what you

want to execute and let me go ahead and

close the parenthesis at the end there

and now let me just try this so for Row

in rows let's iterate over all of the

rows let me go ahead and print out how

about uh row quote unquote and what do I

want here uh let's print out the Tim

stamp of that person for kicks all right

let me open open my terminal window

python of favorites. Pi crossing my

fingers here for

sure

enter uh uh there we go favorit so I'll

type in

Mario okay so I got back it's not very

interesting but I got back all of the

timestamps of students who typed in

Mario that we imported into this

database well what I really care about

is how popular Mario is so let me change

this a little bit let me change this to

count the number of rows and let me keep

it simple let me give an alias like I

proposed earlier like as n where N is a

number so that now down here I can

actually just do this print out the

value of n all right let me go back to

my terminal window run python to

favorites. py let me type in Mario enter

okay 39 now technically I'm cheating

like honestly if I'm executing select

count we've seen before it only ever

returns one row not multiple so there's

really nothing to iterate over but it's

working fine it's just just iterating

once but I'm getting lucky so

technically what I should probably just

do is this I should probably give myself

like a variable called row set it equal

to the very first row and only row that

came back and now print out that rows n

column let me rerun the program I'll

type in Mario again enter and I still

see

39 so this of course I don't strictly

need to do this I don't really need a

variable I can do Row Bracket Z instead

but let me focus on what this library is

now doing so per the documentation what

the C what the cs-50 execute function

always does for you is it returns a list

of dictionaries so if your query returns

nothing like no matches you get back an

empty list like Open Bracket close

bracket nothing in it any Loop is not

going to execute anything useful because

there's nothing in it if though you get

back one row you're going to get back a

list of size one in inside of which is a

single dictionary that dictionary is

going to have keys that correspond to

whatever you selected be it the columns

or the count so when I selected star

before I could have like I would have

gotten all of the columns that's how I

was able to access timestamp here I'm

just selecting count and I don't want to

have to type this down here that would

just look kind of atrocious it would

work but it would look weird to just

keep retyping count peren star close pen

so I just created an alias called n just

to like make this my life easier or

cleaner down here so to be clear the SQL

the cs50 execute function returns a list

of dictionaries when you're using select

and that is how I can now get back the

first and only row and then print out

that Row's n value it is identical to

let me do this let me highlight this

whole line of text let me in my terminal

window run SQL light three of favorites.

DB like we did before break let me just

copy paste this query enter that that's

the table I got back earlier when we

played with SQL manually and so when I

get back this table here's the key

here's the value and I only have one row

which is why I'm just blindly indexing

into rows bracket zero because I know

there's always going to be an answer

there it's going to be zero or one or

more but I know now it's going to be

called n because of this here so what

have I just done well this is SQL down

here and this is just me being like a

data scientist asking questions about my

data just using like black and white SQL

queries this is me now being a python

programmer who wants to talk to a SQL

database using Python and The Bridge

we're using happens to be the cs-50

library but again there's third party

free libraries you can also use as well

ours is just very simple and indeed the

documentation will explain how execute

behaves a little differently for inserts

updates and deletes you don't get back a

list because you're not selecting

anything but you do get back some return

values questions on

this that's the last of our python code

that ties everything together in

spirit

yeah uh what this this one

here yes so db. execute by definition

returns a list of rows and each of those

rows happens to be a dictionary because

it's convenient key value pairs if I'm

selecting the count of rows I just know

from Having learned squel an hour ago

that this is always going to give me a

single row whose column in this case is

called n so if I know it's a single row

I can just blindly just like in C go

into that list or an array in C and go

to the first location and then treat

that as the single row what you don't

want to do is this even if you the human

know the query returns one row you can't

just magically change the variable name

to be singular and expect to have only

one value you will always have a list so

even if there is only one value in it

it's up to you to do something like this

to get at it or if you prefer more

succinctness you can do rose bracket I

bracket n that'll achieve the same thing

without a variable

yeah good so I have been misleading this

whole time and cheating because this is

only ever going to return Mario I'm

ignoring the favorite that the human

typed in here on line five so let me fix

that and that's going to lead us to some

of the problems that arise ultimately

with SQL the right way to solve that

problem let me get rid of my terminal

window here the right way to solve this

problem is not to use an F string like

we did in Python generally because SQL

queries as we'll see in a moment can be

dangerous when you want to plug in users

uh data into a query that you've written

most of in advance you should you must

you had better use a placeholder namely

a question mark in this case this is

somewhat specific to cs50's library but

we just borrowed the convention that

like every other Library uses too in the

world of SQL single question marks are

used as placeholders and the way you do

this is as follows if you want to plug

in a value for that question mark just

like in print F in C you specify as a

second or a third or fourth argument all

of the values you want plugged into this

so in C weeks ago we were using percent

s same exact idea in SQL it's a question

mark that you use instead this now if I

open back my terminal window and I run

uh python of favorites. type in Mario I

should still get 39 but now I can also

type in scratch perhaps and get 44 for

that very first piece at zero and that

one is even more popular here so this

now is correct it would work to use an F

string here and then plug in a value

like favorite here but you'll see in

just a moment don't do that you will

expose yourself to potential hack or

attacks um by trusting the user's input

and so in fact let's transition from

that to exact some of these kinds of

challenges namely two before we wrap up

so in the world of SQL especially when

it's used at scale at the Twitter and

the Googles of the world like a lot of

lots of data is probably coming into the

database all at once because multiple

people are opening their phones at the

same time around the world they're

clicking on the same links roughly at

the same time around the world when you

have thousands of people all using your

site at once like order of operations is

going to be important but unfortunately

in SQL and in other contexts of

computing there's this risk of what's

known as a race condition so for

instance has anyone ever seen or liked

this this is like yes the world record

egg or it's like this thing that was

very popular while back it's still kind

of going strong but if you go to the

Instagram profile for world record egg

uh the goal was to make the most most

liked Instagram post ever and they did

pretty well it's just this it's just a

picture of an egg now at the height of

the popularity like there might have

been hundreds thousands tens of

thousands of people clicking pretty much

at the same time on this egg so it

actually creates a potential problem

with the Integrity of Instagram's data

why well if you're have all these

requests coming in at once how do you

possibly keep track of all of them and

update your counter in a way that can

keep up with all of that traffic why

well let's just hypothesize what meta

formerly Facebook was doing underneath

the hood with Instagram if this were

their code so suppose for the sake of

discussion that Instagram servers are

using a mix of python and SQL probably

not using the cs50 library but they

could absolutely be using those two

languages or two others together um

suppose they do this in order to update

the number of likes for that post they

first execute a SQL query like select

the current number of likes from a table

called posts where the ID of the post

equals whatever the unique identifier is

for that spec specific egg in the table

and then they store the result in this

row variable just like I did and then

they do this they grab uh they create a

variable called likes they set it equal

to rows bracket Z so the very first row

in the result set and they get the likes

key so this is literally what I just did

with the count let me hypothesize that

Instagram does something similar with

the total number of likes why are they

doing this because they then want to

execute a third line of code that

executes update the posts table set the

new number of likes equal to something

where the ID of the post equals this

other thing now notice just like in

printf there's the comma separated list

of values they want to update the

current number of likes from the current

value to the current value plus one so

it's likes plus one and then we plug in

the ID for this so suppose this is what

Instagram's doing unfortunately whenever

you execute multiple lines of code

independently and you're so popular like

Instagram that you have thousands

hundreds of thousands of servers

potentially it is quite possible that if

you and I and everyone else in the room

clicks that egg at the same time it's

not going to be the case statistically

that like three lines of code are

executed for me and then three lines for

you and then three lines for you they're

probably going to get interspersed like

this gets executed for me and then this

gets executed for you and then they get

back to doing work for me and so forth

just to kind of multitask just like a

human might but at a super speed here

the problem though is if these lines of

code get interrupted what could go wrong

well suppose that Carter and I both

click the egg at the same time and

suppose the current number of likes back

in the day is 100 that stores in this

variable the value 100 but if we click

so close in time we might get back the

same answer to this select query as of

that moment in time when David and

Carter clicked it had a 100 likes but

then this last line of code is executed

for me and then maybe Carter because

that answer the state of the database

was stored in this variable then both

Carter and I will result in this line of

code being executed with the same value

update the post table setting the likes

equal to

101 for that post's ID why because again

if each of these lines of code running

on different servers are checking the

value of the current number of likes but

then getting interrupted because Carter

clicked the darn thing too and then

resuming their work on my behalf we

might have a race condition where the

code is sort of racing to finish but

getting interrupted by other users

clicks and the problem with that is that

if you are inspecting the value of some

variable or in this case a database cell

and making a decision based on it like

how to update it you might now lose data

and Instagram probably not good for

advertising if they're losing likes and

so that's probably a problem not to

retain the value 102 and instead insert

the number 101 twice it's actually

similar in spirit to a story that uh was

told in a databases course I took myself

years ago

whereby uh it's somewhat analogous to

kind of a contrived scenario involving

like a refrigerator and this is the

closest thing to a refrigerator we could

get on stage but imagine you've got like

one of these little dorm fridges in your

dorm 2 and your roommate and maybe both

of you as the story was told to me

really like milk and one of you is at

class but the other of you comes home

and you open your dorm fridge and you're

like oh darn it we're out of milk and so

you close the fridge you walk across the

street to CVS or some other store and

you get in line to buy some milk

Meanwhile your roommate gets out of

class they come back to your dorm room

they're really thirsty for some milk

they open up the fridge they say oh

we're out of milk and then they take a

different route perhaps to CVS or some

other store nearby get in line to buy

some milk fast forward some amount of

time in this very contrived story and

what happens oh damn it we now ended up

with two gallons of milk and there's no

way we can fit gallons of milk in there

let alone two of them so that's a

problem but what's the relationship to

this here well both of us yeah did

what

exactly

exactly so to summarize both of us had a

very similar thought process made a

similar decision based on the same

information not realizing that the

information the fridge was in the

process of being updated and of course

in the Instagram World happens like this

in the fridge World it might take a few

minutes but the problem is ultimately

the result of our having made a decision

about the state of the world and the

state of the world was in the middle of

being updated the queries got mingled

with others or in this case someone was

already on their way to the store so

what's the solution in the real world

well you could you know very simply like

take a Post-It note and put like gone

for milk so as to communicate to your

roommate that they should not inspect

the value of that variable and make a

decision on it why because it's not yet

consistent with the outcome that's about

to happen you could be more traumatic

and you could actually lock the fridge

somehow put a pad lock around it or the

like so they can't even get in there and

that would achieve the result uh the

same effect too and that is actually

pretty much the solution to this problem

in code too it's not safe it's not

sufficient to only execute three lines

of code like this rather what you

probably want to do is use additional

SQL keywords that we won't spend much

time on in the class itself but these

there are solutions to this problem you

can begin what's called a transaction

and you can more explicitly commit to

making a decision like updating the

database to 10 1001 or 10 1002 or if you

realize wait a minute Carter's query

inist interrupting mine let me roll back

to the previous state and just uh rewind

let me undo contrl Z if you will there's

also another keyword that's not so much

used anymore in SQL which is locking you

could literally back in the day lock the

entire database table preventing anyone

from updating it or making changes or

even reading it while someone else was

accessing it that was a very

heavy-handed solution because it slowed

everything down but in short

transactions are now a feature of SQL

that you won't necessarily need to use

yourselves that do solve this problem by

doing the equivalent of saying while

David's like counter is in the process

of being updated keep Carter at Bay

ideally briefly and then let his data go

through too it's equivalent too to

putting a note or a lock on the fridge

and indeed I mean lock litter they they

were Once Upon a Time called and still

are in some texts called locks on

databases 2 and the last Pro and the

code for which you might do this is

almost the same you simply wrap the

three queries uh with a transaction

statement and a uh commit and the term

of art here is that this makes your uh

your uh statements Atomic so Atomic

means they're either all executed or not

at all that is they're all very tightly

coupled together without interruption

transactions solves that problem and

avoid having two gallons of milk and the

last problem that arises that Is

tragically so darn common in the real

world today is what's called a SQL

injection attack and it's what I alluded

to earlier with the question mark So

suppose you're in the habit of logging

in uh to Yale websites with your net ID

or password or at Harvard your Harvard

key and password as well suppose for the

sake of discussion that the people that

implemented like Harvard key login allow

you to type in your email address of

course and your password but suppose

that they are using SQL underneath the

hood to check your username and password

to make sure that you are David menen or

Carter zeni or whoever you claim to be I

haven't shown you the syntax yet but it

turns out that in SQL Das Dash is a

special way of indicating a comment it

means ignore everything to the right so

it's just like SL slash and C or the

hash symbol in Python dash dash just

means ignore everything to the right and

we've of course seen single quotes So

one way to wage a SQL injection attack

is to try to inject malicious SQL code

into someone else's database without

them realizing it how do you do this

well suppose I log in as M harvard.edu

single quote Das Dash I'm not double

quoting anything clearly and there's

nothing to the right of the dash dash

anyway but it this imbalance is going to

be useful why because if I'm a hacker

and I'm presuming you know someone at

Harvard probably is using SQL uh single

quotes to wrap the user's email address

and wrap the user's password what if I

try to like complete their thought for

them and close one of those quotes for

them what might happen well we could do

this here for instance let me

hypothesize is the code that Harvard

wrote hopefully not underneath the hood

so they're using CS library and Python

and they're using SQL inside suppose

that they have a query like this select

star from users where username equals uh

question mark and password equals

question mark and then suppose they just

plug in whatever username and password

was typed in and then if they get back

some number of rows dot dot dot they

assume I am David they assume Carter is

Carter if both the username and password

are in the database just end of story

there this is good this has the question

mark placeholders we discussed earlier

but what if you don't quite remember

that you don't quite take that to heart

and you use your more familiar last week

F strings whereby we use these curly

braces to plug in values what if you do

this instead so it's almost the same

idea it's still DB execute but now it's

select star from users where username

equals and now notice I'm doing the

single quotes which is required by SQL

but I'm using F strings with the curly

braces and the password equals single

quote password and then close single

quote the problem is if you're just

blindly pasting equ effectively the

user's input into that web form into the

username field and the password field

there's nothing stopping a malicious

user student faculty staff from

including a single quote in their name

or maybe even an uh uh you know

benevolently if their name happens to

have a single quote as some last names

in particular do so this is very fragile

why well suppose that if we plug in my

malicious value Ma at harvard.edu single

quote-- notice what happens to username

here the username variable inside of the

curly quotes will get replaced with this

and notice single quote which the

Harvard programmer wrote M an

harvard.edu single quote which I wrote

dash dash which I wrote single quote

which Harvard wrote and whatever else

they want after that what's the

implication though of the dash

dash everything to the right is going to

be ignored so the password is never even

checked in this scenario I'm tricking

the server into ignoring everything

after the dash dash but I have

constructed very cleverly very

maliciously a syntactically valid query

why because I provided the single quote

that's going to finish the thought of

that first single quote and now I would

only know how to do this if I saw the

code or if I just randomly try putting

apostrophes into web forms and see if

things break that's often how

adversaries attack systems they type in

potentially dangerous characters hit

enter if something breaks they're not

necessarily into the system but they

know that there might be a vulnerability

and then they start trying more

methodically things like this so This

Then is going to be bad because it

effectively Grays out the rest of the

query and this query is surely going to

return some rows without even knowing my

password and so this logic here dot dot

dot means well if a data came back from

this query Harvard is presumably going

to assume that men logged in show them

uh show him like uh his account or

whatever is being protected here so in

short using F strings bad using any

equivalent like p uh percent s in C bad

when it comes to SQL using question

marks or whatever a thirdparty library

like cs50's prescribed is the way to

solve this why because libraries like

ours are designed to at least be smart

and be paranoid and what we will do is

this when you use the question marks and

the values are plugged in we will escape

any potentially dangerous characters

inside of those placeholders and so

effectively the single quote will no

longer be considered a grammatical

single quote it will just be literally a

character in the username or password so

the library takes care of this for you

because you're plugging in the username

and password as separate arguments and

then we or the third party you're using

actually sanitize that is uh clean up

the data and prevent those bad

characters now this is kind of an

internet meme that went around for a

while um if you've ever uh driven a car

been in a car where there's like the

automatic reader for tolls uh this

person thought it might be funny to try

doing something like this what are they

presumably doing the presumption here is

whether or not it worked as unclear is

that here's like the end of a actual

license plate number but here's an

interesting single quote and a semicolon

that's especially bad because it means

you can maybe execute a second query on

the database this is someone having fun

trying to drop the entire database table

for whatever municipality is scanning

through cameras uh their license plate

code and I would be remiss if we didn't

end on this note at least in computer

science circles um there is someone

named no relation to the TF name we put

in the database earlier um little Bobby

tables which ends with this XKCD

comic and if you chuckle if you laugh

you're now legit SQL

programmers nice

nice like every CS student out there

knows about little Bobby tables so if

you name drop little Bobby tables now

like you're in all right that's it

though for today we will see you next

[Music]

[Music]

time

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[Music]

[Music]

[Music]

[Music]

[Applause]

[Music]

[Applause]

all the way to the top and then you're

passing

down this is for you Yale we love you

Yale we're here to Har

go

har

[Applause]

down it's going to

happen it's actually going to happen I

can't believe this what do you think of

Y they don't think

good can't does everyone have it does

everyone have their stuff does everyone

have their stuff probably that it's

going to beable very

small I

know what

houses how many exra how many

EXT no F forer yeah just make sure

everyone

has pass all the car distributed

all right we can do it

[Applause]

[Music]

now

what do you think of Y

sir go go one more time one

[Applause]

more there goes

[Applause]

again

[Applause]

all right this is cs50 welcome to week 8

last week we learned how to create read

update and delete databases using squl

but this

week Adam everyone happy

Halloween all right so this is cs50 and

this is week eight already my thanks to

Adam on today this happy Halloween uh in

the coming moments we're going to learn

all about how the internet itself Works

which of course is a technology that

like we're all take we all use every day

probably using in some form right now

but we'll see that if you start to

understand some of the underlying

building blocks that power the internet

itself we can actually start to build

interesting things on top of it and a

lot of the apps the websites that you

all use every day should become all the

more familiar things that you yourself

VES can create and honestly when things

go wrong you'll have all the more of a

mental model for how things work or are

not in fact working so that you can

ultimately diagnose diagnose all the

more issues yourselves so if we take a

look at the internet in the early days

it pretty much was just this this

happens to be of course the geography of

the United States and just some of the

first uh points on the internet were

these here this was so-called arpanet

back in 1969 and indeed the internet had

its Origins here in the United States

with just a few computers interconnect

somehow initially that of course began

to grow over time such that we

eventually had the West Coast connected

to the east coast and nowadays what you

can think of these dots on the screen is

representing are these things called

routers sort of computers or really

servers that somehow have wires or maybe

wireless connections between them that

allow data to flow from point A to B to

C and then this of course has been now

magnified across the entire Globe um and

even above ground as well so that we can

connect all the more readily uh to

systems anywhere now in order to Route

the data from one router to another we

need to somehow make routing decisions

and this is the kind of thing that the

internet service providers the isps of

the world just handle for us you and I

plug our Macs our PCS into the network

here at Harvard or equivalently at Yale

or we somehow get online via Wi-Fi or

cellular technology and then some of

these larger entities these bigger

companies or countries handle most of

the data getting from point A to point B

and if you think about what these

routers present they're indeed just

servers somehow interconnected not

unlike this grid of tiles here for

instance back in the zoom days and in

fact here we have I claim a grid of

routers implemented here by the courses

teaching fellows and course assistants

and Tas and if the goal at hand for

instance is for Phyllis to Route some

piece of information maybe it's an email

maybe it's a request for a web page in

the bottom right hand corner all the way

up to say Brian here in the top left

hand corner suffice it to say each each

of these tiles represents a router a

server that can move the data back forth

left and right that packet of

information so to speak from Phyllis to

Brian could take any number of different

possible routes up down left right to go

from the one corner to another so let me

go ahead and hit play on this video here

we're in the teaching fellows play the

same

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role

all right so in this particular case the

data was routed pretty straightforwardly

up and then to the left but suppose that

one or more of the staff were a bit busy

maybe one of the routers is congested

that is to say just got way more

envelopes at a moment in time that it

can handle thankfully the design of the

internet is such that there's often

multiple ways that data can get from

point A to point B maybe going through

Point C or Point D instead and so

there's a resilience there even as some

of these servers themselves might go

down so allow me to propose that we use

the same grid of routers now to Route

the data in a slightly different way

this

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[Applause]

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time

so success you'll see perhaps later just

how many takes it took us to actually

get that routing right but it does in

fact manifest that you can uh travel

different paths in order to get the data

from point A to point B so as we talk

about routers as you think of the

internet I mean think of those humans as

just representing these routers points a

to p and everywhere in between now how

did the teaching staff know to route

that packet up and then down or left and

right in order to get get to the

destination well all of them were

programmed so to speak to understand

protocols it's called TCP and IP

Otherwise Known together typically as

tcpip and you've probably seen these

acronyms at some point in the real world

on the internet on some kind of

documentation a text even if you haven't

really thought hard about it but IP is

certainly the more common of the two

perhaps in common culture so what does

TCP and IP do for us well really two

primary things any computer or any

teaching staff member who understands

tcpip knows how to get data from point A

to point B but how well let's break down

what that problem to be solved is IP

otherwise known as Internet Protocol is

a protocol that computers speak that

allow them to know how to address

computers on the internet and a protocol

is just a set of conventions that

computers adhere to so someone wrote

code that probably has a whole lot of

conditionals that tells the computer

what to do if something happens like if

I receive a packet then send it to the

next server or something like that in

the human world we have protocols too

you know pre in healthier times it was

quite common to sort of extend your hand

to another human in order to greet them

and if they're following human protocol

they would presumably grab your hand and

shake it at least in a a culture like

this one here on campus and now that is

a human protocol in that someone

initiates it someone responds to it and

you both sort of know what to do you're

programmed to know what to do so same

idea with Internet Protocol computers

just know what to do when they've been

programmed to do so so what does this

mean IP had decides that every compter

computer in the world will have a unique

address just like uh a the science

center around the corner might have a

unique address of one Oxford Street

Cambridge Massachusetts 02138 USA IP

dictates that every computer on the

internet have a unique address of this

form and this too is probably something

you've seen in the real world even if

you haven't thought too hard about it

it's a number and what's called dotted

decimal notation which means it's a

decimal number do something do something

do something so four digits separated by

convention by decimal points although

there are newer and bigger versions of

the same and these so-called IP

addresses that might be as simple as

1.2.3.4 uniquely identify a computer on

the internet uh the numbers have to

range from 0 to 255 each and that's a

bit of a hint just as you start to think

more computationally if each of these is

a number from 0 to 255 how many bits

does that suggest each number is

using feel free to shout it

out how many bits gives us 255 256 total

possibilities so eight bits that should

just be sort of a heuristic in your mind

anytime you hear something that's in the

range of 0 to 255 or 256 values total

think back to week zero which gives us

eight bits plus another eight bits

another eight bits and another eight

bits which is to say an IP address

typically is 32 bits in total now if we

do another bit of quick mental math or

think back to week zero if every IP

address is 32 bits how many computers

can we have on the

internet at once give or take

roughly 4 billion is the ballpark and we

don't need to be super precise for

discussion's sake but roughly 4 billion

is how high you can account assuming no

negative numbers if you have 32 bits in

total now that's not terribly uh many

number uh numbers of addresses

especially considering the number of

humans in the world the number of us

that do have laptops or desktops or

devices more generally phones in our

pockets and the like so let me just

stipulate for today's purposes that

there's even a newer and improved

version of Ip otherwise known as version

6 this is version four but still super

popular version six uses 128 bits which

is a huge number of possible

premutations I dare say I can't even

pronounce that number it's so big so

there are ways around even this

limitation already so every computer has

an address like this what does that

really mean well suppose that I was

Phyllis in the story told visually

earlier and I want to send a message to

Brian well both Phyllis and Brian have I

IP addresses and suppose that Brian's IP

address happens to be 1.2.3.4 in that

top left hand corner well phyllis's Mac

or PC or phone would essentially do the

equivalent on this human envelope by

writing the two address in the middle of

the envelope as is our human convention

like this so this is an envelope a piece

of information an email a text message

whatever destined for Brian and so she

would have her computer put Brian's IP

address in the middle her IP address is

Maybe 5.6.7 do8 so just like our human

convention I might write 5.

6.7.8 at the top of the envelope

5678 thereby indicating what the return

address is and this is helpful because

if Brian's computer needs to acknowledge

receipt if he needs to reply in some

form this way the envelope has all the

information we need but in the real

world servers do a lot of things

nowadays not just email but maybe chat

Maybe video conferencing maybe any

number of other services as well and so

it turns out that an address alone might

not be sufficient because how do how

does Brian's computer know when he opens

the envelope so to speak that this

should be interpreted as an email or

interpret it as a chat message or

interpret it as like a video attachment

that Phyllis has sent well we need some

other mechanism some other hint on this

envelope to distinguish one type of

Internet service from another and so

that's where the other acronym in TCI

pip comes in which is TCP so this stands

for transmission control protocol which

is just a different set of conventions

that computers adhere to in order to

solve a couple of different problems one

is this problem of distinguishing one

type of service from another now what

does that mean well humans decades ago

decided as they started inventing all of

these various internet services the web

being the first one how they might or

the web now being one of the most

popular ones they decided to assign

different services that can be used on

the internet unique numbers and so two

of the most common are these 80 is the

number that a bunch of humans decided

years ago will represent what you and I

know as HTTP and we'll talk more

technically in a bit about what HTTP is

but obviously it's the thing that's in

the beginning of every URL nowadays or

https which of course has the S added to

it and that has its own unique number

and for now the S just means secure one

is encrypted or scrambled somehow for

privacy sake and the other is

unencrypted it's a little more

vulnerable to interception so these two

numbers are what the world decided when

implementing TCP shall uniquely identify

those services so what does this mean

well this means that if Brian's computer

in the story from before is hosting not

like an email server but maybe he has a

website and Phyllis is requesting

Brian's homepage or something like that

she would have her Mac or PC or phone

not only write Brian's IP address in the

middle of the envelope but also the

number otherwise known as a port number

that she wants this envelope to be

routed to now 80 would be insecure

nowadays HTTP col is sort of p and we

almost always see https colon now so I'm

just going to go with best practice and

I'm going to add a colon and then the

number

443 at the end of Brian's IP address so

now I have an IP address for Brian the

port number for the service that this is

relevant to and I'm not going to bother

writing it but it turns out that

phyllis's computer would also choose a

port number maybe a random port number

so that Brian can conversely reply and

then the computer can know which

response is in uh coming back for which

request but the most important one is

this one in the two field whereby this

distinguishes this from like an email a

chat message a video conference session

or a zoom or whatnot from anything else

happening on Brian's computer at the

same time so all this time if you've

seen these terms TCP and IP those are

really two of the most important things

that they do but TCP does one other

thing that's super useful too it turns

out that it's

super common nowadays especially to

transmit a lot of media on the internet

internet whether it's an image or maybe

it's a movie file and it would be a

little obnoxious to say the least if

you're downloading a really big file

meant that no one else in your dorm room

or your household could actually

download anything until you're actually

done so of course multiple people

nowadays can be on the internet at once

even if all of the connections are a

little slower but like one person's

usage does not block someone else's now

how does this work well TCP in

conjunction with IP can also allow you

to take like a really big image of a cat

which is the internet of course is

filled with and take a big image of a

cat or a big video file of a cat and

fragment it into multiple pieces so I'm

just going to sort of roughly tear it

down the middle and then maybe tear it

down the middle again so now it's four

different fragments and I'm I'm sorry

but the computer will be reassembling

these for us and what phyllis's computer

could do now if she's like uploading

this picture of a cat to Brian's web

server well she could put one fragment

in this envelope and then have three

separate envelopes for the other three

fragments and what you could then do on

the outside of this envelope is just

kind of number them somehow and in fact

this is something else that TCP and IP

together would do for us this first

envelope now might say something like

one out of four in the memo field so to

speak of the uh metaphorical envelope

here now this should be enough

information because now if Brian gets

all four of these envelopes he

presumably knows how to reassemble the

picture of the cat in order top to

bottom left to right but more

importantly suppose that one of the

routers one of the TFS in the video is

sort of distracted and they sort of drop

one of the packets and that's a metaphor

actually in practice for when a router

gets really busy it's got way too much

data coming in it might metaphorically

drop packets what does that mean in

practice I mean it literally just

ignores the zeros and ones it doesn't

save them to its memory because there's

just no room left so it's equivalent to

sort of dropping the packet so suppose

now that Brian gets one of four three of

four and four of four what can his

computer infer

now after receiving those three packets

one of four three of four and four of

four what's the use there yeah I think

you're you're signaling with your

fingers which one did which one can I

call on you yeah so he's missing two out

of four the second of the packets and

this is useful now because you could

imagine he can send some message back to

Phyllis saying hey please retransmit

number two of four without having to

redownload the entirety of the cat so

there's there's an efficiency there as

well so tcpip allows data really to go

from point A to point B while solving a

bunch of these problems along the way so

nowadays if you ever see mention on your

Mac or PC of your so-called IP address

that is the sort of problem that's being

solved questions now on these protocols

these conventions called TCP and

IP that's the extent to which we'll need

to understand them won't have to

implement them per se we'll just take

them Hereafter for

granted any questions that you've ever

been wondering wondering about your home

network

yeah a really good question uh how does

TCP know that a user got a message

another aspect another feature of TCP is

that Brian's computer by design of this

protocol will also acknowledge the

packets that he's received and it will

do it efficiently if Brian receives all

four packets in a pretty narrow window

of time his computer will send to

phyllis's computer a quick message

saying essentially received all four

otherwise he'll say the opposite which

is that I'm missing for for instance two

out of four and that just ensures

ultimately that all of the data is

indeed uh arrived so that you're not

missing like uh a quarter a quadrant of

the cat in question all right but th

that's not the only problem that needs

to be solved ultimately we also need to

make the internet userfriendly if you

will and it would be really tedious if

you had to visit websites for instance

by way of of their IP addresses right

1.2.3.4 is pretty memorable but there's

like 4 billion other possible addresses

available and it would be super tedious

to remember those it would be bad

marketing to advertise those in fact

most of you probably don't even know the

phone numbers of your closest friends

and family members anymore because you

instead store them in your contacts

you're in your dress book associating

with numbers that are completely opaque

with actual names or strings if you will

the same is goes for the internet too

even though every computer does have and

must have a unique IP address

numerically why well routers or

computers computers just crunch numbers

very readily but we humans work better

with strings of text we need some system

for converting userfriendly strings like

harvard.edu or yale.edu or google.com to

the underlying IP addresses and that's

where the next acronym comes in today

which is DNS domain name system so this

is just another technology that's been

in use for some time now and it's a

collection of servers on the internet

that whose purpose in life is to convert

domain names to IP addresses and maybe

vice versa as well so let me stipulate

for today's purposes there are some root

DNS servers in the world that long story

short know about all of the Dooms all of

the edus all of the dot dot dot all of

the other top level domains around the

world as well as in the US and then

there are some smaller DNS servers owned

by companies owned by universities and

even in your apartments or homes most

likely indeed if you have a home router

plugged into the wall somewhere that's

not only routing your data in and out of

your apartment or home or dorm room it's

also typically serving as a local

therefore faster DNS server a cash if

you will it's sort of locally saving

your most frequently accessed websites

and their IP addresses just to avoid

bothering the bigger more expensive

busier servers all day long so there's

DNS servers all over if you poke around

your settings in Windows or Mac OS or

Android or iOS you'll see mention of DNS

and you'll probably see the IP addresses

of the servers whose purpose in life is

to do this conversion for you but this

is a requisite feature if we just want

the internet to be user friendly and

allow us to use words instead of numbers

alone what's inside of these DNS servers

you know it's essentially a spreadsheet

or if we can say it more geeky it's

essentially like a hash table of some

sort which it has keys and values like

the key is the domain name harvard.edu

gale. edug google.com and the value is

the corresponding IP address or in many

cases IP addresses plural of the

corresponding servers so here already

even though I've drawn it fairly

abstractly like you would on a

chalkboard it's really probably

implemented as some kind of table maybe

a hash table maybe a database table

maybe SQL or something like that or

maybe it's even just a link list or an

array we just have to somehow enable

this computer to convert one to the

other now just to be super precise

DNS servers actually convert what are

called fully qualified domain names

which is generally not just harvard.edu

but more verbosely

www.harvard.edu and

www.google.com so the whole thing that

you would see as a substring of the URL

so that's what DNS does and that's what

your University your company your home

router are doing for you let me pause

here to see if there are any

questions this to is just a technology

now we'll take for granted just

works questions at all all right so

let's now transition among our protocols

really to the last for today which will

set the stage for actually solving

problems with these and writing some

code ultimately um HTTP this is

something that you see or hear all day

long even though you rarely have to

bother typing it anymore odds are if you

go to harvard.edu yale.edu google.com

you don't bother typing HTTP let alone

https manually anymore why because your

browser Auto completes that kind of

thing just to make life easier but it is

officially at the beginning of every URL

you visit either HTTP or the more secure

https whenever you're using your browser

to access some website so

HTTP stands for hyper text transfer

protocol and it's uh easily one of the

most popular dare say one of the most

powerful features of the internet

nowadays but the mental model to have

here is that HTTP or the web more

generally is kind of a service that runs

on top of the internet and maybe Zoom or

Microsoft teams is another service that

runs on top of the internet an iMessage

and Technologies like it is another

service that runs on top of the internet

so the internet is really like the lower

level Plumbing the tcpip stuff the DNS

stuff that just gets data from point A

to point B but now and we're in a a

software development class ultimately

here in cs50 HTTP is the application

Level protocol it's sort of what

programmers use what companies use what

uh developers use ultimately to use the

underlying Plumbing to build interesting

and Powerful things so what does this

mean when it comes to accessing Services

via HTTP or the more secure

https well here is a representative URL

even though you might not type the whole

thing if you poke around your address

bar this is what's up there with that

said a lot of browsers nowadays are kind

of simplifying if not dumbing down what

you see with your human eyes just to

shorten the strings especially on mobile

devices but almost always if you click

the URL or highlight it then you see the

whole thing but on many browsers you

might only ever see example.com but all

of this information is there it's just

getting more and more hidden just for

user interfaces sake well it turns out

when you visit a URL by default

especially if you type nothing after the

do com in this case you're technically

implicitly adding a single slash so a

single slash denotes the root of the

server that is the default page or

folder in the server and the slash

whether or not you type it or not is

implicitly going to be there and that

just means give me the default whatever

is at www.example.com give me that page

or that folder but URLs can be longer

than this and more generally there can

be a path so to speak and this is a term

of art a path is some sequence of folder

Andor file names after a URL like this

and so you might see more specifically

that a URL contains a very specific file

this isn't as common nowadays anymore

though we will begin to today by using

this technique but if there is a file

called literally file. HTML or something

else on the server that file is going to

be what this URL pulls up on the

computer meanwhile you might have slash

folder slash which just means show me

whatever is inside of this folder or you

might have more verbosely folder file.

HTML which will show you that file in

that folder and meanwhile just to give

some other terms of art this is the

so-called fully qualified domain name

and again these vocab don't matter all

that much but you'll hear or see them

over time we generally colloquially just

refer to this as the domain name which

is a little less precise but gets the

job done certainly in conversation and

this part here I described briefly

earlier what's the name for this suffix

at the very end of the fully qualified

domain

name the yeah yeah top level domain or

TLD and this is just some form of

categorization of the URLs now the

internet in got it start within the

United States and a lot of the first

websites of course came from the US and

so For Better or For Worse the sort of

steak was planted in the ground so

generally do indicated at least early on

that it was some kind of commercial

Enterprise a business that owned a

domain name edu is a educational

institution net was some kind of

network. goov was the US government now

there are also country code tlds cctlds

like UK or JP every country in the world

has its own two letter uh TLD um that

might very well be restricted to only

servers or companies or people in that

uh in that country many of them can be

used by anyone you've used a lot of URLs

in this class ending in

cs50. that doesn't mean input output um

it actually is a TLD from another

country that lets anyone on the internet

um pay for and on an annual basis using

that domain. TV for instance you might

see in some cases like twitch.tv and the

like um that too is owned by another

country that allows others in the

English-speaking World in this case to

use it as though it connotes TV but

those are just different types of TLS

that roughly categorize where the domain

lives but it doesn't necessarily mean

it's commercial anymore it doesn't

necessarily mean it's a Network anymore

for the most part there are hundreds of

tlds now for better for worse most of

which are less common than these big

ones um but most anyone can buy most of

them with just some restrictions on

things like edu and goov that are still

very much regulated this meanwhile is

what we might call the host name www

it's obviously a super common convention

like almost every website uses www as

its host name but that's a human

convention it's not a requirement and

indeed some websites don't even bother

having a host name they just use their

domain to advertise their websites this

now is going to be the scheme or the

protocol and this is just going to

indicate via what protocol the computer

your Mac your PC your phone should use

when accessing content at that address

because indeed there are other protocols

you can use but for the most part

will'll only focus on HTTP or

equivalently

https all right any questions now on

those just definitions building blocks

of URLs just so we all sort of share a

common

vocabulary any questions at all yeah

what is the

local sure we'll come back to this

actually later today there's a a

technical term known as Local Host which

is a generic name for your computer your

Mac your PC your phone especially when

you're doing software development and by

convention your own computer has not

only whatever IP address you get from

your University or your internet service

provider it also has a reflexive IP

address one that just always refers to

itself which is 127.0.0.1 and that's

just a human convention humans decided

that shall refer always to your computer

and it's actually going to be useful

today and onward because we can use that

when development on our own computers

ultimately other questions on URLs IP

DNS or any of these building

blocks all right so what do we mean by

HTTP being a protocol when I extended my

hand earlier as a human handshake you

know a typical human in healthy times

would know to respond in turn well how

does Brian's computer know to respond to

phyllis's envelope whatever message is

they're in well assuming that Brian is

indeed still a web server in the store

and Phyllis was trying to upload a cat

or maybe download a cat from Brian's web

server inside of phyllis's envelope

would have been a message literally in

text and it's English for the most part

and it would contain at the beginning of

that message literally one of these

English verbs either get which means

just that get me the homepage get me a

picture of a cat get me a picture of a

dog or a video or anything else or post

which often means post that is um put

that is um upload a picture of a cat or

a dog or something else to the server

instead that's not strictly the only use

cases for these but you can generally

think them as one is just getting

information and the other is posting

from the client from Phyllis to the

server in this case Brian so those are

the two keywords that we might see and

why is this useful well it turns out we

can start to see in our own Mac or PC

some of these very same messages for

instance if Phyllis were visiting not

Brian's but example.com that web server

inside of her metaphorical envelope

there would be a textual message that

literally starts with get slash then the

word HTTP then the version she's using

1.1 is very common two and three are

becoming more common but HTTP generally

looks like this the next line of text in

her envelope would probably say host

colon then literally the fully qualified

domain name of the server she's

accessing just in case and this happens

super commonly especially on small

websites if one server is hosting

multiple domain names multiple websites

this just distinguishes which one she

actually wants and then there's usually

a whole bunch of other lines of text as

well so where can you actually see this

well let me actually go ahead and do

this give me just a moment and I'm going

to open up on my computer here uh an

empty Chrome window in incognito mode

generally speaking incognito mode or

private mode is used when you don't want

there to be left remnants of what

websites you visited and it has the

effect for software developers of just

forgetting any things you might have

tried already within your browser

including things called cookies more on

those another time uh your autocomplete

history and the like so for development

purposes incognito mode is especially

helpful because it's sort of like

starting with a clean slate every time

you open a new private or incognito mode

so there's not going to be like any

remnants of previous testing or code

that you've been playing with and I'm

going to go ahead and do this I'm going

to go ahead and uh right click or

control click on Chrome I'm going to

choose inspect and it's going to pull up

this window sometimes on the side

sometimes on the bottom I'm going to

move it to the bottom just so we can see

it a little more readily and I'm going

to zoom in and it's going to look a

little Arcane at first and I'm going to

just highlight a few of these tabs we'll

see here along the top that there's

elements console sources Network and

whole bunch of other things as well this

is sort of the advanced mode in Chrome

and Safari and Firefox and Edge have

their own equivalent of these features

they've always been there even if you've

never clicked the right button to enable

these features and I'm going to focus

for a moment on network like this

this is a feature of the browser that's

going to allow me the programmer in this

case so the the engineer to just kind of

look at what messages my browser is

actually sending to a server so let me

go ahead and do something like this let

me go ahead and visit uh for instance uh

in my browser here and I'm going to

shrink the window just a little bit so

we can see it exactly I'm going to visit

https uh

www.harvard.edu and now I'm going to hit

enter

and a whole bunch of stuff just happened

along the bottom of my screen and I'm

going to try to pull my window up just a

little bit so we can focus on a subset

of this let me pull this up covering up

really the content of the page focusing

on these lower level details down here

and what I want to see first

is let me oh sorry let me go ahead and

reload this page here after retaining

the log so that we can see absolutely

everything on the screen and to be clear

I just checked because I forgot earlier

preserve log because I wanted to

preserve everything on the screen I want

to see everything all at once and we'll

see this the very first line of output

is completely overwhelming with detail

at first glance but what you'll see here

if I start to scroll down and down and

down and down are the so-called request

headers and let me zoom in here and what

you're seeing inside of chrome inside of

its Network tab in its so-called

developer tools again this is just for

engineering types you'll see all of the

headers all of the lines of text that

magically were sent by my Mac to

harvard.edu much like from Phyllis to

Brian server in that story so I can see

exactly what messages are being sent and

a lot of this we haven't talked about

yet but we do see some mention of get

and we see some mention of Slash and a

bunch of other Arcane details but notice

they're all sort of key value pairs with

the here indicating what the

corresponding value is now most of this

is not going to be interesting and we're

not going to focus too much on the weeds

of of all of this but it indeed gives us

a sense of what's inside of that virtual

envelope now harvard.edu is one thing

but there's other uh websites we might

visit as well um and no matter what they

are we're going to expect ultimately an

HTTP response so in addition to a

computer like my Mac or phyllis's

computer sending a request containing

gets and host and those details too

you'll see here in my slide form just

representative response from the server

and notice that key here is that the

server is responding in the same version

of HTTP in this example it's sending

back this so-called status code just a

numeric code that indicates in this case

that everything's okay and it includes

this header this HTTP header which again

is just a key value pair saying that the

type of this content that's coming back

from the server is text/html more on

HTML in just a little bit but for our

purposes now this just means that

harvard.edu is sending me back a web

page and indeed if we hide all of this

techn techical stuff that's the web page

that we saw up here with all of the

usual imagery and the like and in fact I

can see this if I scroll back up not to

request headers but response headers

you'll see up here that we get back

responses including the date that the

server responded and a whole bunch of

other details as well and honestly this

has always been under your fingertips

and it will soon be useful as we start

making web-based applications ultimately

but this very quickly gets overwhelming

quickly and so better in this might

actually be a tool that we can use

within our code space itself so let me

go back to vs code here I didn't open

any code tabs I'm just going to use my

terminal window for a moment and I'm

going to run a couple of commands that

are going to allow me to actually see

what is going on when I request one

website let me go ahead and use a

command called curl for connect URL and

this is like a command line black and

white program that's going to pretend to

be a browser and it's going to connect

to the URL show me the headers but it's

not going to show me the images are the

graphics which might very well be useful

to the humans but not to me right now as

the developer so I'm going to do curl

I'm going to do Dashi and then I'm going

to do

https www.harvard.edu as though I'm

pretending to be a browser requesting

the home page and what's nice about curl

is albeit overwhelming too you'll get

back a whole response from the server

containing only those header values the

key value pairs inside of the envelope

and we'll ignore almost all of these but

here is the response from the server it

responded using a new and improved

version of HTTP in this case version two

and it gave me back a 200 there's my

content type text/html and then this

Char set happens to do with the encoding

if it's Unicode or asky or something

else and then there's all this other

overwhelming detail for now but this is

the beginnings of my ability to just

kind of poke around and see how the

server works and it turns out too that

we'll be able to see other potential

responses as well so for instance uh

HTTP might not only return 100 what if I

do this instead let me go ahead and

visit c-i

HTTP

colon uh www.harvard.edu so notice I

deliberately use the insecure version of

the URL which maybe Harvard's

Administration system administration

doesn't like anymore well how can they

ensure that I the end user the student

nonetheless use

https even if I didn't type it myself

well let me run just that command with

just HTTP not https and you'll see that

everything is not okay it didn't come

back with a 200 it came back with 301 in

this message saying Harvard moved

permanently but here's where you can

look for another clue among all of these

lines most of which I don't care about

there's a location header colon that's a

little hint to me that says where

Harvard University has apparently moved

too on the web and what's different

about this URL just to be

clear it has the S included and what

your browser will do by default because

Google and Microsoft and Mozilla

programmed it this way whenever it sees

a 301 response instead of 200 it won't

show you any web page it will look for a

location header find that URL and then

automatically quote unquote redirect you

there to so this is why it doesn't

matter what we type in the browser

Harvard can have its server send these

semi-secret messages to our browsers and

then it will just visit a second URL all

automatically and you can do this with

host names as well suppose that Harvard

does not want to standardize on

harvard.edu why they just want it to

always be www maybe it's a branding

thing maybe it's a technical thing we

can see the exact same response here

this first tells me when I visit HTTP

harvard.edu with no www Harvard

minimally wants me to be using a secure

connection if I then okay fine cooperate

let me go ahead and clear my screen let

me add the s but not the www you can see

here that it again responded with 301 up

here and the location now adds the www

so it's just a way of bouncing users

from one place to another and this is

all thanks to http boiling down to

relatively simple messages inside of the

envelope that tell the computer the

browser in this case how to respond now

odds are you've seen others besides 301

even though you've probably never seen

that actual number unless you've done

this kind of thing before but there

probably is a number that like everyone

in this room has seen if if You' never

really wondered why is it that number I

think you're smiling what number are you

thinking of yeah so 404 why is 404 well

404 indicates by convention not found

and now why the world decided years ago

to show us normal humans on the internet

404 is anything significant is unclear

that's sort of like bad design like what

do I care if the status code is 404 but

it's common enough on the internet that

probably all of us have seen it but that

just means that some server when you

visit a URL that's incorrect maybe it's

outdated the URL has been changed if you

see a 404 it just means that the virtual

envelope that came from the server back

to your Mac or PC or phone contains not

200 okay not 301 moved permanently but

404 not found instead and it's usually

accompanied by a technical message maybe

a cute picture of a cat sort of hiding

because it means not found or something

like that the Aesthetics are entirely up

to the server but that's what the 404

means and there's other codes too a few

of which you'll use in the coming weeks

as we transition from commandline

programs in C and python to web-based

ones we've seen a few of these already

and several of them are kind of variants

of the same thing like these 300 ones

here but we'll see others like 403

Forbidden means like you probably forgot

to log in or you need to log in or

something like that uh internal server

error right now no in the coming weeks

you will absolutely encounter 500 errors

and they're always your fault it's going

to be the equivalent of like a

segmentation fault in C a 500 error

means like you or I screwed up when

writing some code so we're going to see

that but it's just going to be an

opportunity for us to fix it if a

server's overloaded you often see 503

like something's unavailable because

something's too popular uh or is maybe

worse getting attacked um this is an old

uh um April Fool's joke 418 is not

actually used in practice but someone

like took the time to write up an entire

formal text technical proposal so that

servers can respond saying I'm a teapot

so it's kind of part of Internet lore

and there's other ones of these status

codes as well but this is useful because

eventually we'll see in code you can use

this understanding uh high level as it

might be of HTTP to do some interesting

and Powerful things so for instance we

can even send fancier URLs to servers it

turns out as we'll soon see if you send

a message like this get/ search question

question mark Q equals cats and then

HTTP 1.1 or whatever version and you

send that message to Google server

www.google.com this is how you can

specify not just the path of a web page

that you want SL search in this case the

question mark it turns out is going to

be a convention in the internet in the

web specifically for passing hum human

uh user input to the server as well in

fact you've probably never paid close

attention to URLs but they very often

have question question marks they very

often have equal signs and indeed even

google.com supports a certain key Q in

this case for query and you can put

anything you want after that in order to

search for actual cats so if I actually

go back to Chrome itself here for a

moment let me pull back my uh pull back

open my Chrome browser here previously I

was using uh incognito mode for

harvard.edu I've gone ahead and Clos

that window and opened a new one so we

can start fresh by visiting Google

normally you and I are in the habit of

going to google.com and searching via

the form or nowadays you just type like

your search query in the browser itself

and it brings you automatically to

Google or Bing or something else but I

can really be pedantic here let me go

ahead and zoom in and I'll manually go

to

https

www.google.com search question mark Q

equals maybe cats now this would not be

a very userfriendly experience if all of

us had to manually type out something

crazy like that but that's what the form

is redirecting you to when you type in

more user friendly cats into like a text

box if I hit enter here we'll get back

indeed a whole bunch of search results

about cats if I zoom back in and maybe I

change it from cats to dogs that too is

going to change and notice it's

pre-populating the text box because

Google has written its code in order to

do so as well now appr propo of the

video with which we began today from

yester year one of the better uh Yale

pranks over the years um has anyone one

actually ever been to uh

safetyschool.org

and to our friends at Yale watching live

hi safetyschool.org so it's kind of fun

if you actually visit it uh depending on

who you are so if I open up a new window

here and I go to

https

safetyschool.org zooming

in enter my oh my look at where it goes

now how does that okay okay thank

you this is not a cs50 thing someone out

there who I don't know who they are for

like 20 years has been paying annually

for that domain safetyschool.org for

just this joke uh but if I now go back

into VSS code here in my terminal window

and a little more you know

sophisticatedly let me do curl dasi HTTP

col andsafety

school.org the browser of course

immediately redirected me to the website

but what's going on under underneath the

hood well 20 some years ago someone

bought the domain configured the server

quite simply to spit this out safety

school.org for years has moved

permanently to

www.yale.edu so just a little

demonstration of what you can do with

just a little bit of understanding of

HTTP that's been a lot already we've

laid the foundation for understanding

how the internet works now we're going

to use it with a language called HTML

hypertext markup language CSS cascading

stylesheets and JavaScript a proper

program in language but first it feels

opportune to take a 10-minute break we

have some wonderful Halloween candy in

the transcept and we'll be back in 10

with those

languages we're headed up to uh

Austin checking out the stadium for the

prank a few years ago I was at a math

conference and I was sitting around at

the table at dinner with a few other

mathematicians and uh one of them went

to Harvard and started telling the story

of this amazing prank that was you know

against Harvard and at that point I felt

I had to interrupt and said well

actually I can tell you a lot more about

that okay okay the idea was perfected in

a dorm room came up with the idea

actually to prank them with signs at the

football game we threw some ideas out

there as far as what what the signs

would say we uh eventually settled on we

suck and my immediate reaction was no

this will never work however the problem

solver in me started thinking well maybe

we can make this

work the problem they had to infiltrate

Harvard Stadium without getting caught

sneak in 1,800 placards distribute them

to unsuspecting Harvard fans and then

convince those fans to prank themselves

it's great we thought about basically

every possible thing that could go wrong

and tried to come up with a solution for

it and then you put two Reds on top of

it they made fake Harvard IDs and fake

backstories

fake placard designs and a 28 member

fake Pep

Squad on November 20th 2004 a fake

Harvard student smuggled the placards

into the game what do you think

[Music]

[Applause]

[Applause]

of but then trouble what houses

how many how many extra

are I you know just showed him the front

of this ID and all of a sudden he just

ran away and he felt so

embarrassed having escaped one

confrontation they couldn't risk another

it was

time this just looks like a total mess

we have absolutely no idea if this is

going to work look at the it's going to

happen it's actually going to

happen I can't believe this what was

once a

prank became a

legend and immediately we started

hearing chance from the other side

you and I think it was at that point in

time that we knew we had pulled it

off on

there it goes

[Applause]

again I really think it didn't matter

that Harvard won because of the prank

for a lot of Yale students in alumni we

definitely won that

[Music]

year all right

happy

Hallen thank you all okay thank you oh

here oh all right little Halloween candy

all right thank you all so glad I wore

the same thing

today so in just a

moment thank you okay so in just a

moment we'll transition to understanding

all the now what we can do with this

underlying infrastructure so again HTTP

and below it TCP IP is all about just

getting the data from point A to pointb

in some standardized way but now let's

talk about HTML this is the language in

which web pages themselves are written

hyper text markup language now some of

you might have used this before to make

personal homepages some of you might

have dabbled even if using some website

to create your own homepage but

understanding this language is useful

certainly for creating the Aesthetics of

a web page can vying data that's of

interest but at the end of the day it

and the language we look at next CSS uh

are not programming languages there's

going to be no functions no Loops uh no

programming logic but we will end today

with a teaser of a proper programming

language called JavaScript via which you

can manipulate all of these various uh

other languages as well so ultimately

HTML has like two features and this is a

language that we spend very brief amount

of time on because it really boils down

to just a couple of basic ideas and then

vocabulary that you'll build out over

time just by Googling looking up

references looking at other Pages source

code but tags and attributes are what

characterize HTML now what do I mean by

that here for instance is the HTML code

via which you can make probably the

simplest of all web pages one that quite

simply says in the uh browser window

hello title and hello body for instance

now what does this actually mean if you

imagine opening up uh this code in a

browser be it on a Mac or PC or phone

you'll see typically like some kind of

rectangular window and there's usually a

tab that has the title of that page and

then most of the rectangular region is

the web page itself what you're looking

at then is the code that's going to put

hello title in the title bar in the tab

at the very top and down at the bottom

hello body is going to be all that's in

the big black and white box that

composes the rest of the browser window

itself now what are the Salient

characteristics here that we'll now

start to take for granted well first

whoops uh first let's go ahead and give

me just a moment here um and actually do

something with this code so I'm going to

go ahead and do this back in vs code

here I'm going to first create a file

called say hello.html and in this tab

I'm going to go ahead and really repeat

exactly that same code now I had this

line first DOC type HTML then I had this

line HTML Lang equals quote unquote n

close quote then I had inside of that

head then I had inside of that title

then inside of that I had hello title

and I'm doing this quickly because we'll

tease apart in a moment what it actually

all means and then down here below that

so-called head I had just the text hello

body so at the moment I that I claim is

the entirety of a web page but it

currently lives in my code space so to

speak in a file called hello.html that's

fine if I want to create it but how do I

how do you how does anyone on the

internet actually view it well to serve

a web page you indeed need a web server

and it turns out that codespaces comes

with one of these pre-installed because

we cs50 staff uh did so for you and what

you can do in a terminal window once you

have an HTML file ready to go that you

want the world to see you can literally

run in your terminal window

http-server single command and what

that's going to do for you is start a

web server that is to say a program

whose purpose in life is just to serve

web pages and even though probably up

until now for years you probably if

you're like me equate server quote

unquote with a physical device server is

really aie piece of software it just

tends to run on big fancy devices so

when we say server we often all think of

in our Mind's Eye you know big expensive

devices perhaps but a server is just a

program whose purpose in life is to

respond to requests with responses and

that's the vernacular there now once you

run HTTP server and I'm going to do a

bit of magic because I set this up

before class just to make sure it goes

smoothly you'll see some output like

this whereby your server is now

available on a very long URL mine here

uh uh is a very long URL that will be

different from yours but what this is is

a unique identifier that your codes

space has temporarily generated so that

you can now access and ideally only you

can access that file using your browser

now if I flip the URL or you flip the

your all to public by right clicking or

control clicking the right features of

vs code you can enable anyone in the

world to visit it but we're not going to

ultimately host our websites in your

code space because as soon as you log

off for the night and the thing shuts

down like the website will go down but

at the end end of the semester

particularly for final projects we'll

show you ways that you can put your own

website your own code on the actual

internet

247365 even with your own domain name if

you want to get one so that it lives uh

independent of your own sleep schedule

and usage schedule of vs code here so

I'm going to go ahead now and visit um

this URL in another tab of my browser

and what I'll see here is this this is

the output of that program called HTTP

server and essentially what it is doing

is it's using TCP and IP in conjunction

with HTTP to just run your very own web

server on gith hubs own servers as well

and that's because of different ports

again we won't go too much into the

weeds of the TCP the IP and all of that

stuff but recall that different port

numbers can allow you to distinguish one

service from another now one of the

services is of course your code space

VSS code in the cloud that we've been

using for weeks but if you want to use

the same physical server that GitHub

controls but actually visit your own web

server that I just ran in my terminal

window in another tab that's fine

they're just going to be using different

TCP ports and you and I don't have to

care what they are but just that this is

a feature that TCP supports so what you

see here is somewhat Arcane this is not

like a thing that most people on the

internet should ever see I'm just doing

this for development purposes but this

is the index that is the directory the

the folder contents of my code space and

because I deleted everything from prior

weeks already all we see right now is

hello.html

which I just created so if I click on

hello.html within this folder listing

you'll be a little underwhelmed and I'll

zoom in just so there's something more

interesting there but now you see hello

comma body but what's interesting

perhaps is that after this long very

cryptic and uninteresting URL notice at

the very end of it and I'll zoom in in a

moment what do you see slh hello.html

which follows the convention I claimed

before break is how a browser would

allow you to to access a specific file

on a server by doing slash and then the

name of the file name everything before

it is very cryptic it would be better if

I you know uh buy a domain name that's a

little more easy to remember and set

that up some other time but for now

let's just focus on only the file names

that I'm actually creating all right so

the code is up and running underwhelming

though it might be with the body in the

middle of the page and let me zoom in up

here too hello comma title is indeed in

the tab just as promised so what's

actually going on with code like this

that we just created well let's go back

to the slide version of the same and let

me just highlight a few of these lines

the very first line is what's called

your document type declaration doesn't

really matter to remember that by

phrasing and this is just something you

copy paste or do from memory at the top

of any HTML file that you create when

making your own web page it's a implicit

indicator to the browser that you're

using the very latest version of HTML

which is version five you don't mention

the number five just browsers now

nowadays are program to look for this to

know that you're using the very latest

version of the language languages just

like human languages evolve over time

we're up to version five of HTML but new

features get added every few years so

indeed this lecture this class has been

evolving over time too so let's now

focus on the next line as well as the

bottom line and you'll notice some

deliberate symmetry here this here is

what we're going to call a tag and it's

technically different from this this is

a document type declaration it's got the

weird exclamation point that's the only

anomaly everything else follows pattern

this is a tag in HTML and it's the HTML

tag and a tag generally both starts and

stops or opens and closes at some point

so this is the so-called start tag or

open tag and this just means essentially

to the browser hey browser here comes

some HTML the language in which web

pages are written this here with the

forward slash after the angled bracket

means hey browser that's it for the HTML

of this page so that's what I mean by

symmetry I started a thought here

finished it down here what's in between

those two thoughts well here browser or

rather let me clarify one thing this

thing here is that other uh keyword an

attribute an attribute is something that

modifies the behavior of a tag so it's

similar to an argument in C or in Python

like a parameter to a function these

aren't functions but it's the same idea

it just modifies the default behavior of

something instead Lang equals you can

probably guess just means that hey

browser assume that everything Hereafter

is in English and that might be useful

for like Google translate or just search

engine optimization so that just the

server the browser know like what human

language you have actual content in like

hello title hello body even though a

good computer can probably infer from

Context often all right so that's an

attribute that's a tag and the whole

thing here everything in between the

start tag and end tag we would also call

an HTML element that just means

everything related to that open and

close tag all right now notice indented

inside of so to speak the HTML open and

close tag are another pair of tags the

head tag and the body tag or the head

element collectively and the body

element collectively and same idea hey

browser here comes the head of my page

hey browser that's it for the head hey

browser here comes the body of my page

hey browser that's it for the body the

head is essentially the tiny little

strip at the very top including the tab

itself the body is like 95% of

everything else the big rectangular

region what's inside of your title at

the at of your head of the web page at

the moment just the title so this

indicates hey browser Here Comes My

Title hey browser that's it for the

title the title of course is literally

quote unquote hello comma title

meanwhile if we bounce back out here is

the uh second element inside of the HTML

tag uh this says Hey browser here comes

the body hey browser that's it for the

body and hey browser this is the

contents these are the contents of the

body itself now the indentation is a

stylistic thing I did it just to be sort

of neat and en TIY because it suggests

what is inside of what but it also

suggests a sort of hierarchy and in fact

we'll use terminology from like the

world of family trees if this is like a

parent so to speak head and body would

be the child elements of the HTML tag

meanwhile title is a child of the head

tag or equivalently tital is a

grandchild of HTML so you can use the

same sort of vernacular as in the human

world when it comes to uh familial

relationships too and that just H set

again the same hierarchy so we have tags

and they include HTML head title body

and that's it for now we have attributes

we've seen one example of them Lang but

we'll see many other examples of the

same idea but these building blocks are

exactly the same generally you start a

thought you finish a thought and you

might do something in between questions

on this basic structure of any web

page any questions at

all no all right so let's now now allow

things to ramp up a little more

interestingly and do something with

these building blocks but so that you

have a mental model for everything

that's going on here on after think of

this same HTML being related in spirit

to week five when we talked all about

data structures if I really wanted to I

could take to heart this idea of

children and parents and grandparents

and really depict this thing graphically

and in fact this tree here if you will

and it's not a binary tree it's not a

binary search tree it's just a tree uh

used week five's terminology if this

special node here represents the whole

document well the root element as I

called it is HTML HTML has two children

head and body the head tag has in turn a

title uh child and in turn has some text

just as the body has some text and so

this is what your browser is doing you

and I the programmers write this stuff

the browser reads this code top to

bottom left to right whenever you visit

a website and inside of the computer's

memory Chrome Edge Firefox Safari what

what not they build this data structure

in the computer's memory so as to know

what it is you have told them to do and

we'll see over time at the end of today

you can write code in an actual

programming language JavaScript to maybe

dynamically add or remove things from

this tree and this is how things like

Gmail work when you open up your Gmail

inbox if you're a Gmail user if you just

stay there long enough you'll probably

get more and more mail and what happens

you don't have to like reload the page

or rebuild the tree per se it just all

of a sudden appears at the top

at the top at the top what's happening

there is that Google wrote some code

that just keeps adding more nodes to

this tree every time they realize you've

got a new message again and again so

that's the relationship now even with

this world of HTML with all of the

programmatic ideas we looked at in the

past all right so let's go ahead and do

something with this that's a little more

interesting than just hello itself I'm

going to go ahead and hide my terminal

window because the server is now running

and all I want to do now is experiment

with uh hello and other examples as well

let me go ahead and actually before I do

that let me go ahead and run uh code of

paragraphs. HTML just so I can keep my

code separate and now I'll hide the

terminal window again um paragraphs.

HTML I'm going to do almost exactly the

same let me go ahead and start with

something familiar and eventually I'll

start copying and pasting just to save

time so doc type HTML is always there

open the HTML tag and now notice I

didn't type the rest of that just like

with C just like with python we try to

save you some keystrokes by closing

parentheses adding quotes the HTML

support in VSS code is pretty good too

and it tries to finish your thought when

it comes to tags as well it can screw

things up if you if it does something

you don't want it to do so sometimes you

have to delete but it's just

autocomplete as we've seen before uh

let's go ahead and let me add Lang

equals as all of my examples today will

be let's add the head tag let's go and

proactively add the body tag and now

let's go ahead and give this a title tag

uh which has a I'll just call this

paragraphs just so I remember which

example is what now notice all of this

white space and all of this neat and

tidy indentation the browser ultimately

is not going to care about this is just

for us humans to kind of keep ourselves

saying when we look at the code it's

just easier to read but strictly

speaking I could minimally delete all of

this white space and I could just move

all of this tag up to the same line both

I think are fine I'll just going to

follow a certain convention but this too

would have the exact same meaning but

we'll see where that detail about whites

space could potentially get us into

trouble later in my paragraphs tag let's

do this in advance I've written up some

Latin like text a really long paragraph

of Latin like text like this it's

actually random nonsense it's not real

Latin even though a couple of the words

might look familiar and so here we have

three paragraphs of text and I've

deliberately hit enter in between them

so that just like an essay in Google

Docs or Microsoft Word hopefully I'll

see three separate paragraphs let me now

change tabs and I'll close hello.html

from before I'm going to go back to my

other tab here I'm going to click back

to go back to that index of all of my

files which I started at earlier and

you'll see now that I have two files

because I obviously just created a

second file called paragraphs. HTML so

let's click on this to see our three

paragraphs of Latin like text and voila

I'll zoom out all right first bug if you

will this just looks like one massive

blob of text not three Blobs of text and

why might that be borrowing the the hint

I offered a moment

ago why are we not seeing break yeah so

we need some kind of line breaks here

because the browser turns out is only

going to take us literally and if you

just give it text text text it's just

going to show you text and anytime

there's more than a single white space

whether it's two or 20 or 200 it's going

to just assume that you did that just to

be neat and tidy and it's going to

collapse them into just one space

visually like this so there are in fact

a couple of solutions one is this here I

could add some explicit line breaks and

it turns out that there's a br tag like

this and just for uh visibility sake let

me do two of them so like hitting enter

enter on my keyboard I'll do it here too

BR for break break and now let me go

back to my other tab nothing's changed

yet but that's because I have to reload

I've changed it on the server but now I

need to change it in the browser by

reloading and now it looks a little

better albeit nonsensical but you'll

know is a curiosity per this BR tag this

is kind of poorly designed it's a little

hackish to just say enter enter and make

the browser do this breaks line breaks

don't actually require Clos tags or end

tags so not all tags need to be closed

at least those that it just makes no

semantic sense to close them right like

the break is there or it's not you can't

imagine like starting to move to the

next line and then eventually getting

around to finishing like it's either

there or it's not so some tags do not

have closed tags as necessary but

there's a more elegant way here I dare

say not just sort of hackish putting in

these line breaks let me do this instead

I'll delete those and let me go ahead

and as the name of this file suggests

let me add a paragraph tag now here I

need to fight with vs codes autocomplete

because I don't want to finish the

sentence uh the thought there let me go

ahead and open the paragraph tag and

close a paragraph tag and just to keep

things tidy I'll go ahead and indent two

even though the indentation itself

doesn't matter let me go ahead and

create another tag for opening this

paragraph and I'll close this one here

and now let me see sometimes it's

fighting with my auto complete but

that's fine because I did this sort of

the wrong way at first and now let me go

ahead and finish this thought by closing

this paragraph tag here and I've

manually fixed all of my indentation so

now on line 10 I have the equivalent of

hey browser start a paragraph and then

it does the Latin like text then on line

12 hey browser that's it for this

paragraph and repeat repeat repeat if I

now go back to my other Tab and reload

again shouldn't be all that different

but semantically it's a little bit

better why because just saying break

break doesn't really mean anything but

by saying paragraph paragraph paragraph

now there's some more semantic

information there now if like Google is

analyzing your page or if the programmer

is trying to understand what it is you

did in the past when writing this code

you just know semantically oh this is a

paragraph This is a paragraph This is a

paragraph just like in a book or or an

essay so it's a little more clear

focusing more on what it is not how you

want to display it any questions then on

these

paragraphs now all right so a few more

tags and indeed these first few examples

will really just be like sort of bang

bang bang just a bunch of different

vocabulary words in the form of these

new tags but we won't go through the

entire laundry list of tags this is

indeed the thing for which web

references and books and the like are

ultimately helpful just like a

dictionary in the real world so I'm

going to go ahead and do this let me go

ahead and copy this let me create a new

file called headings. HTML just so we

have a new file for this to save time

I'm just going to paste that exact same

code just to get me started I'm going to

change the title of it for clarity for

the code online to headings and now just

like a book or an essay or a thesis let

me actually put some actual headings

here now if my first heading like

chapter one I could do something like

this up here I could have a paragraph

like I just learned and I could say

something like chapter one here but

that's not really a paragraph and so

it's sort of better designed to tell the

browser and really tell the world what

it is so it turns out there's another

tag I can use like H1 for heading and

like most important heading and in here

I'm just going to keep it simple and I'm

going to say something like one and in

fact this is so short here's a good

candidate for just keeping this all on

the same line but this has no functional

difference but it'll just make it a

little tur more tur on the screen now

let me go ahead down here and I could

have multiple headings so H1 2 and down

here I could have another one H1

three and if I go back to my other tab I

reload it now we should see just like a

book or an essay now we have some

proper now we have some oops I have to

go to the right file sorry if I go back

to the index now we see the new third

file called headings. HTML and now we

indeed see some fairly pretty if simple

headings as well now if these aren't

three chapters 1 two three but maybe

it's a chapter then a section then a

subsection such that just visually you

want things to get smaller and smaller

well those exist too and in fact you can

do H1 through H6 H1 a little

paradoxically is the biggest and boldest

H6 is the smallest but still bold so it

might make sense to make this H2 both

open and close and maybe this H3 open

and close if again this is a section or

a subsection inside of that chapter if I

reload now notice just gets a little

smaller so it's more similar to what

you'd see on the printed page but this

now is just another three tags that I

might use in my own code all right well

how about lists of things I have three

paragraphs here but let's do this let me

go back to vs code I'm going to copy

this code so I have a starting point I'm

going to create a new file called say

list. HTML here I'm going to copy paste

I'm going to change my title to be list

just for clarity and in here I'm going

to go ahead and get rid of this whole

body because let's move away from these

massive paragraphs and keep it simpler

for now if I want to have a list of

things uh for instance uh if you haven't

seen these already a computer scientist

when they're fishing for just some

arbitrary meaningless words uh they

often use Foo bar and baz just as their

go-to just like a mathematician might

use XY Z for variables so Fu bar and baz

are on three separate lines and maybe

this is like my to-do list or my

shopping list but you can probably

imagine if I go back to my other tab go

back to the index I now see my new file

list.

HTML but it's probably going to look

wrong I think I'm just going to say yeah

Fubar baz all in one breath if you will

the not on new line and you can try to

fight this like you can be like really

want to put some line breaks there go

back and reload it's still not going to

make any change how do I want to fix

this well I can do this in a few ways I

could make them paragraphs but they're

not really paragraphs they're a list so

I'm going to use a different tag instead

I'm going to create for instance an

unordered list using the UL tag open and

close inside of that I'm going to use

the list item tag Li I and I'm going to

say Foo inside of another tag I'm going

to say bar inside of a third tag open

and close I'm going to say baz so it's

getting a little verbose but it's still

relatively succinct Li is all you need

type for list item UL is all you need

type for unordered list so there's some

shorthand syntax here that's adopted if

I now reload you're going to see a

so-called unordered list like not sorted

which means by convention to show it as

bullets though it could be displayed in

different ways visually as well if you

actually change your mind and you

realize oh I'd really like to number

this well you could obviously like just

add one and two and three but that's

going to get annoying especially if the

list grows you want to change something

insert something in the middle then you

have to reorder it I mean we using

computers here they can do this for us

so we can change the UL to any

guesses o maybe for ordered list which

is sort of the opposite here so let's

try changing that to O let me go back to

my browser and I'm just hitting command

r or controlr to reload the page instead

of clicking the button every time now I

automatically get 1 2 and three and you

can even override the Aesthetics using

different numera bles or symbology

instead but that would be perhaps the

most common there as well all right it's

a lot of tags quickly but any questions

on lists paragraphs headings or the like

no all right so let me go ahead and

propose this here um let's go ahead and

create what we'll call a table so let me

copy and paste this into a new file code

of table. HTML and in table. HTML let me

again rename the title to table let me

get rid of that ordered list from before

and let me now use a table tag open and

close this one's a little weird but

inside of a table you typically have a

head of the table so uh I'll say well

let's say the first row we'll keep this

one simple a table row or TR inside of a

table row you would ideally have columns

but that's not the nomenclature instead

you have data so TD for table data and

let me go ahead and just have the first

datm be one I'm just going to

arbitrarily do 1 two three just so we

have something to play with and you know

what just for demonstration sake I am

going to deliberately copy paste this

twice and I'm just going to manually

change the numbers just so we can see

what I'm creating 789 and then maybe

just for good measure if you're seeing

where this is going let me copy this one

more time and give myself a final row

with an asterisk a zero and a pound sign

if maybe you see where this is going let

me go back to my other tab let me go

back to the index there's my new file

table. HTML I'll click that and while

it's not very pretty I'll zoom in it's

indeed a table of data I happen to mimic

like a key uh a telephone keypad but you

could imagine this being much juicier

much more interesting scientific or

financial data or the like laid out into

these rows TRS and these columns AKA

table data as well so we have that

ability as well for structured data now

of course the internet um has lots of

images on it and in fact this is all

just text how can we introduce images

well let me go ahead and do this let me

first uh sort of semi secretly copy an

image file that I brought from uh

earlier just so we have something to

play with and I have in my account here

now an image called harbor. JPEG and I

uploaded this semi- secretly a second

ago into my account so that I can

reference a second file let me go ahead

and copy this HTML just to save myself

some keystrokes

let me go ahead and do code of image.

HTML and let me paste that code and hide

my terminal window I'm going to get rid

of all of this table as just

uninteresting now we're going to make

this even simpler by changing my title

to image to keep all these

demonstrations uh separate and now if I

want to make a web page that when

visited shows us a picture of Harvard

well there's an image tag abbreviated

IMG for short I can specify what the

source of that image is and if my file a

JPEG in this case is literally in the

same folder I can just say quote unquote

harvard. jpeg if it's in a folder I

should mention the folder name in a

slash or something like that if the

image is on the internet somewhere with

a URL I could also have a whole URL

https colon and then the URL of the

image but I upload it in in advance now

this is just going to visually display

on the screen but not everyone of course

can see images screen readers might need

a bit of assistance and even search

engines might want to analyze the page

and know what this is an image of now

machine learning and artificial

intelligence are maybe getting better to

be fair at figuring out just by

analyzing images what they are but

they're certainly imperfect I am a human

I know pretty well what I took a photo

of for instance so maybe what I should

do proactively which would be good for

accessibility is have this alt tag for

alternative text and then literally say

like Harvard University so that someone

who can't see or so that a a server can

actually know with higher probability

what it is uh they're looking at and I

could be even more detailed than just a

phrase I could describe the image as

well all right let me go back to my

index in the second tab let me go back

and zoom back out there's my new file

and there's my new JPEG that I quickly

uploaded before I can click now on

image. HTML and albeit a little

overwhelming that is a really big image

of harvor now apparently it's too big to

fit on the screen so this isn't the best

user experience to have to scroll up

okay so there's the image horrible

horrible design if you will at least in

terms of my code but there's going to be

ways where I can sort of rein that in

and affect the height or the width as

well but for now it's just deliberately

a little overwhelming instead now we can

do something a little more fun and

topical today uh which might be to use

an IM a video instead so let me go ahead

here and very quickly grab another file

today which is uh I brought in advance

and that you might have seen briefly

earlier which is an MP4 an actual video

file and what I'm going to do here by

revealing vs code again is I'm going to

code a file called video. HTML get

another demonstration here I'm going to

change my title to video just to keep

these things straight and instead of the

image tag you might imagine using now

indeed a video tag and this is a

relatively newer tag that has increasing

support among browsers so it's good now

to use and inside of this the syntax is

a little different you specify and this

is weirdly annoyingly in consistent not

SRC for Source you literally say source

and then in Source you use a source

attribute horrible design semantically

but like this is what we're stuck with

halloween. MP4 is the name of the video

we uploaded in advance made by some of

harder Harvard's digital artists and the

type of this video so that the browser

knows for sure is video/ MP4 that's a

so-called content type that you just

know or you look up to figure it out and

just so that this is as animated as

possible I'm going to tell the video tag

with a few attributes to autoplay and it

turns out that attributes often have key

value pairs whereby it's the key the

attribute name equals quote unquote some

value just like Lang equals quote

unquote for English but not all

attributes need values in fact if you

read the documentation for html's video

tag there's an autoplay attribute where

you can literally just say the key and

it needs no value it's just going to

mean autoplay and if you don't want to

autoplay you just omit it alt together

so so you don't necessarily need a value

on or off uh I want the thing to Loop

just so it keeps going I want it to be

muted so that we don't hear any sound in

fact there is no sound but browsers

nowadays for anti-spam and advertising

reasons often will not play a video if

it has sound because it's just kind of

obnoxious if you visit a page and all of

a sudden your speakers start blaring so

I know this from having read up on this

that I should mute it so if I want it to

actually autoplay for real and then I'll

set the width manually for now to be

like 128 pixels across just from some

trial and error earlier and that with

attribute does have a value now I'm

being a little um a little uptight here

by alphabetizing all of my attributes

not at all necessary I do it just so I

can skim things faster and know if

something is there or not so for me it's

just a matter of style let me go back to

my other tab go back to my index and

you'll see two new files again the mp4

file and video. HTML I'll click on the

ladder and if I did this well here we

have thank thanks to our friends in

Harvard U our artistic friends at

Harvard very like an oo would help with

the drama here but okay but we have a

very dramatic nice Halloween type view

here as well so we have videos embedded

as well and suffice it to say there's

ways to embed YouTube videos or Vimeo or

other services as well using yet more

tags too but the web is of course all

about hyperlink Hyper text markup

language where you click on something

and you end up somewhere else and this

is how the web is so powerfully

interconnected

so how do we start creating links from

one website or web page to another

either that I made or someone else well

let me go ahead and open back up my

terminal window and let's create a file

called link. HTML just to demonstrate

what you and I know as a link I'll hide

my terminal window now let me copy paste

just to save myself some keystrokes and

let me get rid of the video tag so we

can focus now on links suppose that I

want you to visit Harvard virtually well

I could say something like visit uh

Harvard period this is uninteresting

because it's just going to be text I

probably want you to actually visit

harvard.edu instead more specifically

and I'll lower case it just to be

consistent with what browsers do in the

address bar all right let me go now to

the video uh back to this video Tab and

go back where we now see my index I'll

Zoom back in and there's link. HTML

unfortunately when I click this and I'll

zoom in you literally just see the text

that I wrote and yet on every social

media platform nowadays except like

Instagram when you type a URL or what

looks like a URL even if you didn't

bother with the HTTP or https it usually

automatically links it for you on

Facebook on Twitter and other sites as

well that's just a convenience Discord

and slack do that too but they're just

doing it to make things more

userfriendly but they have to generate

HTML with the proper tags and attributes

so to get this to actually work it's not

even good enough to say

https www.harvard.edu because if I go

back now and reload now you'll literally

just see all of that as text if you want

the browser to treat this as a link you

need to use the anchor tag it'd be great

if it were called the link tag but it's

not it's called The Anchor tag or a for

short and the way you reference the URL

to which you want to lead the user is

via href for hyper reference this is one

of the earliest tags perhaps among the

most Arcane now but if I then put that

whole URL in quotes and close my tag I

now have to opportunity to finish my

thought in between the start tag and the

end tag for this anchor and what I put

in between the start and end tag is

whatever the human's going to see so

here I can say Harvard I can go back to

my other tab I can reload the page and

now you see the familiar blue underline

this now is an actual link and if I

click it I'll be whisked away to the

actual Harvard websites but there's a

risk here can anyone imagine pretty

simply after like what 60 seconds of the

link tag of the anchor tag how could

someone an adversary misuse this tag

alone How could a website run by an

adversary How could a spammer misuse

this tag do you think

yeah yeah absolutely you could have it

say one thing but lead another lead

elsewhere so I could say Yale in here

nothing stopping me as the developer go

back to the page reload now it says

visit Yale you click on Yale and voila

you end up applying to the wrong place

instead now there's some hints of this I

could hover over this and super small

like this isn't very good for your

anti-hacking techniques but way down

here you can actually see the URL that

it's going to go to in most browsers

indeed do this at least on desktops and

laptops so it's a little bit of a hint

but what you're seeing here even though

this is kind of a silly um uh playful

example this is exactly how fishing

attacks work p i sh iing work whereby an

adversary tells you to log into your

PayPal account but it doesn't go to

paypal.com it goes to some other random

website that they bought and built that

then tries to collect your username and

password and stored in their database so

now they can log into your PayPal

account as you and it boils down to that

simple primitive and you can be even

more manipulative to you can even say

the whole URL for Yale like yale.edu or

Worse htps www.yale.edu reload that and

now who I mean who among you and people

in your lives are necessarily going to

be so paranoid as to not just blindly

click on that URL this is why just being

a defensive real world person nowadays

digitally is just ever more so important

so these same things that can be used

for good or uh benign use cases can also

be used for ill purposes too and it is

literally that simple questions now on

any of these tags thus

far just a few more to offer up any

questions on this

here no well let me open up a couple

that I brought in advance just so we

don't have to type all of them here um

if you for instance have a web page

that's got quite a bit of code let me go

ahead and grab from the website a couple

of examples real fast here namely one

that we'll call how about uh meta

HTML and in this example here give me

just a moment to full screen it we're

going to

have a file So codem Meta HTML I'll open

this up next no relationship to what we

now know as meta the company but rather

this is going to be a page that I copied

and pasted the same chunk of Latin like

text from earlier so it's going to be a

really big paragraph of text and this is

an example where if you were to open

this web page not on my own Mac or your

PC but on your phone the font might

actually be really annoying and

difficult to read why because your

phone's going to try to squeeze all of

the content onto the tiny viewport the

rectangular region of your phone instead

so it turns out there are ways pretty

easy ways to make your website mobile

friendly as well Otherwise Known

technically as responsive and the

easiest way to do this is to include

this tag here a meta tag again no

relationship to Facebook this has been

here much longer and this case here this

meta tag online five has its own sort of

approach to key value pairs this is a

good example of where it'd be nice if it

looked just like everything else but

this is what we have historically

you can have a meta tag with an

attribute called name that refers to the

name of some feature of the browser in

this case viewport is the technical term

for like the big rectangular region to

which I keep referring the body really

of your page the content for the

viewport you can say some AR esoteric

details like this the initial scale

should be one that is no matter who

visits your site it shouldn't start

zoomed in it shouldn't start zoomed out

it should start at just the default

sizing and then this here with equals

device width is a very arcane way of

saying if the user has a small screen

show the text proportional to that size

don't just try to cram it all into a

tiny little window so it's super simple

but if for the next problem set or

future projects as well you find that

just things look really bad on mobile

like this kind of tag is the place to

start meta there aren't terribly many of

these that you'll use but they're useful

for other mechanisms as well in fact let

me go ahead and semi secretly pull up

one other example as well whereby I'm

going to grab another example that uses

more of these tags and in just a moment

I'll reveal it here give me just one

second here I'll have I'll propose that

in this

example of

meta we now add these properties instead

so I copy pasted this from an existing

file just so as to not waste time typing

all of these out if you've ever shared a

URL on Facebook or Twitter or slack or

Discord order any number of websites

nowadays that automatically show a nice

preview where you sudden instantly see

like the default image of the page maybe

a few sentences or words of text or

something like that sometimes those

applications those websites will just

choose like the first image it finds on

the website or the first sentence it

sees and show that but that might not be

very user friendly or search engine

friendly and so a prog a developer might

want to control what it is that slack

Discord Facebook Twitter and other such

sites show by default for that can use

nowadays what are called open graph tags

which is to say there's other uses of

the meta tag and you just look these

things up even I had to look this up to

remember what the key value pairs are

the meta tag can also have a property

attribute that can be these very

specific strings OG title OG description

OG image which denotes open graph which

again is this standard that's evolved in

recent years and what you can do here is

tell browsers and in turn servers what

you want them to show as the default

title of the page P the description of

the page and even the default image just

so you can exercise more control when

sharing things socially nowadays as well

again it just boils down to these key

value pairs this is absolutely the kind

of thing you look up as needed to cross

check but those capabilities are there

and so literally the next time you paste

a link into slack or Discord or any

online site that then displays it in

embedded fashion just know that all this

time a little bit of textual code like

this in HTML has been there by whoever

authored the site

all right let's do one final example in

HTML alone before we transition to just

cleaning up the Aesthetics and improving

the visuals of everything we've been

creating let me go ahead here and close

meta HTML let me code up a new file

called how about search. HTML and see if

we can't draw some inspiration from our

cursory understanding earlier of how

URLs work to see if we can't reinvent

google.com itself so recall that a

canonical URL might look like this here

and in particular if you want to pass in

user input to that URL again you can

potentially have a question mark and

then a key equals value pair or for that

matter you can even have multiple key

value pairs that by convention are

separated by ampers sense these things

are everywhere like later today when you

pull up almost any website in your

browser look at the URL and you'll just

see a lot of this a lot of noise too and

distractions but there's going to be

some equal signs most likely Andor some

ampersands as well and those are just

separating key value pairs now what can

I do here well if you think back to how

we manually searched for cats earlier

let me quickly do this I'll do this one

manually doc type HTML as my very first

line HTML tag with how about my Lang

attribute for English up here and then

inside of this I'll have a head tag

inside of this I'll have a title I'll

call this example uh search and then

down here I'll have my beginning of a

body tag and now let me introduce you to

really a final tag for now uh a form tag

which will create a web form the thing

with text boxes and buttons that you and

I use every day on any number of

websites inside of this form I'm going

to have an input like a text box whose

name is going to be Q for query because

I'm trying to re-implement Google here

uh the uh type of that I want to be a

text box or if I know I'm using this for

search I can actually change this to a

search box and it's going to let me it's

going to generally put a little X there

so you can clear it quickly that's a

nice little enhancement as well and then

I'm going to give myself a submit button

by doing input whoops I'm going to give

myself a submit button by doing input

type equals submit and then I'll leave

that as such here all right now I need

to do a little bit more but let's see

how this looks let me go over to my

other tab let me go back to my index and

if I zoom

out there is search. HTML I'll click it

and there's not much going on here even

if I zoom in but I do indeed have a

really big text box and a submit button

but I haven't in my HTML told anyone

anywhere that I want this input whether

I type cat or dog to go to google.com so

for that I need a couple of more

attributes and I know this from having

done this before and any online

reference will say the same you can add

an action attribute like what do you

want the action of this form to be and

you can put the URL to which you want

this form to be submitted and I know

from tinkering that it should be hdps

www.google.com search I don't need to

put any question marks here myself but I

do want the uh browser to do that for me

so let me go back to my other tab let me

reload and nothing visually has happened

but watch this when I now type in cats

but before I hit enter notice that I'm

currently at some long crazy URL search.

HTML is expected if I now go down to the

submit button and click submit watch

what happens to the URL and the page

itself I'm whisked away to the actual

google.com and indeed there are those

same cats and if I zoom in here you'll

see that my URL has changed to be indeed

SL search question mark Q equals cats so

this is just how web forms work when you

submit any form on the web in this way

the browser automatically goes to that

action URL adds a question mark puts any

key value pairs that you manually typed

into the text boxes and lets the server

do its thing now here's where Chrome is

starting to simplify things Safari does

this too if you double click on your url

now you see the full URL but if any

parts are missing that's just a UI thing

to eliminate visual distractions

nowadays meanwhile if I go back to my

own form if I search this time for dogs

and hit enter now again the URL changes

to be Q equals dogs and it all reduces

to this basic building block of using an

a form tag now I can be more explicit if

I know I want to use get it which is

actually the default I can literally say

quote unquote get in all lowercase even

though the verb earlier was by Design in

uppercase but here now I'm just being

ever more explicit if I don't want the

label of this button to be very

generically submit maybe I want it to be

Google search quote unquote well if you

read the documentation for forms you can

actually change the value of the button

to be quote unquote Google search and if

I now go back here and reload I get a

fresh form and now I get a button that

literally says Google Search and if I

tinker with this further because this

isn't very user friendly there's even

more attributes I can do I can add on my

t uh search input a uh autoc complete

equals uh off if I don't want to see my

own history for whatever reason I don't

want people knowing I'm searching for

cats and dogs on this page I can

autofocus on the text box so that it

shows the cursor blinking in that box by

default and I can even do something like

this I can have a placeholder attribute

that says something like query or some

other documentation for the user and if

I now go back and reload you'll see

notice it says query and it's subtle but

my cursor is already positioned there it

gave it focus and I can type cats now

without having to click in the Box

manually which is just marginally better

for the user's experience any questions

now on all of this

here any

questions all right that too was a lot

why don't we take a casual five minute

break and when we resume we'll take a

look at CSS add in some JavaScript and

then wrap up so 5 minutes only for now

all right we are back so that's

technically it for HTML like here on out

it'll be up to like online resources and

references we point you to just to fill

in your vocabulary for more tags and

attributes but like conceptually that's

it there's attributes uh there are tags

and there are attributes and the rest of

it really is just kind of a laundry list

of capable uh possible features but it

turns out too you'll see over time that

you can even see the HTML for websit so

for instance if I go over to harvard.edu

in my browser which I'll go ahead in

just a moment here here and do here and

do

https

www.harvard.edu

enter it again will pull up today's

version of Harvard's website and if I

rightclick or control click on it again

and go to inspect you'll see those

so-called developer tools and earlier we

focused on the network tab just so we

could see the HTTP stuff going on but

what I glossed over earlier was the

so-called elements tab which actually

shows you the HTML underlying any web

page on the internet and so for instance

here is the underlying HTML for

Harvard's homepage as of right now and

aesthetically some of it's been

collapsed so if I click on these various

triangles I'll see what's actually

inside of that is the children of some

of these HTML tags but here on out if

you're ever curious as to like how a web

page uh made some feature visually you

can just literally use these developer

tools built into your own browser just

to see what the uh web developer

actually did and you can do things too

like this like if you really like maybe

uh let's see if you really like this

menu in the top right hand corner of

Harvard's website you can even

rightclick that or control click that

specifically choose inspect and what

browsers will do is jump to the HTML

corresponding to that visual element on

the page and here you can see though

we've not talked about this tag before

there's a button tag there's an ID

attribute and there's some other

attributes as well that Define that

button um you can do other things too in

the web page let me scroll down for

instance here and let's go actually

let's go to another one like yale.edu

here in today's theme and suppose we

want to do something like uh change the

Aesthetics of this website well let's do

how about this over here life at Yale

let's rightclick on this choose inspect

that's going to jump to that part of the

page and notice what you can do here in

this elements tab we can be a little a

little playful in return today life at

Harvard and voila we've now changed

Yale's website it would seem so have we

really like hopefully hacking is not

actually this

easy what did we actually do based on

today's mental

model like I have changed the page

but yeah just changed how it is for me

right because my browser just like with

Phyllis and Brian from the GetGo

requested Yale's web page I got back a

virtual envelope containing that HTML as

we've now called it my browser has a

local copy it's got its own tree

otherwise known as a Dom document object

model built up in its memory and yeah I

went to town and changed my copy of it

but of course hopefully I've not changed

the actual server and in fact if I

reload Yale's website now hopefully it

will revert back to indeed yep what it

should be instead life at Yale but this

ability in your own browser be it Chrome

or Firefox or Edge or Safari to have

these built-in developer tools are very

powerful because it's going to enable

you to not only diagnose problems that

will invariably arise in the coming

weeks with your own code but is also

going to allow you to learn from other

sites like how you can do things and

Tinker as well but up until now we

focused only on tags and attributes and

on the structure of a web page let's now

focus more on the Aesthetics and

fine-tuning that it turns out that HTML

has very limited support for anything

aesthetic like font sizes and colors and

so forth and in recent years people have

used necessarily a second language

called CSS not a programming language

again a markup language if you will to

just fine-tune the Aesthetics of a page

font sizes colors margins and all of

that so CSS is going to allow us to

define a whole bunch of properties which

is just another group of people's

terminology for key value pairs indeed

ever since week five like key value

pairs are everywhere in the world not

just at like sweet green and restaurants

but indeed under in line code and

languages and Technologies like these so

properties is the new word in CSS for

what a moment ago we called attributes

in H but it's the same idea just

different vocabulary that you get used

to over time a few phrases I might use

now and you'll hear in the coming days

would be these type selector class

selector ID selector attribute selector

which just refer to different techniques

we're about to see that are going to

allow you to control more precisely the

Aesthetics of specific things on the

page and the way we're going to do this

is we're going to take our basic HTML

like we saw earlier and we're going to

introduce in the next few minutes just a

couple of more tags and or attributes

one we're going to introduce you to a

tag called style which nicely named

allows you to um allows you to control

the style the Aesthetics the visuals of

the web page or we're going to introduce

you to a link tag which very confusingly

does not give you a link that you can

click on it just links to another file

that then gets automatically included or

imported to borrow our language from C

or in Python but same idea this will

allow us to include secondary files and

we're going to ultimately show you how

you can leverage third third party

Frameworks libraries that other people

wrote so as to not get stuck in the

weeds of all the fine tuning of

Aesthetics and just make pretty things

fast so you can focus really on the

intellectually interesting part if

that's your choice of building the

content the site out the application out

yourself all right so with that said let

me go back to vs code here and let me go

ahead and create a simple example called

home.html like a very simple homepage

for John Harvard for instance let me

give myself three paragraphs initially

the first of which is just going to have

uh the person name the second paragraph

is going to say something like welcome

to my

homepage uh to greet visitors and the

third is going to be like a little

footer like copyright uh how about John

Harvard 1636 or something like that all

right let me go back to my other tab as

before reload my index there's my new

file home.html and I'll click that and

you'll see okay I mean this is sort of

1636 style web page super simple all

text nothing really interesting going on

there

but we can start to style it a little

differently like if the title of the

page is John Harvard and then it's

welcome to my homepage and then this

less important footer why don't we have

the text be large then medium then small

so something arbitrary but a little more

nuanced so let me go back to vs code

here and in my home.html file let me

introduce not yet the style tag but what

I'm going to call temporarily the style

attribute both indeed exist this one's

simpler and it's going to be correct but

we'll see in a moment not as well

designed arguably as is often our

narrative so inside of the style tag you

can put this language called CSS key

value pairs otherwise known as

properties the only way you know what

properties exist what keys exist is by

taking a class reading a book looking at

an online reference and we're going to

give you just a sampling of what's out

there so suppose I want to control the

font size of this first paragraph I can

literally say font Das size in all lower

case colon and then a word like large or

I can specify 12p point or 18 point or

something more precise like that like

from Google Docs or Microsoft Word and

suppose I want to make this text down

here uh medium well uh I'll do quote

unquote font size colon medium and down

here I'll do style equals uh font Das

size small so I'm going to start with

just these three key value pairs same

key but different values I'll go back to

my page and in a moment I'll reload and

it's going to be somewhat subtle but

watch how the font size do change when I

reload now all right so got a little

bigger middle one's about the same and

the last one is a little smaller what if

I want to center it just like many web

pages have the text like this centered

well I can separate these key value

pairs with semicolons and I'm sorry

semicolons are kind of sort of back with

CSS but I can do text-align Colon Center

strictly speaking I don't need the last

semicolon if there's no more key value

pairs but I'll just do it to be

consistent uh text align Colon Center

and then down here after another

semicolon text align Colon Center all

right let's go back reload now it's

going to be much more obvious the change

and we now have the beginnings of a

homepage still pretty basic but at least

it's a little more interesting turns out

we can do a little better with the

copyright symbol like most computers

actually have support for a circle with

a c in it but you can't just do that

with uh text like this there's different

ways to do this you could copy paste it

from like a website that already has it

so you don't have to figure out the mag

iCal keystroke on your Mac or PC but

there's also in HTML what are called

entities and you can actually specify

using heximal or decimal codes numbers

like this H1 169 semicolon after an

Amper sand and this is a special symbol

that you can look up in any online

reference for like special characters

that are hard or impossible to type

manually at your keyboard and this let

me zoom in just so it's obvious if I

reload now instead of being two

parentheses and a c character

now it's a proper Copyright symbol so

you'll see these out there they're not

necessarily that frequently used

nowadays but it's good to know that they

exist but let me go back now to my code

and propose that while correct uh this

is arguably not very welld designed and

even if you've never seen HTML never

seen CSS before what Instinct might you

have for why this is poorly

designed

yeah there's repetition right in general

in the past several weeks see python SQL

like repetition generally bad and and

sloppy and it's not going to scale well

so the repetition I think you're

probably alluding to is textalign Center

textalign Center textalign Center well

we can factor that out in CSS the C in

CSS means cascading and this means that

if you move some properties to like a

parent or a grandparent uh the children

or grandchildren will inherit those

properties that is they will Cascade

down the family tree so to speak and so

so let me go ahead and remove all of

these since I claim they're indeed

redundant and let me preserve just one

of them by well let me do this it's not

quite right to put a a paragraph inside

of a paragraph like that's just not a

thing in English writing or in writing

more generally so I'm going to do one

thing first it turns out that these two

are arguably not paragraphs right this

is like a header the body the essence of

the page and then the footer so if a

paragraph isn't quite the right English

semantics you can actually use more

generically a tag that's all over the

internet called div for division of the

page and this is just a very generic

term for like a big rectangular region

that divides the page again and again

just so that you can think about

different regions now that I have div

which really has no more meaning than

that it's a division of the page

interpret as you will now I can have

multiple ones of these and let me go

ahead and open a div tag here let me

close a new div tag here and then just

to keep everything tidy I'm going to

highlight everything in between and hit

Tab and that just automatically indents

everything for me now I have a three

divs inside of another div and that's

totally fine this is very commonly done

now I'm going to do this style equals

quote unquote text align Colon Center

semicolon or not and now I have some

cascading capabilities now the parent of

those three children John Harvard welome

to my homepage and the copyright will

now all inherit that property so when I

hit reload nothing aesthetically has

changed whoops sorry

um I should have done reload slightly

earlier when you use a div instead of a

paragraph it actually gets rid of the

par uh the space between those

paragraphs it just sandwiches them a

little closer together I can fix this in

another way but that aside everything is

still centered and the text is still

large medium and small but I should have

called out that change in the paragraph

spacing but we could bring that back

before long if we wanted now what more

could I do to maybe improve this well

strictly speaking I don't really need

that parent div right because these

three divs inside already had a parent

so let me actually get rid of that new

div just undo what I did I'll highlight

this and if you haven't seen this trick

shift tab will unindent nicely which is

perhaps helpful I could just put that

text align Center on the body tag so

text align Colon Center quote unquote

this two would work as well so long as

you go up the family tree so to speak

reload and now indeed there's nothing

aesthetically that has changed this time

but it turns out nowadays the web is

getting a little more sophisticated and

even though you will see so many

examples online and tutorials and books

using div div div div all over the place

there are newer semantic tags semantic

just means they have more meaning than

this generic notion of a division and if

you look up the a documentation for HTML

you'll see that if you want to have a

header on a page not a heading like H1

H2 but a header there's literally

nowadays a header tag and this is

marginally better because it now says

what it is search engines like Google

and Bing can detect oh that's the header

of the page maybe we should use this and

give it more prominence in the search

results you can then have a main part of

the page so literally a tag called main

nowadays you can literally have a footer

of the page and again these are often

useful for screen readers to help recite

things verbally for folks who might

otherwise not be able to read them and

probably these screen readers might

highlight the header and the main part

but maybe not might not spend time for

the user on the footer which is arguably

a little less important semantically

usually um or search engines again now

know what's the header what's the footer

what's the main part of the page so they

know what to search and analyze so this

would arguably be a a better design

nowadays as

well but what else remains as a problem

well this is now getting a little bit

more subtle and takes some experience

but this practice of putting HTML and

CSS all in the same file it's a little

sloppy why because it means I'm

co-mingling my data with the

presentation thereof like the juicy

stuff I care about like John Harvard and

the phrase welcome to my homepage and

all of the Aesthetics that I might want

to change over time and honestly because

everything is currently in one big file

it's going to make it really hard for me

to collaborate with a classmate or a

colleague at work so that maybe I do the

HTML they do the CSS like uhuh not if

you're all working in the same file it

would be a nightmare even if you use vs

code sharing feature like Google Docs

and both are typing at the same time

like you're going to mess up somehow

it'd be nice if we could separate these

two languages well one way to do that

would be as follows let me get rid of

all of the style tags sorry style

attributes that I've added up until

now on all four now of these tags and

let me introduce the style tag that we

saw on the slide earlier instead I'm

going to go up here into the head of the

page which is where technically these

style tags must go so that they're

already loaded into memory before the

body is even analyzed by the browser and

inside of the the style tag I'm actually

going to select the HTML elements that I

want to stylize if you will so if I want

to change the body's Aesthetics I'm

going to literally type the name of that

tag body and then I'm sorry curly braces

are back also from c u inside of these

curly braces I'm going to put text align

Center so the key value pairs are the

same the only new thing I've done is

I've moved some of the syntax up to this

new style tag in the head if I want to

now control the header tag as well I can

use the same curly braces this is

convention to put the open curly brace

on the same line the closed curly brace

on another the browser doesn't really

care but this is a common CSS style

convention I'm going to do font size

large semicolon then for the main tag

I'm going to do font size medium and

then for the footer tag I'm going to do

font size small so same exact thing and

it's admittedly a little bit more

verbose it's taking up more lines of

code it doesn't all quite fit on the

screen but if you scroll back down now

and you'll acquire an eye for this this

is just better like it's just more

compact it's more readable the the

content the data jumps out and there's

no visual distractions like the CSS

properties as before upside here too is

that we don't actually need to uh this

doesn't actually change the Aesthetics

if I reload the same page it still looks

the same but I've taken a step toward

some slightly better design but let me

propose that there's other ways to do

this too we just selected things by way

of their uh type so that was a so-called

type selector when I literally just

specifi the type of tag body header main

footer but there's other ways that now

we can lay the foundation for making

reusable CSS that you and colleagues and

classmates can use and reuse in multiple

files and even in multiple projects so

let me actually go ahead and do this

instead of just very explicitly saying I

want the body to be centered let me

invent an adjective if you will and let

me change this to do centered and this

new uh this new vocabulary word centered

will literally mean texal Center let me

go ahead here and I'm just going to

create a new adjective called large a

new adjective called medium and a new

adjective called small they are

deliberately consistent with what the

properties do but these are now my own

vocabulary words and they are called

classes so a class is just a collection

of key value

a collection of properties that you get

to invent for yourself and what it lets

you do now is this now if I want the

whole body to be centered I can add this

tag which we actually saw briefly in

Yale's HTML class equals centered down

here in the header if I want this to be

large I can say class equals quote

unquote large down here on Main I can

say class equals quote unquote medium

and down here I can have class equals

quote unquote small now I have taken one

step backward by read addding some of

the Aesthetics to the page but it's not

the actual properties it's not the key

value pairs it's now more semantically

nice because now I just know from

reading the HTML what these things are

going to look like whereas the

implementation details for all four of

those adjectives is now relegated up

above and these are literally my words I

could change it to Fu and use class

equals quote unquote Foo but obviously

that would not be the best choice of

words in this case all right any

questions on this this now is what we

would call a class selector by using

literally the dot even though the dot

does not appear elsewhere but dot means

this is a

class these are not always the best

syntactic design decisions that the

world makes all right well one last

trick then notice that this is a little

Annoying that I'm still working in the

same file and if my classmate wants to

clean up my Aesthetics make my homepage

look way better if my colleague wants to

do the same wouldn't it be nice if we

could actually move all of this code to

a different function file like a python

library or a c header file well you can

let me go ahead and delete that whole

style tag let me add a confusingly named

link tag the href of which let's call a

new file styles.css and let's say that

the relationship of that file is that of

stylesheet so this is a term of Art in

the world of web development a

stylesheet is a text file that contains

lots of styles lots of CSS properties

let me open my terminal real fast

and let me do code of

styles.css

enter and in this file I'm going to

paste all of those same lines as earlier

but now they're in a separate file and

indeed if I hide my terminal window and

I give this file to a colleague they can

now work on the Aesthetics of the page

and make things a lot prettier than this

maybe use specific font sizes maybe add

colors and the like whereas I can focus

entirely on the HTML because this file

now will reference that other and if I

go back to my other Tab and reload Cod

the content's going to be exactly the

same but now I'm using some separate

file

instead any questions now about these

techniques

here no all right so with that said let

me show just one example now of what I

called a moment ago Frameworks and this

is where web development gets kind of

fun at least if you like this especially

if you like the sort of logical design

the presentation of information you care

about but you really don't want to

struggle with like font sizes and colors

and getting everything Pixel Perfect so

to speak let me propose that I open up

here an example in just a moment in vs

code that I prepared in advance and this

one is going to be an opportunity to

consider how you might take some of the

data from last week which we had wherein

we collected everyone's favorites and

lay it out in a really big HTML table so

I wrote this out in advance because it

was a huge amount of data but it's the

same data from the Google form from last

week and you'll see already the hints of

a table tag and these TRS and I added a

few other tags for Aesthetics it turns

out when you have a more visually

interesting header for your table

there's another tag called T head

there's another tag called T body these

are not all that intellectually

interesting I just read the

documentation and realized oh to make

things prettier I need a t head a t body

and so forth but what's interesting here

is that if I go to my index here and

open this file called favorites. HTML

here is all of the data from last week's

Google spreadsheet which we exported as

CS and I manually before class converted

to just HTML it's indeed a table but

it's really not pretty like the columns

are really close together it's kind of

hard to distinguish one row from another

but this is just raw HTML written by me

now I could use CSS and some of the

tricks we just saw to maybe change font

size there's ways to change color

background color and a lot of things

like that but honestly other surely

other people in the world have presented

tabular data in pretty ways right I've

been to many websites that have prettier

tables than M can I maybe use someone

else's framework someone else's CSS

include it in my page but then stand on

their shoulders and just make my stuff

look prettier well I dare say I can let

me go ahead here and semi secretly open

up vs code again and let me grab a

slightly different version of favorites.

HTML that I also opened in advance

wherein I add this line of code instead

give me just a moment to foreground this

version and the data is is all the same

as before but I've added one of these

link tags and I'm not linking to my own

styles.css I'm using a popular Library

called bootstrap and bootstrap is just

one of many popular libraries out there

free at that that has a whole bunch of

CSS files and soon JavaScript files that

you can just use for free in your own

projects personally or professionally

that just make things look and behave

better without you having to reinvent

Wheels now to access their CSS I had to

read their documentation and grab this

very long URL here but it's the same

idea link a forre equals quote unquote

something and I read their documentation

and they told me to add this they told

me that if I want my tables to be

prettier I have to add a class attribute

to my own table tag and specify a little

weirdly but this is what bootstrap told

me to do a class called table and that

will make it a prettier bootstrap table

and if I want to stripe it like every

other row is gray instead of white just

to make it pop a little more visually I

can also add a second class separated by

a space called table striped that's all

I did I added line five and I changed

line nine and that is it the rest of the

hundreds of lines in favorites. HTML are

the same but if I go back here now and

reload the browser now thanks to

bootstrap voila like it's much prettier

now I can zoom out and that changes the

font size just locally for me and even

if you don't love their Aesthetics I

mean this is easily better than my own

there and it turns out we can can do

even better by adding interactivity to

this too but to do that we're going to

need one final language for today and

this one is an actual programming

language and we won't use it all that

much in cs50 but we introduce it here as

we begin web stuff because there's just

so many free libraries and professional

libraries that you can use just to make

your web applications fancier and more

interactive mobile applications as well

increasingly use HTML CSS and JavaScript

to power our iPhones and Android devices

as well so a quick tour some syntax and

then we'll conclude with just some

hopefully inspiring examples to give you

a taste of what JavaScript can do so

JavaScript supports conditionals just

like C and python before it if we rewind

to our scratch days here of course is a

conditional here is the corresponding

JavaScript code as of today it's pretty

much identical to see with the syntax

here uh if we had an if an if else in

scratch it looked like this in

JavaScript it's going to look like this

instead so it's a bit of a regression V

ofv python like the the parentheses are

back the curly braces are back the

semicolons I mentioned in CSS are also

back in JavaScript potentially but it's

familiar is the point here and it's a

different language that's frequently

used for the web whereas you can't use

python in the ways we're about to use

JavaScript it just wasn't designed for

that purpose meanwhile if you have an if

El it's else if else in scratch well in

JavaScript just like in C it's going to

look like this instead variables in

JavaScript of course are a thing too and

in scratch we might have initialized a

counter variable to Z in JavaScript a

few different ways to do this and just

the for now the keyword is let it's sort

of a polite way of asking for a variable

let uh counter equal zero semicolon so

you don't mention the type but you do

use a keyword here in this case called

let if you want to increment counter by

one few different ways in JavaScript you

can do this just like in C in JavaScript

you can do this just like in C and in

Python in JavaScript you can also get

this so plus plus is back so maybe that

counterbalances the other syntax as well

that was was not the case in Python

Loops are back of course in JavaScript

whereas in scratch you could repeat

three times like this in JavaScript it's

pretty much just like C the only

difference here is that you say let

instead of int for an example like this

meanwhile if you want to do something

forever in scratch in JavaScript just

like in C you say while true in this

case so this is to say we're sort of

comfortable spending relatively little

time on JavaScript at least for today's

purposes because syntactically it's

really the same as we've seen before

with maybe a slight variance here or

there but what's interesting today

arguably is just what kinds of things

you can do with it so with that said

what kinds of things can we do it's all

comes back to this picture if this is a

simple web page on the left and this is

the corresponding tree or Dom document

object model on the right that is the

tree the browser automatically creates

in memory or RAM for you JavaScript is

now a proper programming language that

lets us dynamically manipul ulate like

read data from this change this and this

is how Google for instance implements

your inbox they might have uh in your

inbox it's like a table so TR TR TR TR

probably something like that or heck

maybe div div div div using JavaScript

anytime they realize someone sent you

new mail they can create a new node a

new rectangle in memory and you the

human see a new div or a new TR again

and again and again so with JavaScript

you just have the ability to control the

user's experience and instead of like

I've been doing constantly hitting

reload in the page to see some new

content to see some new content

JavaScript can be running 247 so that

you can actually see all of these

changes live all right let's go about

writing some JavaScript code now instead

of writing it on the server and

executing it on the server we're going

to actually use a very common Paradigm

whereas JavaScript is actually executed

in the browser client side that is to

say we can actually start writing some

JavaScript code inside of our own HTML

file so that when a user visits that web

page with their browser not only is the

HTML and any CSS downloaded to the

user's browser so is that JavaScript

code so that it's executed indeed client

side on the browser rather than server

side as has been the case with python in

previous weeks well where do we go about

writing some JavaScript code let's go

ahead and revisit hello.html which

previously was a completely static

example that literally just said hello

title hello body indeed if I open this

up using HTTP server and view it now in

a separate tab all it said was exactly

that hello title in the tab and hello

body in the main part of the viewport so

to speak well let's make this example a

little more Dynamic so it doesn't just

say hello body but maybe says hello to

an actual person so let's go ahead and

do this let's go ahead and remove the

hardcoded hello body and let's actually

go ahead here and use a form tag but

we're not going to use this form in the

usual way whereby the data gets sent all

the way back to the server we're going

to Leverage control over this form

client side instead so I'm going to go

ahead and create this open form tag

close form tag inside of that let me

give myself a text input that's going to

have autocomplete equals quote unquote

off just to ensure that what I

previously type in my examples doesn't

reappear accidentally we're going to

autofocus it so that the cursor is

blinking right there in that text box

and this time I'm going to go ahead and

give it how about a uh placeholder of

quote unquote name to make clear that

prompting the user for their name and

then the type of this text box will be

the default or more explicitly here text

and then I'm going to have as we've seen

before a button the type of which is

submit also our typical default and then

inside of this button is going to be the

label how about something like uh how

about we'll call this greet so that's

what the button will actually say well

let me actually go back into my browser

tab let me reload this page and we

should now see a relatively simple form

whereby I have the cursor blinking on a

text inp input prompting the user for

their name and then a greet button that

I can click but if I click this button

now it's not going to do anything useful

because I haven't written any code to

tell the browser what to do when I click

that button but it turns out there's all

sorts of events in the world of

JavaScript that you can listen for so to

speak in fact here's just a list of some

of them anytime something changes in a

form field anytime the user clicks or

drags on something anytime the user

presses a key and maybe lifts their

finger up anytime the mouse goes down or

over or up on top of something or

anytime a form is submitted those are

events in the same way that we talked

about events back in week zero in

scratch and in JavaScript just like in

scratch where you can do something when

green flag clicked in JavaScript you can

write code that actually listens for any

of these events or more so with that

said let's go back to vs code here and

let's make a couple of changes instead

let's go ahead and add to this form a

new attribute that's not the best way to

do it but it's perhaps the simplest for

version one here and let's say onsubmit

do the following so onsubmit is an HTML

attribute and curiously its value inside

of the quotes there can actually be some

JavaScript code and let's go ahead now

and let's assume there exists a function

in the world called greet and what I

want to do is call that function right

then and there well now in JavaScript

how do I go about making that function

exist it doesn't come out of the box

just like print might or say might in

python or scratch respectively but I can

do this let me go up into the head of

this page inside of a script tag here

both open and close let me actually

write some JavaScript code and just so

it stands out I'm going to give myself a

couple of blank lines though not

strictly necessary and let me Define a

new function in JavaScript called greet

and this is the syntax in JavaScript for

creating your own function similar in

Python instead of saying defa in

JavaScript you just say function then

the name of the function and any

arguments within the parentheses

thereafter but I'm not going to pass in

any here then inside of curly braces

what I'm going to do is use a built-in

JavaScript function that comes with any

browser called alert it's not the best

or prettiest user interface but for now

it's going to get the job done what do I

want to say to the user well let's first

just say something simple like hello

comma World close quote semicolon

thereby alerting the user with precisely

that message now what I'm going to do

down here is make one other change I

don't want this form to actually get

submitted to the server just like we've

seen in the past when you submit a form

it often goes to something like

google.com/ search I actually want my

JavaScript code to take control over the

entire user experience of this form so

that I'm just using the form as a user

input mechanism not to actually send via

get or post this data to some other

server including my own so this is going

to look a little ugly but after calling

greet I'm actually going to do this

which I've read the documentation about

and I know that if you actually hardcode

return false here that just tells the

browser please don't actually submit the

form only call the Greet function all

right well let me go back to my browser

here let me reload this because I need

to download the latest version of the

JavaScript code and I'm just going to go

ahead without even typing my name I'm

going to click on the Greet button and

you'll see that Al be it a little

cryptically at the top we see an alert

that says hello world there's my ugly

URL of my code space there at the moment

but we do indeed see that string but

what I haven't of course done is taken

any actual name from the user so how can

we go about doing that well ideally I

want to alert the user with hello comma

David or hello comma Carter whatever a

name I type into that box so how can I

go about doing that well let me create a

variable called name and let me set it

equal to this function call

document. query

selector that comes with JavaScript

itself let me then in parenthesis pass

in an argument that is going to be huh

the ID the unique I need a unique

identifier for the thing I want to

select so let me actually go back to my

HTML code here and instead of giving

this form field a name like Q for query

let me actually use another HTML

attribute called ID where now I can call

this anything I want and for clarity I'm

just going to call this input element

uniquely name now up here in query

selector just like in CSS where you can

use hashes and dots and other symbology

in order to select certain nodes in your

Dom that is rectangles in that memory

tree well I can go ahead and select hash

name which again is just the Syntax for

uniquely selecting the element whose ID

is in this case name so you have the

hash up here you don't need the hash as

the value of the attribute down here on

line 20 and now if I want to actually

get the value of that text box I

literally just say do value so document

refers to the whole web page itself

query selector is a function that's

built into that object so to speak and

the value accessible via value just like

a c stru or even a python class allows

me to go inside of that text field and

get whatever the value the user has

typed in now as I've uh been able to do

in languages like python pretty Le

fairly readily I can concatenate this

name onto the string hello comma space

so as to form a complete phrase and

you'll notice here that I'm actually

using single quotes in my JavaScript

double quotes in my HTML this is perhaps

a common convention in JavaScript the

language does not care if you use double

quotes or single quotes but I dare say

single quotes are just more common and

so that's what I've done here all right

now as always I'm going to cross my

fingers go back to this page I'm going

to reload because I've changed the

JavaScript and I need my browser to

download it and now I'm going to type in

my name for instance David click greets

with fingers crossed and voila now I see

hello comma David all right so it turns

out that while functional this isn't the

best design and co-mingling your uh HTML

with your JavaScript script code AS with

this onsubmit attribute isn't

particularly clean it's better as with

CSS to keep your HTML over here your CSS

over here and your JavaScript now over

here so to speak and better still

perhaps even in some separate files so

how can I go about changing this a

little bit well let me go ahead and

actually let's go ahead and delete all

of this code for just a moment and let

me go and get rid of this on submit

Handler down here and really just

distill my HTML only into the HTML and

the attributes therefore and what I'm

instead going to do now is do this I can

use JavaScript to achieve the listening

for that submit event or that onsubmit

event I don't need to actually use HTML

for that I can use JavaScript entirely

so it turns out I can access some other

uh member of this document by doing

document. query selector again but this

time let's select the actual form tag

and it doesn't have an ID because it has

no ID in its HTML but it does have a tag

name so just like in CSS when you can

Target Elements by way of their name I'm

just going to select the one and only

form on this page by using that same

query selector function and now I'm

going to use another function that just

comes with JavaScript in the context of

browsers whereby once you select an

element like that form I can call add

event listener which is similar in

spirit to scratches when green flag

clicked or any block like that you can

then tell the browser what event you

want to listen for I want to listen for

the submit event so you don't say

onsubmit here now that we're in pure

JavaScript you just say submit and now I

can do something like this I can go

ahead and say call the following

function and I'm not even going to

bother giving this function a name and

that is allowed to in JavaScript as we

saw briefly in Python and what I'm going

to do now inside of curly braces after

that keyword function is the same kind

of code as before I'm going to do let

name equals document. query selector I'm

going to select that same IDE uh same

Name ID as before and get its value and

then I'm going to do alert and then pass

in hello comma a single quote again

after that concatenate with that the

name and then semicolon but I need to do

one other thing it turns out that this

function and if you read the

documentation for this technique

actually takes automatically a special

argument called by convention event and

this is just an a variable if you will

that refers to whatever event just

happened in this case it's of course

going to be submit but in other contexts

it might be a click event a mouse down

event or something else entirely so this

allows me now to do this which is a

little cryptic but you get used to these

conventions I can use that event

whatever it is and then prevent whatever

the default behavior is by calling a

special function called prevent default

with a capital D this is the alternative

to that Messier return fall semicolon

that I had inside of my HTML before so

all I've done here now is I've left all

of my HTML as pure HTML down here and

I've put all of my JavaScript code as

pure JavaScript up here this sort of

separation of concerns similar to what

we started doing with CSS just a bit ago

in order to keep those two languages

separate too well let me go back to my

browser here reload the page and

unfortunately there's a subtle mistake

I've made here let me go ahead and type

in David and click greet and

unfortunately nothing actually seems to

happen well maybe it's just my name

Carter greet and nothing seems to happen

that alert does not come up well why is

that well let me go back to vs code here

and point out that order of operations

in zscript matters similar in spirit to

C because on line seven I'm selecting

the form and trying to add an event

listener for submitting that form

unfortunately the form had better exist

at that moment in time but it doesn't

because just like in C and in some cases

python where the compiler or The

Interpreter reads the code top to bottom

notice that the form doesn't actually

exist and therefore get loaded into the

computer's memory until line 19 so we've

got to kind of reorder these somehow now

maybe the simplest way to do this would

just be to perhaps do something like

this let me scroll back up to my script

tag and perhaps a little more explicitly

move it into the order in which I want

it to be executed so I'm going to go

below my form and inside of my body

which is actually okay for JavaScript

here and just use that same code and

assuming I didn't make any typos let's

go back to the browser click reload

again to get the latest typee in my name

again using that purely JavaScript

solution and the only change I made was

I move the code from up here to down

here clicking greet now and wow it's now

back we get the alert with hello comma

David so those kinds of things those

kinds of principles matter at least when

we're back in this world but there's

other Solutions too and just so that

you've seen it because it's a common

convention in libraries as well let me

undo that change and put that script tag

back in the head or really anywhere else

in the page where it might be

and let me propose that there's one

other way to solve this problem to

postpone that code on line 7 through 11

getting executed until really the whole

Dom the tree is ready to go and the

Syntax for this might be as follows I

can do document and I can add to the

document a an event listener that's

going to listen for something a little

special and I always have to look this

up myself to remember the spelling and

the capitalization but it turns out that

the browser itself once it's done

loading all of your HTML top to bottom

left right it will raise an event called

Dom content loaded capitalized exactly

as such and if you want to call some

function and I don't even need an event

argument in this case you can open curly

braces just as before and put inside of

those curly braces the code that you

want to execute only once the dumbs

content has been loaded top to bottom

and now let me just finish my thought

with a closed curly brace Clos

parenthesis and semicolon it gets a

little Annoying to visually line all of

this up but I think I'm still good and

now even though this code is at the top

of my file or really above the form tag

itself I think we're okay so let's go

back to the browser here reload the page

type in David and click greet and we

still get the same correct behavior and

so this is just a very common Paradigm

to use these kinds of events to listen

and listen and listen for something to

happen and then only do something once

that thing has transpired all right well

let's take one more step with JavaScript

code before we take a look at what's

really fun about this language and what

you can do with browsers in particular

by just cleaning things up a little bit

further I'm going to go back into the

code here and I'm actually going to

remove uh or cut all of this code out of

the hello.html file itself and I'm going

to change my script tag to have nothing

in between the open and close tag but I

am going to give it a source attribute

and let's go ahead and call this for

instance hello.js sojs would be the

convention for the file extension for a

Javascript file and even though this is

a little weird that we have the script

tag and a source attribute then nothing

in between the open and close tag this

is indeed the convention when you want

to put all of your code in a separate

file and let me go ahead and do that let

me go ahead and open my terminal window

create a new file called

hello.js and then in that file I'm just

going to paste the very code that I just

cut from the previous file so no changes

to the code all I'm doing is factoring

it out and now I'm doing something just

like our CSS factorization before which

confusingly use the link tag this uses

the script tag this just now allows me

to collaborate with someone like Carter

or someone else so that they can do the

JavaScript code I can do the HTML maybe

a third person can do the CSS and indeed

maybe we can build even grander things

by uh designing things in this way all

right well let me go back to my browser

again reload the page I shouldn't see

any visual changes but if I type in my

name again David and click greet this

still now works and what my browser has

just done underneath the hood is not

only download the uh hello.html file as

always because there's now this script

tag that's referencing the source of

another file just like an image tag

might reference the source of an image

the browser is automatically helping me

out by loading that into its memory as

well and now how about one final example

and for this one I'm going to go ahead

and not write it live but open it up as

prepared in advance just to show you

what you can do by listening for some of

these other events as well like the key

up the finger going down the finger

going up and listening for exactly that

so as the user is typing something you

can do something interesting as well I'm

going to go back into my directory

listing here and I click on this Source

a directory which has all of the

examples that I wrote here in advance

and I'm going to scroll down to one

called hello 5. HTML and in hello 5 now

we've gotten rid of the button and we

just have this text box but notice now

what happens if I start typing my name

as d a v i d D I'm not typing enter at

all and in fact if I start deleting and

I change my mind and start typing

Carter's name notice now that the web

page the Dom inside of the computer's

memory is now automatically updating

itself so it's not even listening it

would seem for a submit event anymore

but maybe for a key up event instead so

let me go back to VSS code here open my

terminal window and in my source a

directory let me open up hello 5. HTML

inside of the script tag you'll see some

code that's similar in spirit to before

whereby I'm adding an event listener to

the document waiting for the whole Dom's

content to be loaded but then inside of

that function I'm now doing this I'm

creating a variable called input and

selecting from the document the one and

only input tag that we saw just a moment

ago I'm then adding on line 11 in event

listener for key up which is exactly

that gesture so that I can execute some

additional code anytime the human lists

their finger from the keyboard after

typing a key what do I then do well I'm

going to go ahead it seems and declare

another variable called name and I'm

just going to select some P tag on the

page and now we didn't really see a P

tag so I think it's time to look at the

HTML if I scroll down to the bottom of

the page where my actual HTML is you'll

see that there's just a form tag and no

onsubmit Handler anymore as before

there's just an input tag and no button

at all but there is on line 29 here an

open and close P tag just so I have an

empty placeholder in which to put

something like hello David or hello

Carter so that's why now on line 12 I

can define a variable called name and I

can select that P tag so that what do I

want to do well if inside of the input

there's a value so this is essentially

checking for null or the absence of a

string so if input. value does not equal

nothing that is there is a value in that

text box well this syntax here on line

14 is a very clever way of doing the

following go into that name tag that is

to say the empty par paragraph change

the inner HTML of it the HTML inside of

it to be literally this hello comma and

then just so you've seen an additional

piece of syntax this is similar to

Python's F strings the syntax is a

little weird in JavaScript you actually

use not single quotes not double quotes

but back ticks which on a US English

keyboard are typically the top leftand

key of your keyboard or thereabouts and

using dollar sign curly braces just like

the curly braces alone in Python allows

us to plug in whatever the value is of

that input however if there's no value

there it looks like I'm just going to

say hello whoever you are and in fact we

can see that behavior now is I delete

delete delete delete delete delete and

nothing's there now that if condition is

no longer true and so we see this

default value instead so this is only to

say that by harnessing these various

events that are constantly happening on

most any web page we can now register

code just like we did way back in

scratch to actually listen for those

events and do something with them now it

turns out we can do some interesting

things even using third party coat and

just as we used bootstrap a bit ago to

make our table prettier allow me to

propose that we also take a look at this

version of favorites as well let me go

back into my source a directory and open

up favorites 2 which I made in advance

which looks almost the same though I've

zoomed in here a bit but you'll notice

somewhat subtly over the leftmost column

in this table you'll see now this arrow

in Gray pointing up and pointing down

previously those were not there all I

had was a static HTML table with all of

this data sorted in whatever order it

was inputed the other day in that form

but now notice what I can do if I want

to sort in uh one order I can click this

Arrow or the other order I can sort in

this Arrow so essentially doing it

chronologically forward or backward now

how is that sorting happening it's

presumably based on all of the

timestamps that were registered when we

submitted that Google form just a bit

ago um uh when it was live but now using

JavaScript it turns out that we can use

some logic somehow and sort this data by

the same and you don't get that

automatically just by using HTML alone

now how did I achieve that well it turns

out if I go ahead and close these hello

files and in vs code let's open up uh

favorites 2. HTML you'll see that all of

the HTML is actually the same if I

scroll down and down through this file

but I added a little something

interesting at top I copied and pasted

the appropriate URLs and HTML tags from

bootstraps documentation and you'll see

here that I have a file called not only

bootstrap.css but also

bootstrap.min.js as well as a couple of

other things as well that allows me

ultimately using third-party libraries

to add some special HTML attributes that

those libraries told me to add and then

as soon as those libraries detect the

presence of these attributes now on my

own raw data they do their thing and

javascrip defy not a technical term the

entire table and now make it interactive

and not static so you'll see here that

the Aesthetics of the table are as

before table and table striped but I'm

adding now another HTML attribute called

Data toggle whose value is table and I

know that only from the documentation of

these libraries indicating that's that's

how I can now enable this table to be

interactive as I can too by adding data

sortable equals quote unquote true on

specifically the timestamp column and

the only thing unfamiliar here perhaps

is I'm using t P for table heading as

opposed to TD as I do elsewhere but

that's all that it takes to now focus on

the raw data you want to present and let

someone else do the heavy lifting of

actually implementing the logic well

let's end with just a look at what more

you can do with Java script and just how

powerful it is when you combine a

language like this with the data and the

uh user interface you want to convey

let's go ahead and open up within Source

a directory something called background.

HTML now this interface here is quite

simple it just has three buttons RG and

B and the background of course noticed

by default is just white but when I

click on the r the background turns red

when the I click on the G it turns green

and the blue it turns blue and again and

again so how is this working well if you

think back again to the available events

perhaps I'm just listening for a click

on those buttons and then doing

something with maybe the CSS of the page

to allow me to see those different

colors so in fact let's go back to vs

code here and let's open up background.

HTML and in here you'll see some simple

HTML at the top just three buttons but

I've given each a unique ID so that I

can reference it in code and then inside

of a script tag here below because I

didn't bother with the uh Dom content

loaded event here notice that I'm doing

the following I'm creating a variable

called body that lets me select the body

tag I then have in these three line some

code that handles red what am I doing

well I'm selecting from the document

whatever h HTML tag has unique ID of red

and then I'm adding an event listener

for any click on that button and anytime

someone clicks on that red button I call

this function anonymously it doesn't

even have or need a name and this syntax

here is powerful because now in

JavaScript I can alter the CSS of my

page by doing body which is the tag that

I selected two lines ago accessing its

style accessing its background color

property and setting it equal to quote

unquote red and I do the same down below

for green I do the same down below for

blue and the only thing worth noting

here is that in CSS it turns out it's

the case that the CSS property for the

background color of a page is actually

background Das color in all lower case

with a hyphen in between unfortunately

in the world of JavaScript a hyphen

would be mistaken for subtraction like

background minus color which would be

wrong so the convention in JavaScript is

when you're trying to manipulate CSS you

take whatever the property name is uh

font size background color and you

change it into so-called camel case here

you get rid of The Hyphen and you

capitalize the subsequent words like

color in this case here all right how

about another well it turns out back in

the day back in my day there was a HTML

tag that would actually allow you to do

this create blinking text on a screen

it's rather unpleasant at this rate

certainly but how might this work well

it turns out in JavaScript if we take a

look at the blink. HTML file here we'll

see that you can in your HTML do

something literally as simple as hello

world in the body but then you can call

this function here turns out just like

document there's another Global special

variable you can use in JavaScript and

browsers called window which refers to

all things related to the window itself

the window comes with a set interval

function which lets you do exactly that

set an interval in milliseconds and

after every expiration of that interval

some function will be called for you so

in this case it's saying every 50

milliseconds but let's actually slow

that down now to 500 milliseconds or for

one half a second call a function called

blink notice I do not have parentheses

after the blink name because I don't

want to call blink now I want to tell

JavaScript to call the function called

blink every 500 milliseconds now we'll

see in a moment what that code looks

like but let's go back to the page and

reload and you'll see now that it's a

more pleasant blinking if that's even

the case every half second because I'm

now firing that event that is I'm uh

calling that function now every 500

milliseconds instead how am I doing that

well this same script tag I've invented

my own blink function this is has the

distinction back in the day of actually

being an HTML tag and among the few tags

that the world removed and got rid of so

that it's no longer used because it's

not all that user friendly but down here

what am I doing I'm getting the body of

the document itself with this variable

and then I'm checking two CSS properties

that we didn't talk about before but it

turns out that you can check and set the

visibility of an element in JavaScript

by going into that tag checking its

style and getting its visibility uh

property and if it happens to equal

hidden the next line of code here 13

sets it equal to visible instead else if

it's not hidden it must already be

visible and so line 17 flips it the

other way and changes it to Hidden here

left hand right hand clearly not talking

no idea why the opposite of visible Is

Not Invisible it's indeed visible and

hidden but this just allows you every

time this function is called to change

the property from one value to another

achieving that blinking effect you can

do even more powerful things that you

and I take for granted every day let's

go into Source 8 and go to autocom

complete. HTML which I wrote in advance

and this is a page that also loads into

memory a really big dictionary that you

might recall from problem set five and

if I go ahead and type in something like

c a t you'll notice dynamically an

unordered bulleted list appearing below

the text box that shows you all of the

words in that dictionary from pet 5 that

start with c and then CA and then T just

like the autocomplete you see every day

on your phone in Google or websites like

it how is that working well probably I'm

listening for the key press going up as

soon as that key is pressed I'm probably

searching through a big array uh really

of all of those words maybe using linear

search maybe binary search if uh faster

than that and then looking for any

string in that array that starts with C

or CA or c a t and then I'm generating

automattic IC Ally the HTML therefore

but perhaps most familiar nowadays is

just how much your phone and your laptop

know about you and let me go into a

final example here in Source 8 called

geolocation HTML which is a term of art

that just refers to figuring out your

geography for instance your GPS

coordinates now here we have a third and

final Global variable that your browser

provides you with called Navigator and

in Navigator there's a special object

called geolocation that comes with a

function called get current position

so if you call Navigator geolocation dog

getcurrent position and then pass in a

function of your own that doesn't need a

name but does take an argument in this

case I called it position as soon as the

browser or as soon as the phone has

figured out where in the world that user

is with that browser at some latitude

and longitude the browser will

automatically call that function for you

and you can do anything you want with

the position that comes back the

latitude and longitude respectively so

I'm going to use a function that's not

often that help but for our purposes

today it's just going to write to the

document itself to my big rectangular

region whatever the latitude is then a

comma and then the longitude as well so

if we go back with this final flourish

into Source 8 open up geolocation HTML

you'll see that my browser first wants

my permission to let this website my own

know my location I'm going to go ahead

and click allow crossing my fingers

because it might take a moment for the

phone or the laptop to figure it out and

it looks like according to Google I am

right this moment with my Mac at

latitude longitude 42375 comma

7111 let's go ahead and highlight and

copy that let's go to a website like

Googl maps.com crossing our fingers if

you've never done this you can search

for GPS coordinates too let's hit enter

and amazing we are indeed in Sanders

Theater roughly there standing on this

stage on Halloween and that then is week

eight we will see you next time happy

Halloween

nothing

go buffering

okay Josh

[Music]

nice it's Moy no oh

wait that was amazing

Josh uh um Sophie

[Music]

amazing that was

[Music]

perfect I think I

hey to

you

[Music]

oh that was amazing thank you all so

good

[Music]

[Music]

[Music]

[Music]

[Music]

all right this is cs50 and this is

already week nine which is our second to

last indeed this is really the last week

where you'll learn in this class how to

program but indeed it's this week that's

really meant to be the the pedagogical

climax of like all of these various

languages we've been looking at all of

these various techniques all of this

syntax so that at the end of cs50 in

just a few weeks you indeed feel that

you didn't take a class on C and you

didn't take a class on python but you

really more generally took a class on

programming because indeed we know

already about half of you uh will go on

to study computer science further but

half of you will not and indeed all of

your programming chops here on out

theoretically will have a foundation in

what we been doing these past many weeks

but here on out it's really going to be

up to you to learn some new fangled

language when it comes out or to follow

some new trend when some language

eclipses the ones we've been using as

more popular as more appropriate for

problems you want to solve and so today

really is about synthesizing so many of

the past few weeks but doing it in the

context of web programming which For

Better or For Worse is so very much

invogue nowadays both on our laptops and

phones and indeed the languages we

looked at in recent weeks are used not

only to make websites but also full

fledged applications and app stores and

the like so this really will be the

culmination of those past several weeks

and indeed we'll even talk about some

familiar Concepts like shopping carts

when you're on Amazon and these things

called cookies when you're visiting

websites all of those topics too will

come into play and you'll have an

understanding of what all that means

from the ground up so how did we get

here well just last week we focused on

HTML and CSS primarily which are not

programming languages they're just about

Aesthetics structuring your data

presenting your data and so forth um and

we served the web pages we wrote using

this program HTTP server this is just

one such program there's dozens hundreds

of different web servers that you can

use out there this is just a super

simple one we pre-installed in your

codes space for you in vs code so that

you can just serve up web pages at the

end of last week two though we teased

JavaScript a full-fledged programming

language that you can use to manipulate

the users experience for the better to

make things more Dynamic and interactive

by actually running code in the user

users's browser on their Mac their PC

their phone as opposed to server side

which up until now is where all of our

code in C and python has been written so

you're writing code on a server you're

serving code from a server but now with

HTML CSS and JavaScript it's getting

executed in a browser but today we're

going to give you one final feature of

python or really languages like it that

you can also use code on the server to

generate automatically dynamically the

HTML uh the JavaScript the CSS that you

actually want the user to rece receive

you don't have to hardcode everything as

you have when making your own homepage

well let's consider what what some of

the building blocks were last week so

here's a sample URL and over here slash

is sort of the default page on any web

server it might be index.html it might

be something else that's just a

convention but it refers to whatever the

default actually is you can visit of

course when any browser like a URL that

ends in file. HTML or something else.

HTML and that literally means your

browser wants this file on this server

or of course we saw that it can be a

folder and inside of that folder is

presumably some default file name like

again index.html or you can be more

explicit like folder file. HTML and

these more generally we just called

paths and indeed a path is just a

location on your Mac your PC or on a

server of some piece of information but

today we're just going to rename this

only to use other common terminology but

they're really just synonyms today we're

going to refer to those same things as

routes because now today we're going to

ultimately replace HT PTP server which

just serves up static content that you

all write with your own web server like

now you will be the ones controlling

what it is the server does in response

to the user in uh requests so that you

can respond interactively and

dynamically but we're still going to see

techniques like this these were our

so-called HTTP parameters they're

everything that comes after a question

mark in a URL and it can be like key

equals value and an example was what

when we played with Google what was the

key and what was the value

that I first tried any

recollection I was searching for cats

and so the key I figured that was Q

because that's what Larry and Sergey who

created Google years ago decided the

name would be of the HTML text box that

you type your query into and if I type

cat for cat the value of that would end

up in the URL for Google as being

question mark cat equals value and I

mentioned that it's often the case that

you want to send two different inputs to

a server and this is why I propose that

you just keep an eye out for Amper Sands

and and Ampersand separate these key

value pairs but again this is the same

darn Paradigm as before and we've seen

this so many times right key value pairs

in dictionaries in Python we've seen uh

HTML attributes and their values we've

seen CSS properties and their values

it's all the same thing associating

something with something else even

though every language every person seems

to have their own uh vernacular for it

it really is just the same idea this

associating of something with something

else we'll continue to see and here to

be concrete were the

HTTP lines of text that were in those

virtual envelopes if you will if I were

indeed selecting trying to search for

something like cats on Google this

recall was the message that got sent to

the server by my browser in order to

tell Google to please search for not

dogs but in this case cats now what is

HTTP server been doing for us well it's

just been serving up HTML files CSS

files maybe some js or JavaScript files

but it has been ignoring any HTTP

parameters like HTTP server does not

take user input why well what's it going

to do with it because you already wrote

the HTML you already wrote the CSS like

there's no decisions to be made until we

introduce a proper programming language

on the server and so we're going to move

away now from this simple HTTP server

program and introduce you to your own

server that's going to handle the

parsing that is the extraction of these

key value pairs so that you and I don't

have to write python code all of a

sudden that like analyzes this stuff

figures out what pages requested the key

value pairs all of that were still going

to get for free by just using the right

framework and so today we revisit python

uh which we've now used in some form the

past few weeks and indeed it's kind of

been the glue that allows us to stitch

together some of our own logic we saw it

with SQL we're going to now see it with

HTML CSS and even JavaScript if we want

and we're also going to see another

language today not a programming

language called Ginga and this is going

to be a common Paradigm in the real

world whereby different languages

different libraries different Frameworks

often like borrow from each other or

they use uh technologies that someone

else wrote just so they don't have to

reinvent that wheel so flask is just a

framework that is a third party Library

it's pretty popular nowadays it's

relatively simple which is why we use it

in cs50 if you've programmed before cs50

Jango is another popular framework or

library in the python sta space but it's

a little more complicated so we focus on

flask and Ginga we'll see is not a

programming language it's just going to

be some syntax thankfully familiar with

curly braces that allow us to use

placeholders in our actual web pages so

again you'll wrap your minds all

eventually around where the lines are

among these various Technologies but

these are not the interesting ideas the

interesting ideas are the ones we'll

focus on in code but starting today

instead of running HTTP server to serve

up a static website we'll have you start

running literally flask space run in

your terminal window to run your own web

server that's implemented in Python

using this flask framework so bootstrap

was a library for making your CSS and

JavaScript prettier and more interactive

flask is a framework or library for just

making your python code more pleasant to

use since you're borrowing features from

someone else all right so how can we go

about doing this well if you are to

write your very own web application your

own amazon.com your own google.com in

Python using flask minimally you need to

have a file called app.py by convention

which is where all your python code goes

and then a folder called templates which

is where all of your templates go and

for now your templates are just your

HTML files so if we're going to now

start building more interesting

interactive things like google.com or

amazon.com we need to be able to execute

code on the server and so this is the

convention it's not index.html anymore

necessarily it's these two things at the

top level with that said we'll quickly

see that there's some other conventions

and in my examples online and in the uh

problem set nine you'll see another file

called requirements.txt which is just a

text file that allows you to enumerate

all of the thirdparty libraries your

application might want to use it's a

convention so that the server can like

automatically install things for you

without you having to do it manually and

then static is going to be where

literally your static content goes so if

you've got images for your web

application if you've got JavaScript

files CSS files by convention that goes

in static these are just conventions

like all of this can be changed but this

is like the way to do things so we'll

introduce you to the defaults all right

so what does this mean how for instance

could I go about implementing my own web

application using python that somehow

spits out a message like Hello World all

right well turns out just this now we'll

tease this apart in just a moment but

this is the content of a sample app.py

file that apparently uses some Library

stuff like familiar syntax from

something import something else we've

seen that before with csvs and other

libraries this is somewhat new syntax

but it's kind of copy paste for now this

is definitely new syntax and kind of

weird with the at sign here but we'll

see this again and again today and it's

just copy paste initially until you

understand what it's doing for you but

at least there's some familiar stuff

here like index.html is still going to

be with us but it's going to be up to us

when and how to show it to the user so

let's make this more real let me go over

to vs code here and let me go ahead and

create a how about we'll do this in

hello let me do makeer hello to make a

new folder called hello and I'm going to

CD into it just to isolate all of these

files to the same directory so that we

have different apps today and different

folders and now I'm going to do code of

let's do this actually let's do our

maker templates. HTM ah sorry not

templates. HTML let me rename that to

templates using the MV command this has

nothing to do with web programming this

is me making typos so if I type LS now

I've got a folder called templates all

right in there let's create a file

called index.html that is going to be

super simple and pretty much copy paste

from last week let me hide my terminal

window and let me just very quickly whip

up a simp simple hello world page using

my HTML tag Lang will equal English then

inside of this I'm going to have a head

tag inside of this I'm going to have a

title tag and I'm just going to call

this thing hello uh I'm going to then

have a body and in this I'm only going

to say something simple like hello comma

world and just so this is mobile

friendly recall that we touched on these

meta tags so just in case you after

class play with your mobile device

instead of your laptop I'll do name

equals quote unquote

viewport uh viewport and and content

equals and I never remember this I'm

literally reading it off of a cheat

sheet initial scale equals 1 width

equals device width and this is just

this magical incantation that says to

the browser like size things

appropriately for the size of the device

it blows up the font sizes a bit all

right so that's what I would have done

last week and I would have served this

web page by running HTTP server in the

same directory and boom I would see that

HTML but let's now start to take some

control over the user's experience and

for now it's going to be underwhelming

it's just going to always say hello

world but in a moment version two is

going to say hello David or hello Carter

a bit more dynamically and we'll quickly

escalate from there to just more

interesting applications as well

culminating with things like cookies and

shopping carts and the like so let me go

back into my terminal window and as

promised let me create another file

called

app.py and this is where now I need to

implement the web server I'm going to

run using this flask framework and for

now I'm just going to kind of do some

copy paste from uh what we saw on the

slide a moment ago from the flask

Library which we've pre-installed for

you I'm going to import a uh function

called flask capital F it's subtle but

it's important there and I'm also going

to import a few other things a function

called render template and another

variable called request and the only way

I know this is from having taught this

before read the documentation followed a

tutorial like you wouldn't know this

unless someone told you or you read how

to do this this but what this means is

that this Library called flask has three

things in it a function called flask

capital f a function called render

template and a variable built into it

called request and this is going to be

all the building blocks I need to

implement my own web server the

convention in flask when you want to

create a web app in Python is you create

a variable by convention called app and

then you assign it the return value of

that flask function capital F and pass

into it underscore uncore name uncore

underscore which is weird but we have

seen this before a few weeks ago anyone

recall when and why we mentioned uncore

uncore name uncore

uncore yeah I think was

it name or something yeah if we wanted

to check if the name of the file was

itself main so that we avoided a

situation where if you're writing your

own Library code you don't want your

code to be executed automatically you

want to potentially execute the main

function and that was a solution to that

problem here for today's purposes this

is just the way you do it underscore

underscore name underscore uncore refers

to the current file and so this is just

a little trick that says turn this file

into a flask application that's all it

is and for now uh that line suffices all

right what do I want to do after that

well now I'm in charge of the web server

I need to write the code that decides

based on the browsers request what file

or files I'm going to send from the

server to the browser last week http

server did all of this for us just based

on the file name but today I'm going to

take over control over that process and

the way I do that is as follows I say

app. route with weirdly an at sign in

front of it this is known in python as a

decorator and it's a feature of python

not a flask that we just didn't

introduce in weeks past but it's a

special it's a handy trick to do what

we're about to do the route I want to

Define is quote unquote slash so that is

here is code I want the server to

execute whenever I user visits forward

slash the default page of the website

well what code do I want them to execute

well I want them to execute a function

and I can therefore Define in Python a

function I can technically call this

thing anything I want X or Y or Z but

because they're accessing the default

page otherwise known as the index of the

site I'm going to just more reasonably

call this function index but just a

convention you could call it anything

you want but X Y or Z is probably a bad

stylistic choice it doesn't need to take

any arguments in this case and the only

thing this code this function is going

to do is for now let's go ahead and have

it return hello world quote unquote and

that's it all right now let me go into

my terminal window let me go ahead and

do flask run in the same directory that

has appy and hit enter I'm going to see

some cryptic output but including a URL

of my code space and if I open that URL

after hovering over it I'll indeed see

hello world as you might hope but let me

do this let me go ahead and rightclick

on the page and click view page source

which if you haven't done before shows

you all of the HTML for a page however

pretty or messy it is and that's it

there's no HTML that I've spit out it's

just quote unquote hello world well if I

actually want to spit out a full web

page which is not a big deal here

because who cares it's just the text

anyway but if I want to spit out a whole

file let me do this I want to return

essentially the contents of index.html

which have all of the tags I want the

mobile friend stuff and all of that well

I can't just return index.html but I can

return this render template quote

unquote

index.html and per the documentation for

flask this render template function will

go find that file for me in my templates

folder by convention it will open it up

and then it will spit the whole thing

out to the browser for me so I can keep

all my HTML in one place and all my

python code in this one place so now if

I go back to my browser and reload I

don't think I'll really see a difference

because it's the same text ultimately

but if I view page Source now notice

that ah there is all of the HTML that

was just sent to the browser so this is

only to say we have the building blocks

the puzzle pieces if you will via which

to now store all of our HTML in one

place and presumably CSS JavaScript and

so forth but then serve up whatever we

want even though I'm just blindly

spitting out

index.html so before we proceed any

questions on this which again I claim is

like my manual version of what HTTP

server was doing for us automatically

last week but this is how you do it

yourself any

questions all right well let's make it

more interesting which we could not do

with HTTP server and HTML alone why

don't we go ahead and do this let me

visit the same URL and I'm going to zoom

in and your url will differ from my code

space but it's going to end similarly

here I'm going to do slash question mark

name equals David for instance or Q

equals cats or name equals Carter any

key value pair I want I'm going to

append after a slash and a question mark

thereby providing user input to the

server albe it in a very user unfriendly

way no one's going to normally do this

in their browser enter nothing changes

here it just says hello world but

wouldn't it be nice if it says hello

David or equivalently if I zoom in here

again and change David to Carter and hit

enter wouldn't it be nice if it says

hello Carter instead so we need some

dynamism there and here's now python is

going to be our friend if I want to

access the HTTP parameters that the user

has provided via the URL be it Q equals

cats or name equals David I can use this

special variable I already preemptively

imported earlier and I can do this if

there is an HTTP parameter called name

in what I'm going to call request. args

then I'm going to go ahead and create a

variable called name and I'm going to

set it equal to request. args bracket

name else if there is no quote unquote

name key in this special variable called

request. args I'm going to just assume

that the user's name is World by default

now what's going on here well it turns

out that flask provides us with this

special variable called request. ARs and

in there is all of the key value pairs

that might have come in via the URL so

if you had to guess what type of data or

what data type is request. args that's

its name and here is in context line n

might provide a clue in Python what data

type might request. args

be

yeah uh it's not going to be an array or

a list because those are always in every

language we've seen numerically indexed

but you're

close someone else it's a dictionary so

a dictionary is similar syntactically to

a list in Python but instead of numeric

indices like 012 you can literally use

strings like quote unquote name now

that's a bit of a white lie it is a

dictionary but it's flask's special

fancy version of a dictionary but the

syntax via which you can access it is

exactly the same and I actually this is

a typo I didn't mean to say names there

I meant to say name singular but

otherwise I think the code is correct

this is going to on line eight check if

there's a key called name in request.

ARs and if so it's going to set it equal

to that value otherwise it's going to

default to world I deliberately did not

do this I added this IFL

and did not do this why what error might

happen if I just blindly grab

name exactly if there was nothing at the

end of the URL that was of the form

question mark name equals someone then

there would be no name key and this is

uh you know a couple weeks back but this

would give you one of those annoying key

errors when you get a trace back because

you screwed up because you used a string

that doesn't exist that's why I'm just

proactively trying to avoid that

situation just like I'm might have a

couple of weeks ago so even though it's

more verbose this is just much more

defensive so that I don't accidentally

index into a dictionary where there is

no key but we'll see how we can tighten

this up to be not four lines but one but

I think now I can do this wouldn't it be

nice if now in my index.html file which

recall is in my templates folder

wouldn't it be nice if I could do the

equivalent in C of like a percent s here

for instance or in Python something like

this name well it's close and this is

just because different humans invent

different languages invent different uh

Frameworks the Syntax for this in flask

is to actually do whoops two curly

braces and then name of the variable

inside of it why it's just probably

someone figured what are the odds that a

normal person is ever going to use two

curly braces at once versus just one so

this is probably decreasing the

probability that people actually want to

Output literal curly braces like this so

it's similar in spirit to Python's F

strings it's similar in spirit to C's

percent s it's similar in spirit to

sql's question marks same idea slightly

different syntax and this there is

ginger so it's not programming code per

se it's just a template and indeed

that's why this folder is called

templates it is sort of like a a

blueprint for what I want to be spit out

to the user but I've got these

placeholders like this variable that I

want to plug into that value now this

alone is not enough watch what happens

if I go back to my other browser and I

reload the page after changing up here

let's do name equals David again enter

nothing outputs after the hello comma so

it seems that the name variable doesn't

exist yet and that's why indeed if I do

view page Source you can see what was

sent to the browser something's wrong

with my placeholder but I just need to

be a little more explicit as to what I

want to send where so it turns out that

the render template function takes not

just one argument the name of the

template you want to spit out but it

takes after that with commas all of the

placeholders you want to plug in so for

instance if you want the placeholder to

be this literally placeholder inside of

those curly braces you can then specify

as the second argument to rep uh render

template a placeholder named argument

equals whatever the name is so name is

the variable in the lines above

placeholder is the name of my literal

placeholder in the curly braces and so

now if I go back to my browser and

reload this with still quote unquote

with still question mark name equals

David in the URL now I indeed see hello

comma David and if I zoom in here and

let me move over here let me type in

Carter and hit enter now I see Hello

Carter instead now this is a little

unnecessary to explicitly call the

placeholder placeholder especially if

you want to have two or three of them so

you can actually call this anything you

want and I'm going to change it back to

name which is a little more

straightforward the only weird thing

here is that now you'll see that you're

writing code like this and this is

correct and this is the norm it just

looks weird but the thing on the left of

the equal sign is the placeholder you're

using in the template the thing on the

right can be any value you want

including a variable so even though I'm

naming them exactly the same which looks

stupid admittedly like this is what

people tend to do just because it's uh

simpler than introducing another word

like placeholder any questions now on

this

any questions on these

placeholders no all right well let's

tighten this up a little bit and see if

we can't get things to be more Dynamic

still let me propose now that instead of

outputting instead of using this

condition which made a very simple idea

like very verbose with four different

lines it turns out there's an easier way

to do this you can actually still create

a variable called name and you can set

it equal to request. args but instead of

just blindly indexing into to that

dictionary it turns out that request.

ARS comes with a function as well called

get you can pass it an argument that

tells you what value you want to get and

by default if there is no key called

name in that dictionary this function

will not throw a key error it's just

going to return none an O the special V

the Special Value in Python so it avoids

a bug in your code but it tightens up

four lines into one but even nicer if

you read the documentation

the get function can also take a

explicit default value so if you don't

want none to be on the screen like hello

comma blank or I mean that would be

weird too you can just put in a default

value per the documentation of this

function like world so now we've gone

from four lines to just one so arguably

it's better designed and if I go back to

the browser now still with Carter in the

URL and hit reload same thing happens

but we notice this suppose I uh get rid

of the name parameter altogether and hit

enter now it goes to the default instead

world so it's just a little better a

little better designed than doing it the

other way instead all right how about we

take things up one more Notch and how

about we introduce multiple routes and

actually introduce perhaps a form to the

mix because again no normal person is

going to like visit a URL and add a

slash and a question mark and their name

like that's not how browsers work uh

well that's how browsers work that's not

how humans interact with browsers you

and I use a form to quickly instead so

now things can get a little more

interesting when making our own web

application cuz maybe we could do

something like this let me go and zoom

out again let me go back to my code here

and let me move this around and focus

now on the index.html file instead of

just this placeholder why don't we go

ahead and give ourselves a form like

we've played with a little bit in the

past be it for Google or something else

and let's do this uh form and inside of

this form let's have an input and the

name of this input will be quote unquote

name so that too is confusing but inputs

have name attributes but this is a

person's name so I'm saying name equals

name here so just a messy world of

semantics and let me go ahead and make

this a text box by default and then let

me give myself a button whose default

type will be submit and the name of this

button will be greet for instance so

let's see what happens here but let me

change app.py to just be the original

simpler I'm not passing in any

placeholders now and I'm going to even

get rid of this I'm just going to rewind

to the first version of this for

Simplicity let's now change the url to

get rid of Carter and myself so we just

go to slash and hit enter and now we

have a super simple form again all right

this is not super userfriendly but

there's some nice enhancements we can

make for instance like we can uh for

instance turn off autocomplete

especially if I want to type David and

Carter manually and I don't want it

finishing my thought during class uh we

can do autofocus which puts the cursor

there BL linking by default which is

nice cuz then the human doesn't have to

deal with that um and then we can even

have a placeholder attribute placeholder

equals name so that it's like built in

instructions for this thing and so now

if I go back to the other tab nothing's

changed yet because I have to download

the HTML again reload okay now it's a

little more user friendly it says name

and light gray the cursor is blinking

and I'm sort of ready to go but this

form hasn't been wired up to go anywhere

yet and so let's do this let's for

instance say that the action of this

form

is not going to be something like

google.com which we did last time with

for cats I am now going to be both the

front end and the back end of this

website the front end is what the human

sees the web page the graphics the forms

the back end is the stuff the human

typically doesn't see the python code

the SQL code the server itself but now

I'm in control of both sides of the

experience the HTML and also the routes

so let's just propose that we invent our

own route and instead of calling it SL

search like Google does let's call it

SLG greet and let me specify that the

method this form will use which is

technically the default will be get and

confusingly it is lowercase get even

though in the envelope we keep talking

about virtually it's actually capitals

again left hand wasn't talking to right

hand when these things were decided all

right so all I've done is create a web

form that's going to submit whatever the

text box value is to a route called SLG

greet by default because there's no HTTP

or htps or no domain name SLG greet is

going to be assumed to be not at

google.com but whatever my own serers

URL is so whatever my code spaces URL is

that's going to be the implicit prefix

this SLG greet is just the route so now

let's go back to VSS codes app.py file

how do I now Stitch this together well I

think we're good to go with index.html

if index. html's purpose in life is just

to spit out this form we're done with

one of my routes but if I want to have a

second route greet that actually spits

out some greeting to the user well let's

prepare that template too let me go

ahead and highlight all of this HTML let

me go back into my terminal window and

into my hello directory and then into my

templates directory and let me create

another template called greet HTML whose

purpose in life will not be to show a

form but to greet the user with hello so

and so so in this file I'm going to

paste all that same HTML but I'm going

to get rid of the form and essentially

revert to our previous version hello

comma and then using the ginger syntax

name so one template index.html is for

the form the second template now is for

the greeting of hello comma so and so

but otherwise these files notice are

almost the same except one has the form

one has just the hello so now let's

finish this up in app doop High let me

go down here after a couple of blank

lines stylistically let me do app. route

quote unquote SLG greet but I could call

this route anything I want I'm just

using a a reasonable verb then let's

define another function I could call the

function anything I want X Y or Z I'm

going to call it more reasonably greet

no arguments and then now is the code

where I want to render the template so I

do return render template greet HTML but

but I need to do one more

thing what else do I want to do if I

want greet HTML to have access to the

human's name just to

recap I think we solved this already but

I deleted it but what do I have to add

back

yeahh yeah so I got to pass in the

placeholder somehow so I can do this a

couple of different ways I I'm going to

keep it a little more elegant this time

I'm just going to put my name uh

argument there and I'm going to set it

equal to request. ar. get quote unquote

name comma world before I used a

separate variable but I only used it in

one place so that's not strictly

necessary so this is fine too but if

this gets a little overwhelming notice

that I can alternatively do this I can

create an actual variable called name

and then I can pass in an argument

called name with a value that is that

variable but again what's really the

point here it was kind of prettier all

on one line so these are the exact same

things I'm just trying to tighten things

up further here all right so what just

happened if I go back to my form this is

still index.html if I reload it nothing

has changed if I type in my name to this

form notice again the URL I'm currently

at this is Chrome hiding things it's

technically slash by default even though

many browsers are just hiding

unnecessary uh characters these days but

Watch What Happens now if I scroll over

here and I click greet on this new

form notice my URL my route changed to

SL greet question mark name equals David

and the body of the page at top left

says hello comma David so this is

exactly how google.com works and it's

how we implemented search. HTML last

time but instead of submitting the form

to Google via the form I'm submitting it

to myself my very own route so I'm

implementing my own backend for this

same front end all right any questions

just yet much less interesting than

Google certainly but we kind of have all

of the wiring

now any

questions no

all right so what can we do to further

uh tighten this up and adhere to some

conventions well let me propose that in

this version we solve one problem and

even if you've never done this sort of

thing before I dare say we have enough

weeks of cs-50 where if I show you

index.html again and greet HTML again

odds are to someone's mind there's a

opportunity for

improvement why is this web app super

simple though it is arguably poorly

designed at the

moment and the answer lies somewhere in

these two templates index.html and

greet.tolowercase

probably did necessarily for your

homepage why because when you have HTML

only maybe CSS and even JavaScript

that's all you can do is copy paste copy

paste and just make sure that you have

the same structure maybe you have the

same CSS file the same Javascript file

the same third party libraries but it

makes it very very annoying as you might

have realized already to just m make a

change that affects everything so

wouldn't it be nice to like factor out

all of this and all of this and just let

the body change so here too is something

that flask and really other equivalent

Frameworks let us do it allows us to

create what we're going to call

conventionally a layout instead so I'm

going to go ahead and do this I'm going

to copy one last time all of the same

HTML I'm going to go into uh my terminal

window and I'm going to create by

convention a file called layout. HTML

this is truly going to be a blueprint of

sorts and in layout. HTML I'm going to

paste all of that same code but I'm

going to use now some ginger syntax to

indicate that I don't want to plug in

just a variable like name here I want to

actually plug the contents of a whole

other file so instead of just using

curly braces two of them left and right

I have to use slightly different syntax

to say I want a whole block of HTML here

from some other file and the way to do

this even though the syntax is a little

non-obvious is you use open curly brace

percent sign block then you can call the

next word anything you want it just has

to be a special type of placeholder for

an actual file not for just a variable

I'm going to call it body only because

I'm in the body so I'm want a

placeholder to be the entire body and

then outside of this you then say in one

word no space end block so it looks kind

of stupid honestly and why do we have

yet more ugly syntax again just

different software developers in the

world are all choosing their own Syntax

for their own libraries so they all kind

of look different but are all kind of

similar in spirit and you just get used

to seeing the different syntax this now

is not nearly as pretty as the pair of

curly braces for variables but this is

how I can say plug the contents of an

entire file Here and Now what does this

let me do I can now go back into my

index.html file which at the moment

still looks like this but almost all of

this is copy paste the only lines that

are interesting and different are these

four lines here in the body so what I

can actually do now is I'm going to

highlight that and cut it and then I'm

going to highlight everything else and

just delete the entire file and I'm

going to use some of that same syntax

and say curly brace percent sign extends

quote unquote layout. HTML and then I

close my thought with a percent sign and

close curly brace so this syntax as you

might just be inferring is now saying

please extend whatever layout. HTML

looks like that's the original blueprint

the mold out of which I want to make

this web page and now here the syntax is

a little weird too but similar at least

from before I can now say the block the

body block that I want you to plug into

that layout is going to be everything

between these two tags which we already

saw earlier but in layout. HTML they're

sort of giving a placeholder in

index.html this is what I'm going to

plug in to those other placeholders as

well so I'm just going to give myself

some extra white space I'm going to

paste the HTML that was there if I want

to make clear what's going on I can

indent it although this has no no

functional impact but it just makes

clear that just like in HTML you can

open a ginger tag and close it but in

ginger here we have this here hey uh Hey

python here comes the body of this page

hey python that's it for the body of

this page and all of this stuff should

be plugged into this main parent layout

if you will so super ugly admittedly but

now at least things get way less

redundant because I'm going to do the

exact same thing over here in greet HTML

it looks like this but now I'm going to

do this extends layout. HTML also just

as before uh the body the uh the body

that I want to plug in is going to be

everything inside of these tags here and

this body is just going to be hello

comma name in curly braces like that so

again ugly syntax got really ugly fast

but it's really just following these

patterns now and we have two types of

placeholders two curly braces for

variables and now this kind of syntax

with the percent signs and the single

curly braces for like contents of actual

files and so now in this world or in the

world of a homepage if you were using

flask and python to make your personal

homepage with all of those various Pages

you would probably design one main

layout with all of your pretty logos and

colors and fonts and like what you want

the site to look like and then each of

your smaller Pages would now be

distilled into just these smaller

fragments and whether you're using

python or Java or Javas script or other

languages too all different programming

languages have popular Frameworks that

do things like this the idea is the same

across all of them all right let's see

if it works let's go back into the

browser let me go back to my slash route

there's that same form let me type in

David and type and click greet and

indeed I see hello comma David I see

that greet was automatically added to

the URL by the browser when I submitted

the form followed by the key value Pairs

and if I view the page source as I did

earlier you'll see that you have the

entirety of that layout with hello David

plugged in meanwhile if I go back to the

form and view this page Source you'll

see the exact same layout but with the

form tag plugged in and here's where you

can be a little less uh nitpicky with

styling okay yes this isn't technically

indented inside of the body but it was

relative to the original file so at this

point in the game you don't need to

worry about your outputed HTML looking

super pretty you want your source code

that the see to be pretty not the

browser this is not a stylistic

concern okay questions on these

capabilities then a

flask or problems that we've just solved

and why

yeah uh okay so if the files in question

are in different folders for instance if

I go back into my uh index page which

has the form um the routes here are

entirely dependent on what is an app.py

there's no notion of a folder when it

comes to implementing a web application

anymore they are more generically routes

however and we've not done this yet you

can put your static content your images

your video files your CSS files in a

folder called Static and there can be

subfolders therein and that would affect

what you use as your Source attributes

for images or your Source uh tags for

video or any of those kinds of assets

and we'll see that eventually in the

home in the uh the problem set next

other questions on what we've just done

here

yeah good question how do I how did I

ensure that the web app starts on the

form and then goes to the hello page so

whatever you decide your default index

route is like the implicit slash that is

what is going to be pulled up when a

user visits the domain name where your

website is hosted so if I go back over

here to app Pi because my slash route is

designed to return index.html that's

exactly why that response came back good

question all right so beyond this let's

consider now issues of privacy and we'll

touch on this too as we get to issues

like shopping carts and cookies if I

zoom in on the URL here even though the

URL itself is a little long and ugly and

cryptic but when I type in my name and

hit uh greet of course we keep seeing

name equals David in the URL in what

sense might this be bad

design or in what kinds of web apps

might you not want the name to show up

in the URL like that because this is

what Google does this is what my app

does

yeah yeah so if I'm logging in with a

username and password I I could imagine

that they show up in the URL after the

question mark where username equals

mailin and password equals 1 2 3 4 5 but

then all my you know like nosy siblings

need to do is go through through my

browser history and boom like it's right

there for them to copy paste so that

doesn't seem particularly uh secure or

if someone's walking past you in a cafe

they can just look at your url if it's

revealed by the browser and they see it

too so get is not necessarily the best

verb to use even though it's the default

when submitting forms typically when

you've got anything remotely sensitive

or anything large so be it a password or

credit card number or like an image or a

video that you're uploading to uh

Instagram or to YouTube or any such site

like that you don't want the data going

into the URL and thankfully there's

actually an easy way to fix this I can

go into my form which is currently in

index.html and I can just change the

method from get to post in lower case

here but the verb in the virtual

envelope we discussed last week would

itself be capital p o St now

unfortunately Watch What Happens here

let me go over to my original form by

going back to the Slash rout and I'm

reloading the page to make sure I get

the latest freshest HTML and just as a

um just to confirm here yep if I view

Source method is now post so let me go

ahead and type in David now and click

greet and before we saw hello David but

now I get method not allowed and this is

somewhat subtle but in the title of the

tab notice that it's a 405 error which

is not familiar probably almost all of

us have seen 404 file not found turns

out 405 a little more Arcane is the

method the HTTP verb is not allowed why

because by default my app.py only

currently supports get by default how do

I support post well I just need a little

bit more syntax so let me go back into

vs code here let me go into app.py now

and after changing the form I just need

to inform flask that you know what the

method I want this GRE route to use

should not be the default which is only

get I want it to use these methods and

it takes a second argument called

Methods the value of which is a list the

default of which is quote unquote get so

that's the default this has not made any

changes but if I want to support post

instead I can explicitly pass a list

with one string in it P instead and now

what does this mean we didn't talk about

this in any detail last week but inside

of this virtual envelope typically is

that line like get slash search Q equals

cat after the ex after the question mark

if you want to hide that kind of

information for privacy sake or because

you want to upload like an image which

just doesn't make sense to put in the

URL essentially the part of the story

would be well the computer looks deeper

inside of that virtual envelope and

anything submitted via post goes below

the htdp headers like deeper in that

envelope so they're still there they're

just not obviously visible uh for prying

eyes in the user's own browser so just

by making that change in the HTML

telling the browser to submit the data

via post and changing app.py to tell the

route to expect the data via post I can

now go back to my other tab let me go

back to the original page let me reload

just so I've got the latest HTML and

indeed view page Source it's still yep

it's still post but now when I type in

DAV ID and click greet now it works but

but but notice the Privacy implication

I'm at the SLG greet route but where's

my name it's not actually there it's

still went to the server but it's not in

your autocomplete or your history now

for privacy sake questions now on

post yeah oh No Just scratching all

right can you the programmer see this

well let me show you a couple of other

features of Chrome's uh Chrome and

Safari and other browsers as well I keep

going to view page source which just

shows you like a readon version of your

HTML but recall that last time I

actually right-clicked and went to

inspect or viewed developer tool tools

and this brings up a much fancier

version of the developer tools and under

elements here you see everything and

it's nice and pretty printed it's

hierarchical it collapses things into

these clickable triangles but it's the

exact same thing it's just more

interactive but notice what I can do

today is this if I go to the network tab

here and let me zoom out a little bit

let me go ahead and re uh load the form

here and type in David again and click

greet notice now in the network tab of

Chrome's developer tools I see a few

things as we saw before one I see that

the request method is post two I see

that the server automatically without me

writing any code for this returns 200

when it's successful but I can scroll

down down down down down and you'll see

that eventually after all these cookies

more on those later if I click on

payload the second tab next to headers

you can see as the developer what was

actually sent to the server so indeed

this is going to be super useful like

when doing problem set 9 maybe your

final projects if you want to see what's

going from browser to server you have

complete control over all of that

information even if you're using htps

because your browser and you the

developer can certainly see all of this

so again these developer tools even

though there's a lot of tabs and buttons

you probably won't need anytime soon

some of them like elements and network

and with JavaScript console are going to

be super useful to start to get familiar

with all right any questions now on this

implication of

post anything at

all no okay how about one final hello

example that ties a few of these things

together how about now we try to tighten

things up further only in anticipation

of something like problem set 9 or

really more complicated web apps where

you might have not two but 20 or maybe

even more different routes it might

might be ideal to just minimize how many

total routes we have so we don't get a

little too overwhelmed and I dare say

that these two routes are so short maybe

I can combine them into one and maybe I

can keep the user at what seems to be

the same URL but just a kind of Tidy

things up so let me propose that we do

this instead let me get rid of my greet

route and let me go into my form in

index.html and let me go ahead and just

have the action of this form still slash

so I want the form to be visible at

slash the index of the site but I also

want the form to submit to itself if

only because I don't want to introduce

another route like SLG greet which

eventually indeed will be compelling so

you don't have one route for everything

you want your website to do so

technically this is the default to and

if I omit action the exact same thing

would happen as well but let me rewind

and let me now go into app.py to see how

we can make this happen well if I want

my one and now only route to support

both methods I can say methods equals

and then a list with both get and post

in any order but I'll keep them

alphabetical like this this now tells

python hey this route should handle both

get and post requests at the same place

let's now go into this function I kind

of want to say the equivalent of this if

get then I want to return the form else

if post I want to then return render

template of greet Doh HTML with the

user's name but this is not yet complete

code but I think I can do this I'm going

to go ahead and say the following I'm

going to go ahead and say if

request. method equals equals get then

indeed return

index.html L if request. method equals

equals post then go ahead and return

greet HTML this isn't quite enough

though because I still want to pass in

that placeholder so let me again add

back name equals request. args doget

quote unquote name and then a default

value of world what does this Now do for

me well let me go back to my other tab

here let me close the developer tools

let me go back to the form here let me

reload to make sure I have the latest

let me view page Source just to make

sure I have the latest and yep I have

the latest because it still says post

but it now says slash and let's see what

happens now if I type in my name David

previously this submitted bya post so I

didn't see any name or value thereof in

the URL but I did end up at SLG greet

but if the action is now slash and I

click greet notice that it still kind of

works I see hello comma World although

that didn't quite work so we'll come

back to that issue in a moment but

notice the URL ends in just slash and

again Chrome is hiding the slash because

that's all that's there but it does not

end in name equals David in this case or

name and equals world now notice this

too if I reload I'm going to get this

warning do you want to confirm form

resubmission the page you're looking for

used information that you entered

returning to that page might cause any

action you took to be repeated do you

want to continue you might have seen

this on websites you've actually visited

where you hit reload and you're prompted

wait a minute do you want to do that

odds are you've been prompted to reload

explicitly because why whatever you just

did was post instead of get and by

convention besides post being used for

privacy to like hide your username your

password your credit card number or the

like besides being used to upload bigger

files like images or videos post is also

used by convention to make changes to

the server to add something to your

shopping cart to add something to the

database whereas get is the name

suggests is all about getting

information not posting or sending

information instead so this is Chrome

being a little careful because you know

if you just checked out on Amazon and

then you hit reload you don't want to

accidentally like buy the same book

again so to speak even though Amazon and

fancy websites have other defenses for

this too to avoid this issue now there

is a bug though here it says hello comma

World instead of hello comma David and

it actually would have said the same a

moment ago and I just didn't retest the

code and reveal as much to you or if I

did I didn't even notice it said hello

world instead of hello David it turns

out that request. RGS is only used for

get when using get request. args is a

dictionary that contains all of your key

value pairs but somewhat confusingly

when using post with flask you have to

go into request. form I have no idea why

these are not sort of more obvious

opposites like request.get or request.

form and sorry request.get and request.

poost would be sort of sensible names in

this case though we have request. args

for get and request. form for post all

right that's an easy fix though if I go

back to vs code here let's change

request. ARS to request. form let's go

back to my other tab let me just reload

and you know what I'm going to say okay

continue to resubmit the same form

because the form was okay it was my

python code that was buggy hitting enter

now it's accessing David okay but watch

this again if I hit reload command r or

control r i get the same warning are you

sure you want to submit the form yes if

I do it manually with the reload icon I

get the same warning as before but if I

want to manually induce a get request

well that's fine don't hit reload and

send the same request instead go up to

your URL and just put the cursor up

there and hit enter and now notice same

URL is a get by default so anytime you

and I have typed URLs into browsers get

is always the default only when you

click on a button typically that the

programmer has configured to use post

are you actually adding things to your

shopping cart or the like all right so

we are back and if I go way back in time

myself like this is actually like the

first web application I made back in

1997 I believe uh so at the time this

was would have been what my sophomore or

so year I had taken cs50 I took a

follow- on class called cs51 which is a

different type of programming and then I

pretty much taught myself a language

called Pearl which is somewhat less

popular nowadays but it's another

language like python like Java like

JavaScript like others that can be used

to make web-based applications and the

web was very young at the time and the

process via which students my classmates

could register uh for the first year

intral sports program AKA Frost's was to

grab a piece of paper and like write

your name and email address on it and

walk it across across the yard to

Wigglesworth I believe where the Proctor

lived and you'd Slide the piece of paper

under the door and like that was how we

submitted forms in my day um so this was

an opportunity even back in 1997 is to

like move things online and the website

went on to live on until I think like

2007 I found this online and then it's

become something else since um but this

was a website via which people could

register for sports and people could log

in the scores for various games and

whatnot and so underneath the hood I

didn't even know anything anything about

databases at the time it was just like

CSV files that I was storing the data in

but there were HTML forms and there was

with pearl the language at the time the

way to do the exact kind of stuff that

we've just been doing already with flask

and so what I thought we'd do is

Implement a slightly less ugly version

of this um repeating uh graphical

backgrounds were invogue in like 1997 as

you can see here um but this is where

these were the Aesthetics of the day uh

including the the so-called blink tag so

let's at least focus on the

functionality of this website and not so

much the Aesthetics and see if we can't

Implement some of the plumbing for

actually solving like a real world

representative problem be it for

freshman inal sports or something else

like it where you're getting data from

users and processing it somehow so let

me go over here to VSS code let me

create a new directory called fros IMS

just so we can keep all of this code in

its own directory let me CD into fros

IMS let me proactively make another

directory called templates in which our

templates our HTML files do need to live

and eventually I'm going to go ahead and

create a two files minimally app.py and

index.html so let's do the first of

those app.py will live in my Frost im's

directory and I'm just going to recreate

something very simple like we have

previously so from flask in lowercase

import flask capitalized render template

and also request so same first line is

before let me then give myself a

variable called app set it equal to

calling the flask function capital f

with underscore uncore name underscore

uncore and then let me give myself a

route for slash as before with an index

function though again I could call that

anything I want and just for now let's

return render template of quote unquote

index.html as though that exists so this

is not really a web application as much

as it is at the moment just a recreation

of HTTP server for one file let's now in

another tab create a templates file uh

called called

index.html and I'm going to save myself

a few keystrokes I let me copy paste

from earlier almost all of the layout

from before I've change the title in

advance to frosts instead of hello but

this is essentially the same template

and for now though because I'm in an

index.html I'm not going to use extends

or any of that fancy block stuff yet I'm

just going to go ahead and create a

relatively simple form via which back in

the day my classmates could have

registered for interal Sports so let's

go ahead here and I'll propose that we

do this um in this page we'll have a

form the action of which will be a route

called SL register though I could call

that anything I want it'll be somewhat

private so I'm going to use post instead

of get just so that people don't

accidentally maybe register twice by

hitting reload uh without warning uh

inside of this form let's go ahead and

give them an input uh where autocomplete

will be off as always for demonstration

sake autofocus so the cursor goes there

initially the name of this field will be

literally named because I want my

classmates's name if they want to

register for some sport the placeholder

will again be quote unquote Nam just so

they see some gray instructions and the

type of this field will indeed be text

as before and then I need to give them

the ability to register for a few Sports

why don't we keep it simple like back in

the day basketball soccer and Ultimate

Frisbee were three of the sports that we

supported and so let me do this and you

might not have seen this before uh

unless you dabbled further on with forms

on your own but I can create a select

menu otherwise known as a drop-down menu

in HTML inside of which are a whole

bunch of options and each option

typically follows this Paradigm the

value of the option and then the actual

text that the human sees so the value of

these options will be how about we do uh

basketball as one and I want the human

to see literally the same thing though

just like with a link in HTML they could

be different but I'm going to keep them

the same another option will be

uh let's say

soccer and whoops let me fix my quotes

and this human will see the exact same

thing though it could say something else

and then lastly the value will be quote

unquote Ultimate Frisbee and the humans

will see the same thing there ultimate

frisbe all right so this is going to

create as we'll soon see just a drop-

down menu with three separate options if

I want the students to be able to submit

this now let me give them a button the

type of which is submit and this button

will be like the word register on it so

I think we're pretty much good to go

like this is all just HTML no python no

flask per se except for the rendering of

the same template so let me go into my

terminal window let me do flask run

inside of this directory because I need

to serve this app instead I'm going to

see some ugly output including my own

URL and if I hover over that and then

open that URL I should now see a more

interesting form it's got not only a

field for their name but also this

drop-down menu with all three Sports now

this isn't maybe the best user uh

experience thus far because I feel like

I'm biasing people to registering for

basketball maybe because it's checked by

default I mean a lot of forms nowadays

have like a blank placeholder for the

form so this is just an aesthetic thing

but I can do this let me go back to the

same form and let me give myself just a

a blank option at the top that in fact

I'm going to disable so you technically

can't select it proactively but I am

going to select it by default and so we

probably haven't seen those HTML

attributes before but if I want to

create the equivalent of like a a a

title for this dropdown I'm going to

literally create a disabled option

that's automatically selected called

sport so that you can't select it per se

but it is there at the top so if I go

back now to my other tab reload you know

it's just marginally prettier than

before and I'm not biasing people toward

accidentally registering for basketball

alone and if I click on this you'll see

that sport is great out and therefore

not manually selectable but I can select

any of these other three still all right

well un unfortunately if I type in David

and I try registering for instance for

soccer and click register I do end up

atreg and there's no question mark or

name or sport so it's probably indeed

post instead of get those are hints but

not found notice the tab here very uh uh

uh succinctly says 404 not found well

why is that just to be clear why did SL

register give me a

404 what's the logic

here perhaps just State the obvious

or it doesn't exist right we haven't

done that step yet all right so

something as simple as that and so I

actually U sort of belabor that point

because as you're learning like a lot of

these conventions and some of this new

syntax like honestly you're just going

to make stupid mistakes something's not

going to work but again go back to First

principles why is it not found all

right/ register should be a template

maybe called register. HTML oh I forgot

my app. route so that should be the kind

of of thinking as you try to diagnose

these problems moving forward all right

so let me go into app. and let me give

myself a second route here uh so app.

route quote unquote SL route then let me

Define a function called anything I want

but I'm going to call it oh sorry not SL

route SL register let me call the

function just to be consistent register

so but I could call that anything I want

and just for now let's not do anything

too interesting let's just return the

rendering of a template called success

.html let's just pretend for now that

registration is successful no matter who

you are or what you do now I need that

template and I only have index.html at

this point so let me actually now do my

best practices let me copy all of that

let me in a separate terminal window let

me do code uh let me go into my Frost

im's directory and let me create a new

template called layout. HTML just like

before let me paste all that same code

let me delete the form and just put in

that big placehold folder so block body

and then end block is all I did earlier

this is just kind of boiler plate now

convention everything else I'm going to

leave the same but if I wanted to make

it prettier I could add my CSS up top if

I wanted to add like this crazy

repeating background I could probably do

that up top too so I could make every

page look as ugly as it did back in my

day but we'll focus just today on the

text all right so now that I have

layout. HTML let me clean up index.html

I don't need all this redundancy I don't

need all of these tags at the top

instead recall I think I just need

extends quote unquote layout. HTML with

the appropriate percent signs and curly

braces I then have the appropriate block

body though I could call Body anything I

want but I'm going to stick with my

convention earlier and I'm going to

delete the tags down here that I no

longer need why because if I go into

layout. HTML I already have all my open

tags all my Clos tags the only stuff I

want in index.html is is going to be

which belongs in the body so end block

down here and just to be pedantic let me

go ahead and highlight all that hit

shift Tab and that will like unindent it

just to line things up just to be tidy

all right so better even though it looks

a little cryptic now but now I've laid

the foundation for making a third page a

fourth page that don't have all of that

same copy paste all right so now let's

go back into app.py success. HTML is

where I left off so okay let me open my

terminal window let me code up a

template called success. HTML whose

purpose in life is literally just going

to be like to say you are registered

just so that we see some informative

message on the screen so this part I do

still need extends layout. HTML so

there's a little bit of copy paste still

which is a little ugly but so be it

block body for this template and I'm

just going to say you are registered

exclamation point all right and then end

block so super simple it's just an

informative message claiming that the

Stu the student is registered all right

let's go back to the original form which

is this let me reload to make sure my

HTML has reloaded type in David I'm

going to register again for soccer and

click register and oh interesting method

not allowed so I'm not getting a 404

anymore I'm getting

405 at SL

register what's the deduction here how

did I screw up this

time 405 is progress yeah the

placeholder uh so it's not the

placeholder I think is okay this is now

about the underlying HTTP stuff the

method was disallow was not

allowed say again so get purposes post

thing too so by default all of these

routes in flask just by default assume

get because it's safe it doesn't allow

you to send information to the server in

quite the same way but if I do want to

support post recall that we changed this

to be methods equals and then a list

with quote quote Post in it so I just

need to enable support for that method

that that is that HTTP verb all right

let's go back to the form reload just to

make sure I haven't screwed up type in

my name David select soccer from the

dropdown click register and now I'm not

only at SL register in the URL it claims

that I am indeed registered now of

course I'm not I've done nothing

interesting there's no database there's

no CSV file we'll get to that in a bit

but at least I now have the plumbing in

place to do something Dynamic based on

that sport all right well how can I now

improve upon this how about we go ahead

and implement store the actual

registrant in a dictionary in the

computer's memory so instead of just

claiming that they're registered let's

actually make a notation and the

simplest way as we did weeks ago in

Python is just store things in like a

variable in memory like a list or a

dictionary a set anything like that all

right well let me go back into vs code

and in app.py and I think what I'm going

to have to do here is change my register

route to actually do some useful

information but before I register the

user let's consider where I want to

actually put them and so let me propose

that how about we do this at the top of

my file let me go ahead and declare a

global variable called registrant uh and

set that equal to an empty dictionary so

we've done this before when we were

playing around previously with using

dictionaries to store key value Pairs

and I'm going to propose that we store

the registrant as a dictionary why cuz

I'm going to keep it simple like the

name is going to be the student's name

sorry the key is going to be the

student's name and the value is going to

be whatever sport they registered for so

David and soccer and Carter and

basketball and so it kind of makes sense

for like a two column dictionary so to

speak as we often depict it on screen so

how can I use this dictionary well let

me go ahead and do this down here under

SL register let me go ahead and

initially do this how about we get the

user's name from request. form. Get and

set it equal to whatever the value of

name is and I'm not going to give a

default value now because I don't want

to call the student world or something

strange like that I'm just going to

assume for now that it's there let's

then create another variable called

Sport and do request. form doget quote

unquote sport to get these students

Sport and then let's go ahead and do

this in the

registrant dictionary let's index into

it using the student's name and let's

set it equal to whatever the sport is so

I've got these variables just to keep my

code tidy and I'm now putting a key

value pair in that uh into that

dictionary all right well what do I want

to now do and I'll I'll go ahead and say

success. HTML sure let's go ahead and do

that but now I think success. HTML means

that so let me go back to the form

reload let me type in David and soccer

register okay let me go back and say

Carter and basketball registered okay

now let's see what I want to do next how

about I go

into um Let me give myself another route

and let's play around here so app. route

let's give myself another third route

called registrant whose purpose in life

is just to show me who all of those

registrants are just like you would

expect from a website like this and then

let me Define a function called

registrants or anything else and then

let me return the rendering of a

template called

registrant H ML and let me pass in this

is kind of neat I can do registrant

equals registrant which again looks

weird but what am I doing I'm presuming

to pass in a placeholder called

registrant the value of which is this

dictionary that I've been collecting all

of the registrations in so similar to

the name placeholder before but it's a

little more powerful because now it's a

whole dictionary not just a single

string so I think now let me be creative

here let me go into my

templates under uh my templates folder

and let's do this uh let's go into uh my

terminal window let's create another

template called registrant HTML that's

actually going to do this displaying of

all of the registrant for us so extends

layout. HTML just so I can borrow all of

the same HTML as

before and let's define block body just

like before and inside of this and block

I want to put I don't know like a

bulleted list or an ordered list list of

all of the registrant so how can I do

this well let's do uh an unordered list

ul and here's where Ginger and flask

more generally get kind of interesting

like I want there to be something like

this An

Li uh and then like the students's name

and then an L uh maybe Li yeah like that

and then maybe like sport something like

this but I didn't pass in a name I

didn't pass in a sport I passed in the

entire dictionary of registrants now in

Python if we were just doing something

at the black and white terminal window

and doing a command line program you

know I'd probably have some kind of for

Loop in Python Ginger does allow you to

do this so a templating language tends

to come with very lightweight mechanisms

for doing placeholders doing Simple

Loops doing Simple conditions so python

like syntax and it's almost identical so

watch what I can do inside of this

unordered list let me not start to

manually output a single Li let me use

this syntax the same Ginger syntax that

I used for Block so curly brace percent

sign and I'm going to say this for name

in registrant so this is just like

python Syntax for iterating over a

dictionary and now this is going to look

stupid but the opposite of that is end4

so in HTML you use the slash in ginger

you literally use the word end no space

and then the name of the keyword so end4

is how you close this but this is where

templating gets really cool you can now

do Li and in here I can do something

like that student's name and that's it

I'm going to leave it like that and what

I'm doing here is using really a

template as templates are intended I've

got like the basic building blocks of

what I want this output to look like but

thanks to this little for Loop here

thanks to Ginger syntax the curly brace

and the percent sign I'm going to

iterate over every dictionary printing

out name name name name and so if I've

got two kids registered now I'm going to

see two liis David and Carter

respectively so let's see let me go back

to uh my Frost IM tab here and I don't

have a link yet so I got to do this

manually uh like a developer would let

me go to slash registrant and I'll zoom

out and hit

enter and you'll see what you'll

probably see two when making mistakes

for the first time in this world so

where is the error message unfortunately

internal Ser server error is not all

that useful but we do tell you you see

terminal window so if I go to the

terminal window I haven't been paying

attention to this for quite some time

and in fact I have two terminal windows

open so that I can still use commands at

the prompt but if I go back to my first

terminal window AKA bash there you'll

see in your terminal window When

developing web applications like all of

the mistakes you made in the terminal

itself this is one of those python

tracebacks that's related to me screwing

up here now let me go ahead here and

let's see uh type error function is not

iterable

and block for name function is not

iterable all right so what mistake did I

make well this is what happens when I

don't follow my notes and make changes

on the fly so I have this variable on

line five called registrant in all lower

case but what did I then do on the Fly

here in line

22 I defined a function called

registrant so like newbie mistake like I

shouldn't have done this I can't have a

variable and a function of the same name

name because the symbols are literally

identical so just to make clear that

this variable up here is actually Global

we'll use our convention like we did in

C often when we had a global variable

we'll capitalize it all just to make it

stand out like a a constant value up

there and so down here what I'm going to

do is pass in registrants in all cap so

that was stupid didn't mean to confuse

there but the reason for that error to

be clear is that you can't have a

function that's the same name as a

variable I could just change the

variable name alt together I'm going to

go ahead and just capitalize it to make

it really stand out that this is in fact

a global variable up top all right now

I'm going to go back to my browser let's

do David and

socer all right but there's going to be

some other mistakes here so on line 17

let me go ahead and change this variable

to be capitalized there because indeed I

want to put the key and the value in

this newly named variable as all

capitals registrant let me now go back

to vs code here let me go back to the

form and let me start adding some data

fresh let me register David for soccer

clicking register now and we should see

you are registered but hopefully now

it's indeed in the computer's memory let

me go back and register now Carter for

basketball clicking register again and

hopefully it's now registered if I now

change my route manually to B SL

registrant which is this newly added

route that I made and hit enter now I

see thank God now I see the unordered

list containing everything in the

computer's memory so when I say you are

registered I kind of mean it now because

the server is still running and in the

computer's memory is in this registrant

Global variable a dictionary of key

value pairs of course we're only seeing

the keys at the moment so it might be

nice to actually see the values in as

well so let me go back to VSS code and

let me go into registr trans. HTML and

I'll just do something a little messy

I'll just say uh how about let just make

it a sentence is registered

four and now another placeholder I'm

going to say registrant bracket name so

just like in Python if registr is itself

a dictionary registr bracket and then

the key you want to index into is

perfectly valid syntax as well so now

let me go back to SL registrant let me

click reload again so why isn't it

working everyone what's the bug that I

introduced earlier if David is

registered for none and Carter is

registered for none but David and Carter

are in the dictionary like that's a good

thing so some of the data is in there so

why are there no Sports Associated well

the first thing I literally just did in

front of you all was I went to app.py

and I stared at line 17 thinking like

how did I screw this up I'm putting

sport as the value of the key which is

the student's name all right line 17

looked fine to me a few seconds ago so I

looked then with my eyes at line 16 and

this too looked okay my first thought

was oh did I use request. args instead

of request. form instead because that

would have assumed get instead of post

but no like that looks okay too so then

my final Instinct was oh my god did I

screw up the HTML form and so that's why

I went back over to my tab here I went

to the original form here I then view

page source and this might not be as

obvious to you if you've never seen the

select menu before what is apparently

missing here that might explain my

mistake yeah

yeah I didn't name this form field quote

unquote sport now to be fair you haven't

seen me do this as a select menu before

and it's different from this input when

you have an input tag you literally say

name equals whatever on the input tag it

turns out I don't know why I skipped

this earlier I probably meant to come

back to it the select tag also can take

a name parameter so if I go back to the

name

parameter here and go back and add the

name parameter let me go into that

template which is

index.html let me add name equals quote

unquote sport in all lowercase which is

different from the visual aesthetic of

this temporary disabled option that's

just there to make things prettier for

the human now let me go ahead here and

first I'm going to go into my terminal

window and I'm actually going to hit

control C to stop the server alt

together because I want to throw away

the contents of memory and therefore get

rid of that dictionary that had David

and Carter and those nun values so this

is sort of me clearing the computer's

memory I'm going to rerun flask run I

get that same URL as before so I'm going

to hover over that and open the new tab

and just to be sure I'm going to do view

page source and here I see now okay now

the form has both a name and a sport in

it all right now I'm really going to

cross my fingers because I intend for

this now to work David will register

again for soccer register claims we are

registered I'm going to go back and do

it again for Carter and basketball

register we still don't have a link so

I'm going to manually go up to the URL

and change/ register to registrant as

before zooming out and hit enter and

thank God now I'm actually registered

properly for this so oh thank

you so what is it like 20 years later

I'm still struggling to implement this

site okay so um so here now we have for

the first time in Python and web stuff

like now we have a proper web

application and it's not just echoing

back hello David hello Carter this could

now work for any of you and it's

currently served privately but if I made

this URL public I could put this on the

web now and let anyone in the world

register but there's kind of some issues

here there's some security flaws

potentially and so for instance let me

go back to the web form here and let me

open up the inspect tab the developer

tools and just remind you that anyone on

the internet not only you the developer

but a an adversary can see all of your

HTML see all of your CSS see all of your

JavaScript but more importantly because

this is all client side in the browser

there is literally nothing technically

stopping them from changing the HTML or

at least their copy of it and I did that

last week with Yale I changed their

website but no I changed my copy of

their website but when forms get

involved you could maybe be actually

malicious now because even though this

drop-down menu only has basketball

soccer and Ultimate Frisbee suppose I

really want to register for how about uh

let's say uh name your favorite sport

volleyball we really want to register

for volleyball but like this website

won't let me well there's nothing

stopping me from going under the

elements tab in my browser going into

this select menu here and you know what

no one ultimate for me let's change this

to volleyball and let's change this to

volleyball enter I'm going to close the

inspector now and as requested now we

support volleyball in the form now now

it's not changed on the server to be

fair but think about how HTTP works when

I fill out this with uh say let's see

Bernie's name Bernie really wants to

register for volleyball as well at the

moment my code is just going to trust

that what's in request. form is what was

in the original form itself no matter

whether the human adversarially actually

changed it so if I actually submit this

form and click register for Bernie and

volleyball even though that's not one of

the supported available Sports if I now

go to registrant my website nonetheless

has trusted that uh Bernie and perhaps

you are registered for volleyball so

what's the implication of this this has

surely happened in the past when like

really poorly implemented websites um

allow you to uh specify the price of an

item for instance in your shopping cart

and they just trust that when you click

submit or add to cart it adds the price

to the backend server if you're not

validating the price and making sure as

with a database that wait a minute that

price is valid valid or wait a minute

those sports are valid like who knows

what people are going to do to your site

and it's that simple to actually hack a

website accordingly now we can very

easily fix this with some um some week

six style python we really just need to

do a bit of logic here and so let me

propose this let me go into app.py here

and at the very top let me also create

how about a uh Global variable called

Sports in all caps and I'm going to set

that equal to in square brackets the

list of Sports I actually want to

support so I'm going to put in

basketball here I'm going to put in

soccer here and I'm sorry no volleyball

officially I'm going to put in ultimate

frisbee here so I've got this Global

list of supported Sports now think about

how I made this form a while ago I just

hardcoded these Sports here well I don't

have to do that I can sort of draw upon

my own official list of sports instead

so let me scroll down to my

index.html rendering template here let

me say that the sports I want to support

are are these so just using the same

placeholder trick as before but I'm now

telling the template what sports we

currently support now if I go back into

index.html I don't have to manually do

any of this let me get rid of all three

of those options which I manually

inputed earlier let me use my new trick

with ginger syntax and say for sport in

sports then let me proactively say end

for just to finish that thought and then

in here let me do option value equals in

curly BRAC is Sport and then so that the

human also sees the same words I'm going

to say sport out here so I've completely

changed what was hardcoded manually

typed to something now that's completely

Dynamic so now it's not going to stop

someone adversar like me from changing

the HTML but watch this the behavior on

the form if we go back is still now the

same drop down as before so

aesthetically it looks the same but you

know what why don't we be clever now and

let's go into app.py and the/ register

route and why don't we say this uh if

how about sport not in sports then let's

return render template uh failure. HTML

now this template doesn't exist yet so

let me just quickly make this real fast

I'm going to copy that code from before

let me create a code file in uh failure.

HTML I'm just going to paste this here

so I have a super simple error message

and I'm going to say you are not

registered just to that's what we mean

by failure and now in app.py consider

what logic I've added Sports in all caps

on line 22 is that same Global list as

before by asking pythonic if sport not

in sports well then you hacked me like

you tried to inject volleyball or some

other sport into request. form so I'm

just going to say no failure not letting

you register and I can do this a little

more uh verbosely too why don't I also

say this if if not name so if the name

is blank let's similarly return a render

template of failure. HTML in other words

if you didn't give me a name you left it

blank that's not useful for me running

the sports program let's also consider

that to be a failure so if I go back to

this tab now I'm going to reload just to

make sure I have the latest client side

let me be lazy and just click register

enter you are not registered because I

didn't give it an actual name all right

well let's go back how about I now type

David but no no I'm not going to choose

a sport I just want register myself nope

that did not work now let me go ahead

here and choose soccer this I think does

work let me go back now and try this

hacker trick whereby I go into the drop-

down menu I go into the select menu I

change as before Ultimate Frisbee to

volleyball and I'll change this one here

to

volleyball let me close the tab now this

looks like it's available now but when I

click register this this time it says

you are not registered and this is much

better than relying on other techniques

you might see or have seen online with

regard to HTML because there's also this

trick let me go back to the screen here

let me go back to index.html and you

might have seen online or you might

eventually see online that there's other

attributes you can use like required you

can literally tell the browser uhuh this

field is required you cannot leave it

blank if I go back to the browser now

reload and I again presume to be lazy

and I don't type in any name and click

register okay so that's kind of nice

like now the browser is being a little

more helpful for me saying no no no this

is required you have to fill this out

but again if you know what you're doing

okay well I disagree with your requiring

a name of me let me go in here let me go

over to this tag let me delete the

required attribute and now I slip

through but I didn't slip through on the

server and so there's a difference here

and an important distinction and so many

people in the real world still screw

this up there's client side validation

like actually checking that the data is

as you expect on the client side the

browser and there server side validation

and even though client side validation

like adding that required attribute

makes things more user friendly right

like that was a pretty little popup it

tells me that it's required it just

looks better than the previous version

it is not trustable you cannot trust any

input that ever comes from the user

because clearly with like a an hour or

so of cs50 like they can learn how to

turn all of these defenses off so even

if you like the user interface better

client side you have to have to have to

do server side validation always users

are not to be trusted and as soon as any

app or website you make becomes popular

unfortunately then you have to deal with

all of the adversarial possibilities as

well oh good question could the

adversary potentially access things

sensitive like app.py theoretically no

like if flask itself is buggy then sure

maybe if you're running some other

software on your server on your laptop

then sure uh it it's possible however if

your server is properly configured

theoretically they should not be able to

get access to that with that said we'll

soon see or You might with your final

project if you do something web based

you're never going to want to write like

usernames and passwords in your actual

code you can put them in what are called

environment variables so sort of in the

computer's memory but not in your code

just in case you or someone screws up

there are uh still ways to defend

against those kinds of possibilities

however slim

yeah a good question and this comes back

to First principles just like in C and

in python as soon as you return from a

function that's it nothing below that

line of code executes and so to

summarize the question even though I'm

returning this failure. HTML template

how am I making sure we still don't

accidentally put volleyball in that

Global dictionary it's because for

instance if you don't give me a name on

line 22 at the moment I'm returning the

failure template and that's it lines 23

24 25 26 27 never execute in particular

26 never executes and that's where I

would have been saving the name

similarly if we do we get a invalid

sport that is either blank or not in the

original authoritative list we return

failure. HTML is a template in line 25

we never get to line 26 so it just boils

down to return and what that means in

Python 2 good question other questions

as

well no all right how about we make one

more enhancement here or so because the

problem with storing everything in this

Global dictionary might be what we've

got it all working finally but why is

probably a global dictionary not the

place to store Frost's registration

data what's the implication of

this okay might slow the whole process

down but that actually Ram is actually

good memory is actually generally a good

thing so not going to be a deal breaker

here why might I not want to store that

data though in that

variable you can perhaps infer how I fix

something

earlier yeah and

back yeah it gets it's the memory gets

deleted garbage collected if you will as

soon as flask stops running so if you so

much as hit control C like you've just

lost all of your freshmen who registered

for the sport probably not a good thing

I did this deliberately a moment ago and

I hit control C because I did want to

clear the dictionary but trusting that

your server will never crash and your

code will always work and the power will

never go out like that's not the right

way to build any kind of web application

with persistent data so what we probably

want to do is reintroduce csvs and we've

played with those in C and in Python

could totally use csvs but we also now

have p uh SQL at our disposal and let me

propose that we do this in SQL in

instead and for this let me go ahead and

open up a version of the program that I

wrote in advance so let me go ahead and

close these templates which will look

very similar but a little different from

the ones I wrote in advance and let me

go ahead and open up uh in today's let

me go into Source 9 let me go into Frost

im's how about version four technically

in the versions online and let me go

ahead and open up app.py as follows so

here is an already made version that

does just a little something different

at the very top I'm importing cs50 SQL

Library which you might recall we used a

couple of weeks past just to write

python that talks to a SQL database and

this feels like an opportune moment to

bring that idea back down here on line

eight I'm creating a DB variable that

opens up a file called frost. DB using

syntax that we've seen before I did

create this FRS DB file in advance of

class just so that we have a couple of

columns in which to store names and

sports and such here's that same Global

array a global list called Sports and

let's just see what's going on down

below if I scroll down to Index this is

the same as before as we wrote together

on the Fly let's skip deregister for a

moment and go now into register so this

one's a little different but let's see

what I've done I've got some comments in

here because I wrote it in advance and I

think this logic is pretty much the same

though I tightened it up and I'm asking

two questions at once using on line 38

the or keyword here just to say if

there's not a name or the sport is not

in sports that is what we'll call now a

failure but what's fun now is that on

line 42

I'm using the cs50 SQL library to

execute some actual SQL and I'm going to

insert into a table called registrant

two columns name and Sport what names

and Sport well these two values with

placeholders plugging in name and Sport

notice I'm using the question marks

absolutely necessary so we don't get one

of those SQL injection attacks because

that too could be possible if someone

typed in some dangerous words or

keywords like delete or single quotes or

semicolons in the form here I'm letting

the library sanitize the data and then

this is a trick we haven't yet seen and

it's really going to start tying things

together I can also use a redirect

function in flask that has the effect of

doing the the if you will uh

safetyschool.org trick again whereby

after the user registers if I want to

automatically show them now everyone who

is registered atreg I don't have to

manually expect that they'll change the

url like I've been doing for the past

few minutes I can just redirect them

anywhere I want on my app or heck I

could redirect them to any URL on the

internet using this function call and

it's just a nice way to send them to a

different route if you want them to see

in this case those registrant so let me

do this in the same directory let me

increase my terminal window size let me

do SQL light three of frost. DB and let

me type schema and you can indeed see it

wraps on to two lines here that each

registrant has an ID which will be

automatically assigned one two three on

up a name which is not null text and a

sport which is also the same and the

primary key is just going to be this

unique identifier so that I made in

advance but if I do select star from

registrant semicolon there's no one

currently registered for any sports but

let's try now running this let me go

ahead and close my old version which we

wrote together and I'll close that tab

let me do flask run in this version four

here all right I'm going to see some

similar output I'm going to open the URL

now and you'll see that I made a couple

of Chang changes before instead of using

a select menu I used what are called

radio buttons now which is a reference

to Old School radio buttons that were

mutually exclusive in cars back in the

day and we'll see how to do this but

it's just an alternative to a select

menu and I'm going to go ahead and type

in my name again here so I'll do David

I'll do soccer by selecting this radio

button and I'm going to click register

now and notice what happened it's a

little ugly the formatting but so again

was this 20 years ago here I have now

now at the slash registrant route

instead of an unordered list I'm just

using a simple HTML table so I'll show

you what this looks like in just a

moment too and I'll show you this

deregister button which is sort of

unnecessarily large I also have

functionality we'll soon see for how you

can unregister someone from a sport as

well so take your name out of contention

well let me go back to my terminal

window here and I'm going to click the

plus to give myself s a second terminal

so I can go back into Source 9 Frost IMS

4 I'm going to do SQL light of frost. DB

I'm going to do select star from

registrant now and now you'll see that

indeed there's David registered for

soccer and in fact if I quit the flask

program with control C and rerun it

again no big deal because that next

version of flask will just use the

database as well so I'm persisting

keeping the data in SQL light whereas

I'm actually grabbing it using my python

code in flask all right let's put one

more person in here so we can delete one

of us too Carter for basketball register

and now we see both of us here all right

so let's see how we did this let's go

back over to VSS code let me shrink down

my terminal window let me go into the

actually let's go into the templates

directory and let's look at for instance

index.html so previously we were using a

select menu turns out radio buttons use

the input tag but instead of having

input of uh type equals text like for

the human's name you have type equals

radio and so long as each of your radio

buttons has the same name the same name

the same name that's what makes them

mutually exclusive so checking one radio

button turns off the others because they

have the same name the value I want to

assign to each of these radio buttons is

just the sport placeholder this is what

the human sees on the screen so it's

almost the same as the select menu it

just looks aesthetically different but

there's my same button so that's all the

difference I made there and I added a

heading tag H1 just to say register to

make clear what it is but let's take a

look at another file uh this one now

being how about the SL registrant route

so if I open up registr trans. HTML here

now it's way more verbose than my

unordered list but this is just kind of

boring HTML here's my table tag table

head table row table heading this makes

things bold as the first row of the

table name sport are my two columns I've

got a third empty column just so I can

fit that button as we'll soon see again

T body for table body here's the same

for Loop trick again so that I can

output for every registrant a whole

table row and there's this weird form in

there but we'll come back to that but

there's the registrant's name there's

the registrant sport but notice the

slightly different syntax here recall

that cs50's select uh cs50's execute

function when it returns to you a list

of dictionaries you can then get at the

individual columns by way of those keys

so let's go to the/ registrant Route let

me go back to app.py scroll down here

and it's actually super simple here I

have a SL registr route that first

executes select star from registrant so

just old SQL stuff give me everyone from

the registr table let me then render the

template called registr trans. HTML and

just pass in this list of dictionaries

and we haven't quite done this yet but

if you go back to register. HTML how do

you iterate over each dictionary in that

list well the syntax is just for

registrant in registrants that makes

this a dictionary one at a time in the

list just like in Python so registrant

name and registrants sport is just

another Syntax for using the square

bracket notation it's just a little

cleaner and slightly more succinct than

having quotes and square brackets

everywhere and then the rest of this is

just

HTML so what happens now if I want to uh

like Carter has been cut from the

basketball team if you will so how do we

do that well we want to click this

button deregister next to Carter's name

but how does this work and think about

now any website you visited that has

something like a shopping cart uh where

you can remove things from your cart or

update quantities or add more quantities

to your shopping cart on Amazon or

anything else well let's actually look

at the HTML that my app has spit out

let's actually look at this here and

we'll see the following we'll see that

we have here in the HTML that reached

the user not only is David in the first

column socer in the second notice that

my register. HTML form is also spitting

out a tiny little web form of its own

it's ugly but I only care about its

functionality for now and notice what

I'm doing here every registrant in this

database gets their very own deregister

button and that form has a button that

says deregister but notice what else

each of those forms have there's no text

box there's no drop down menu there's no

radio buttons rather you have a hidden

input field here so there is a way with

HTML to have a form that will submit

information but you don't have to give

the user the ability to change that

information you can just go ahead and

tuck it inside of the form invisibly if

you will hidden in fashion and so what's

going to happen is if I click the D

register button next to Carter his

primary key is two

mine is instead one so what's going to

happen if I click his deregister button

it submits a form with a

ID parameter whose value is two and it

submits it to the deregister route so

what do that mean well if I go to VSS

code and I go to app.py let's look at

the deregister route that I skipped over

so if you access the deregister route

via post this code gets called I grab

from request form the ID that was

submitted in Hidden fashion if there's

indeed an ID that is it's not blank it's

not zero it's an actual number like one

2 three or more I execute delete from

registrant where ID equals that value

with a question mark Place holder and

then I redirect the user back to

registrant now if I go back to this form

here I click deregister we'll see that

in action gone is now Carter and in fact

if I go back to my terminal window here

I open open up uh SQL light 3 of frost.

DB and rerun select star from registrant

Carter is now gone so again using very

simple HTML forms you can get buttons

and links and other such UI mechanisms

to like do things on the server that you

want but there is a danger here this now

is really meant this example is like an

administrative website like it was some

20 years ago just for us internal staff

to be doing things technically this is

dangerous what I've just done

two even though Carter's ID is two and

hidden and mine is one in Hidden what

could this allow an adversary to do if

they had admin access to the same

site any

thoughts

yeah

yeah yeah they could change the value of

that hidden attri by opening up chrom's

like developer tools change the number

in the HTML they could delete anyone

deregister anyone want from the database

now in this case I claim this is fine

because this is only meant for us staff

who were running Sports back in the day

but it's indeed a risk so wouldn't it be

nice if we could actually ensure that

only those users who are authorized are

allowed to execute certain actions I

think for this capability we're actually

going to need to introduce something a

bit more and so here of course is an

opportunity to talk briefly about really

what you and I do all day long every day

we log into one or more websites or apps

or at least until uh you're logged out

automatically and you have to do it

again so here for instance is a

screenshot of Gmail when you type in

your username you type in your password

maybe your to factor code that gets

texted or sent to your phone then you're

logged in and thankfully you're not

prompted to log in again typically for

number of hours or days or weeks

depending on the website like Gmail

keeps you logged in for ages your bank

probably logs you out within an hour or

so for safety sake so that is completely

configurable on the server but how does

g Emil know how does Google know that

even as you're checking different mails

again and again and again how do they

know that you're still the same person

who logged in well it turns out that

using these same building blocks as

today HTTP and HTML and more you can

actually implement the notion of a login

feature by doing the equivalent of

something with something called cookies

essentially what happens when you first

log into a website for the very first

time successfully with your username and

password a cookie so to speak is planted

on your computer and metaphorically this

is kind of like taking a hand stamp and

your hand is now stamped in this case a

smiley face so that every other time you

click on a link on that same website

google.com gmail.com whatever

unbeknownst to you your browser is

constantly presenting that handstamp

just like going into a club or an

amusement park or something like that

where they don't want to check your ID

again they don't want to check your

ticket again they just want to quickly

see the same hand stamp and so that is

one of the things that a browser is

always doing for you once you're logged

in any cookies that have been planted so

to speak in your Mac or PC or phone are

constantly represented to the site every

time you click a link or make another

request to that website and mechanically

how this works not just metaphorically

and ink essentially this is what happens

here is a example of an HTTP request to

something like Gmail and suppose for

instance that you've logged in typically

as of last week we said that coming back

from the server would be another virtual

envelope containing like a 200 okay

message and then like the actual web

page or the picture of a cat or whatever

it may be but Google can also if they

verified that you have some username and

password correctly inputed they can do

the equivalent of stamping your hand and

the way they do this is they send an

additional line of text in that virtual

envelope from the server to your browser

literally using another HTTP header not

content type which just mundanely tells

you what kind of content has come back

they literally send an http header

called set-cookie and then they set a

key value pair on your Mac or PC this is

the technical equivalent of this smiley

face handstamp and what your computer is

designed to do because your computer and

intern browser are supposed to implement

HTTP anytime you click another link on

Gmail or click on another mail or the

like your browser unbeknownst to you

presents that handstamp and how it does

it technically is in the envelope it

sends to Google from your browser it

doesn't send set cookie it just sends

cookie colon and the exact same thing

and so long as Google is smart and they

have a database or something of all of

the session Valu session is the

technical term for this maintenance of

information across HTTP so long as

Google has a big database that knows

that my cookie value is one 123 and your

cookie value is 456 they can infer from

this virtual handstamp whose emails they

should be showing mine or yours or the

like so this is just scratching the

surface but if I really wanted to enable

only Carter to deregister himself I just

have to make sure that I log him in

somehow username password and all of

that I stamp his hand or really put a

cookie on his computer and only if his

cookie lines up with the user ID he's

trying to deregister should he be

allowed to in fact do so so all of this

is quite possible and indeed the

technical term for this is session and

what we thought we'd do in our remaining

time Today show you some examples of

exactly how some of the most familiar

web functionality is implemented today

some of which you'll use in your own uh

problem set 9 uh which itself will be a

web app or perhaps even your final

project so let me go ahead and do this

let me close my previous tabs and all

things for ashs and let's move on to

implementing some notion of login so in

just a moment I'll switch over here to

VSS code and what I'm going to do is

indeed in my source 9 directory I'm

going to go into a login directory and

if I type LS here you'll see app.py

requirements. text which just refers to

libraries I want to automatically

install and a templates folder as well

I'm going to go ahead and stop the

previous server and close that terminal

window and I'm going to open up this

version of

app.py so there's a few new lines here

and we'll give you these lines for

problems at nine but I've got some of

the familiar stuff up here including

this new redirect function we just used

and I have a session variable that comes

with flask 2 so what's nice again about

flask is that it deals with all of this

cookie stuff for you it sets the cookie

it checks the cookie and what flask does

for you is it gives you the abstraction

of a variable called session so that

anything you put in the session variable

which itself is a dictionary will be

there again and again and again whenever

that same user comes back a session is

how you implement essentially like the

proverbial shopping cart uh if I'm

logged into Amazon you're logged into

Amazon Amazon knows which of us is which

by way of that cookie and Amazon if

they're using flask provides the

programmer with a dictionary called

session and flasks make sure that when

Carter is visiting the site the code

uses his session object when I'm

visiting the site it uses my session

object but it's all implemented with

those same cookies this is the same as

before these lines are new and you'll

see these in problem Set n this is how

we enable sessions in a web application

and I'll just wave my hands at the

detail there's different ways to

implement sessions whether you use

cookies on the server cookies on the

browser or other things these just

ensure that we're storing the the

session information the shopping cart on

the server itself now down here let's go

ahead and do this let's go ahead and run

flask run so I can see what this app

does if I do this and visit the URL that

gets outputed you'll see a very simple

web page here and if I type in for

instance my name I'm not going to bother

with a password and click log in you'll

see that you are logged in as David and

now I can log out so I'm going to go

ahead and click log out and now it seems

to know that I'm not logged in again I

can log in as Carter because I didn't

bother implementing passwords for

Simplicity but now the site knows I'm

logged in as Carter better yet if I

reload reload reload or click this

button again and again notice it still

knows that I'm Carter uh until such time

as I log out all right well how is this

working well let's go back to vs code

here and let me scroll down to first

this route this is a very common

Paradigm here whereby I'm checking for

the index route if there is not a name

in the session redirect the user to SL

login now what does that mean well let

me go back to vs code here let me go to

the slash route so again your url will

be different but I'm just going to go to

slash and hit enter notice that I got

automatically redirected to login and so

many websites do this if you go to a

website and you're not logged in you're

very often redirected to SL login or SL

account or something like that where

you're prompted the code for doing that

is right here if there's no name in the

session if there's no name in the

shopping cart if you will go ahead and

return the user to login that route

otherwise implicitly if they are logged

in show them index.html so let's go down

that rabbit hole let me open up in VSS

code a second terminal window let me go

into Source 9 and go into this same

login demonstration and let me open up

the template called

index.html and here's all this is

there's some layout but who cares at

this point it's just the boiler plate

generic HTML here's the body block I

have this and this two is ginger because

of the curly brace and the percent sign

if there's a name in the session this is

just python syntax then say this

sentence you are logged in as whatever

name is in the session in the shopping

cart if you will and then I just have

this HTML link for logging the user out

else if there is no name in the session

logically just say you are not logged in

and give them a manual link for logging

in instead so that's all this particular

template does but how does the slash

login work well let's go into this other

template code of login. HTML to which

I'm redirected super simple this is just

copy paste from HTML before I've got a

login form that's going to have an

action of Slash login submits for

privacy sake just via post and then the

rest of this is just a simple form and

I'm using using an input type equals

submit instead of button equals type

equals submit but same idea here too and

if I go back to app.py well let's see

how login works all right it's a lot all

at once but they're relatively simple

reapplications of the same idea so if

the user visits SL login via get or post

call this function login if the user has

submitted via post and we saw this

technique before go ahead and do this on

line 23 store in this special session

variable that comes with flask a name

key and store in it the user's own name

so quote unquote name will have a value

of David or Carter or the like and as

soon as you do that redirect the user

back to slash just so they see the

homepage again and this is how Amazon

and all these other websites work too

otherwise if they visit this page

implicitly via get and even though I

didn't say equals equals get anywhere

that's sort of the implication because

if you can only get here via get or post

and we already handled post logically

All That Remains is get well then just

show them the login screen instead but

there's half a dozen ways we could

express that same logic and then for log

out this is kind of straightforward if

the user clicks that log out link and

ends up at SL logout this route well

just change the value of that key in the

session to be none effectively no Carter

is gone David's gone there's no one

logged in so that is all that's required

to actually implement the notion of

logging in and logging out of website

plus the password thing which should

probably involve a database but one

thing at a time and really session is

sort of like the code version of a

shopping cart whereby if I visit the

same code I get my own session object if

Carter visits the website he gets his

own session object and the way flask

Keeps Us straight is they put one cookie

on my computer a different cookie on his

computer and uses those to line up with

making sure the right session gets shown

to the right actual

user questions on this notion of

sessions no all right how about a couple

final examples just to tie this all

together let me go back into vs code

here let me quit my previous version of

flask let's go into Source 9 and go into

store which is a separate app Al

together and let's start by just running

flask to see what it does let's hover

over the URL and open it in another Tab

and this is pretty ugly too let me zoom

in but it's a very simple bookstore like

an early amazon.com for each of these

seven books here Each of which seems to

be like maybe this is H1 this is H2 H2

H2 H2 and then there's a button

underneath each well now let's use this

as an opportunity to kind of infer like

for any website how this thing works let

me go ahead and do view page source and

you can do this for any website on the

internet let's try to figure out how

this bookstore adds things to a cart

well here's the H1 tag uninteresting H2

H2 H2 so the juicy part is in these

forms each of these forms has an action

of SL cart so that's the route that's

going to be interesting in a moment and

it uses post for privacy sake each of

these forms like the deregister feature

for Carter has an ID attribute an ID

parameter that's hidden visually that

has a value of one or two or three so

like the unique uh uh like barcodes for

the books if you will but super small

numbers in our case and then each of

these other forms just had each of these

other books has an identical form except

for the value of this year now in this

case this isn't such a big deal that a

user could technically hack the HTML of

this bookstore amazon.com and change the

IDS because whoa what's the worst

they're going to do like buy more books

by adding more IDs to their shopping

cart like that's not a problem there's

no prices here it's just the unique

ideas of books so whereas the deregister

was maybe worrisome because you're

changing the server I think it's okay

because the user can only at worst buy

more books than then uh they might via

the buttons alone so how does this now

work well let's go into vs code again

here let me give myself another terminal

window and in Source 9/ store let me

open up app.py which is where all the

logic is so I'll flip through most of

this quickly because we've seen this

before these Imports are pretty much the

same as before this line is the same

this line is almost the same but the

database now is called store. DB instead

of frost. DB this is the boilerplate

code for just enabling sessions this

notion of a shopping cart and so let's

see how the index Works how is that I'm

seeing all seven books at once well in

this index function I'm using on line 19

select star from books to get all the

books from the database and then I'm

rendering a template called books. HTML

passing in as a placeholder all of those

books all right let's go down that

rabbit hole for a second let me open up

books. HTML in my templates directory

and here again even though it's you know

new today it's probably increasingly

familiar syntactically here's the H1

here is my Loop here's the H2 which is

going to Output the current book's title

in that Loop here's the form here's the

ID of that book that's going to be

outputed in the form so I didn't

manually type out seven long forms I

just did one in this template so that it

gets generated automatically literally

what a website CL Amazon would do to

show you 10 books at a time or 10 search

results or more at a time all right well

let's go to SLC cart which was the juicy

one and this one's longer but let's see

if we can reason through it if the user

submits via get or post to/ cart well we

first do this and this is just necessary

in some boilerplate rewrote we're going

to use the session object to store a

variable called cart that in this case

is going to be a list so session again

is just a dictionary you can put

anything in it you want previously we

put students names and sports now uh

what I want to do is actually sorry prev

previously we put uh the student name

the user's name in it now I'm going to

actually store a cart key whose value is

a list why because I want to aggregate

more and more books in this list all

right so that just makes sure that I

have at least an empty shopping cart the

very first time the user does this if

they visit this form via post let's go

ahead and get the ID of the book that

they posted if it is not empty if there

is a number like one or two or three

let's go ahead and go into the shopping

C cart which is a list per this line and

just append that ID so this list of

books in your shopping cart is going to

contain like one comma 2 comma four

comma six whatever books you're actually

buying and then the user gets redirected

to cart what if though the user got here

via get and not post well this one's

relatively straightforward if you just

visit slart we select star from books

where ID is in okay so this is

interesting this list and and this is a

syntax you might not have seen before

but if you read the documentation for

cs50's Library if you select something

and use a question mark placeholder and

the placeholder itself is a list we

output a comma separated list of values

just like you would use maybe in problem

Set uh seven for doing SQL queries on

your own so this just means show me only

those books in my shopping cart not

Carters not someone else's not in the

whole database only show me the books in

my shopping cart and then

render it as such so we only saw what

the catalog here looks like at slash

books Let's go ahead and in slash let's

go ahead and add maybe the first book to

my cart and now I see at SLC cart only

that first book whose ID is one let me

now go back to the bookstore here scroll

down to maybe the seventh book and add

that to my cart and now I see this here

too meanwhile all of this information is

stored in my session and so when I

reload this cart again and again the

reason I'm only seeing my two is because

we're checking only the list in my

session and flasks make sure again that

my session is different from your

session is different from Carter's

session as well but you write the code

once and it works for thousands millions

of people in

parallel any questions on

this this yes in the

back sorry say a little louder

uh so to recap so users will never have

the same session values theoretically

the cookie that gets planted does not

look like a smiley face for everyone

each of us gets a big random number

that's assigned to us so it' be like

each of us gets a completely unique hand

stamp that no one else can see the

reason no one else can see it is because

if the website's using https every time

this hand stamp is shown every time this

cookie is sent back and forth It's all

encrypted as well so each of us can even

if we have the same contents by

coincidence because we like the same

books they will be separate cookies

separate memory separate sessions behind

you

yeah really good question when does the

session end totally configurable

typically it ends when you close the tab

or when you quit the browser or you can

also configure cookies to themselves be

persistent for a day for a week for

longer so for instance when you log into

um uh say Gmail they plant a cookie on

your computer probably for a week a

month a year something like that because

it would be annoying and probably drive

you to like Outlook or something else if

you kept having to log into your account

whereas your bank account might actually

wait for you to just close the tab and

then for your own Financial safety they

just automatically delete the session

far sooner but totally configurable by

default as I'm using it it will

typically be thrown away when the

browser itself quits and here too is

another reason to develop websites using

incognito mode because if you want to

just throw throw away all of your

cookies you close the incognito window

mode open a new one and now you're

starting from scratch you don't have to

manually delete all your cookies which

could log you out of websites you

actually care about

yeah a good question when using sessions

if someone maliciously changes the value

of sub forms could it affect other

people theoretically no because the

worst you can do is like add books to

your own shopping cart that you don't

want there so at that point even though

it's on the server it doesn't affect you

or Carter or anyone else unless there is

something more globally happening like

registering or deregistering for a sport

or removing books from the Amazon

database that would be problematic but

in this case we're removing things only

from my own session that the website is

giving me all right the last topic for

today is this thing here which is sort

of everywhere nowadays these things

called apis or application programming

interfaces and this is a very generic

term in fact because any function you've

used in C in scratch in Python in SQL

are all apis like there is a standard

way of interfacing with those functions

they all have names they sometimes have

return values they sometimes have

arguments and an API is just how you use

a function or more generally an API just

specifies how you interact with some

service and so nowadays there's a lot of

web-based services that you can use to

get back data like the weather or the

current time or the database of Amazon

books for instance all might have apis

often web-based that allow you using

URLs or some other technology to just

get data from someone else as though

it's a function you're calling remotely

but HTTP is very often the mechanism

that's used to actually get data from

servers and the way the data can come

back can be as follows let me end with

one final example using some of our

familiar shows from uh weeks past let me

go ahead and close the old flask version

version go back into Source 9 and go

into how about an example called shows

and the first version of this zero I'm

just going to go ahead and run with

flask run I'll hover over my URL and

open it here and you'll see now that I

have a very simple form as we keep doing

today I'm going to type in like o f f i

c office into this search box and click

search and you'll see now that I ended

up at a URL ending in/ search question

mark Q equals office so this is like my

own baby version of google.com but I

implemented it myself and for any title

of a TV show from a couple of weeks past

that matches o i I spit it out into an

unordered list how is this working you

can maybe imagine even if you might not

be able to program this off the top of

your head certainly so soon let me go

into Source 9 let me go into uh show

zero let me open up

app.py and in this file you'll see that

that I'm grabbing a uh file called

shows. DB which is like a simpler

version of the one from a couple of

weeks past uh here is why I see the web

form my first route my index is super

simple it just spits out that form and

my search route like you can think of

this as google.com there only like four

lines of code so if the user sends data

to SL search this function called search

is called I declare a variable called

shows I execute a SQL command that is

Select star from shows where title like

question mark and the syntax here is a

little crazy but I want to prefix to the

user's input percent sign and suffix it

with a percent sign as well putting in

the between those two values the actual

input why in SQL what is it mean if you

have a percent sign to the left and to

the right nothing to do with Ginger

today yeah it's a it's a wild card so it

means match zero or more characters on

the left or match zero or more

characters on the right you have to do

the concatenation

as the second argument to this function

you can't do something clever like put

it here around the like the is the

placeholder that you plug these values

into but this just means hey SQL show me

all of the titles that have o f i

somewhere in them that gives me back an

a list of dictionaries I pass that in as

a placeholder for a variable called

shows and if we look at search. HTML

let's look at that in my templates

directory there's something called

search. HTML super simple I mean this is

like the essence of google.com search

results I'm using an unordered list to

keep things simple but I iterate over

every show in the shows list that came

back and I output An Li with each of

those shows titles and that's it now

Google has like Blue Links and like

little previews and other text the first

sentence or so from each page but like

that's the idea like this is really

similar in spirit to what google.com

search does for you now how is this

working there's no API involved here yet

this is just very basic HTTP I submit

the the form I go to another route and I

get back the results but check out this

version Let me close these tabs here and

open my first terminal window let me go

into shows one from today's Source 9

directory and do flask run this time let

me go ahead and hover over that URL and

open it here and gone now is the submit

button now I'm going to make an user

interface that uses a technique called

Ajax for asynchronous uh JavaScript and

XML which is somewhat of a data term

because we're not using something called

ml anymore but Ajax is a technique

whereby you don't have to submit forms

anymore to get more data from the server

you can use JavaScript per last week

listen for an event like the key press

coming down or up and as soon as you

hear such an event you can secretly in

JavaScript code send a request to the

server to get back more data and then

plug it into the Dom the tree in the

computer's memory and this just makes

for more seamless experiences like

autocomplete on any website so now let

me try typing o okay we got auto

complete super fast f f i c e and you'll

see every time I add more keys to my

input I'm doing another search another

search another search and the data is

changing now how is this working well

let me go back to vs code here and in my

other terminal window let me open up uh

app.py and in app.py you'll see that

there's still a

search route down below that turns a

search template but watch this let me go

into templates search. HTML and notice

here that we're indeed getting back an

unordered list of shows again and again

and again and this HTML that's coming

back let me go here let me open my

terminal oh sorry this is the wrong

version uh sorry I was in the wrong

folder let's fix this and shows one code

of app.py it's almost the same thing

inserch

here okay well I changed this slightly

let me show you this version of search

if I open up app.py here's my search

route I'm getting a variable called Q

giving it the value of whatever request.

RX has from the user like Q equals

office and then I'm checking if the user

actually typed something in execute this

SQL query select star from shows where

title is like that using the same and

this time just to keep things efficient

I limited the total results to 50

instead of an infinite number otherwise

if the user type nothing just to be

super safe here I'm setting shows equal

to an empty list so if you don't type

anything there's nothing to show and no

matter what I render this template

called search. HTML well let's look at

that if I open up templat search. HTML

this time there's no layout there's no

inheritance of that layout. HTML I am

literally generating just a whole bunch

of Li fragments why well let's see what

what's happening in the browser and this

is the beginning of an API if I wanted

to make this API available this

application programming interface I

could tell the world that if you want to

search for TV shows in my database go to

URL that's something something/ search

question mark Q equals office or cat or

dog or anything else and what I will

return to you is this enter I will just

give you a whole bunch of Li tags which

almost looks the same but let me view

the page source only am I going to hand

you back a fragment of HTML I'm not

giving you an HTML tag a body tag a

title tag a head tag I'm not giving you

a web page I'm giving you a fragment of

HTML that you can now do whatever you

want including insert this into your own

unordered list so notice what happens in

this actual app if I go back to vs code

here let me open up my index template

here and you'll see some JavaScript

magic so in JavaScript here in my form

that only had the text box and no button

what am I doing in a script tag here I

am creating a variable called input and

I'm using this function called query

selector that just gets me a reference

to the input text box on the form so I

can see what the human typed this is a

little different today but I'm using

input. atevent listener which is a way

in JavaScript to tell it just like in

scratch listen for something to happen

like the green flag being clicked but in

this case listen for an event that

involves input that is like typing on

the keyboard whether it's by a copy

paste manual input or anything else then

whenever that happens call this function

and async stands for asynchronous this

is a term of art which means that this

this function might take like a split

second maybe even a second or two to

execute so it's going to do it behind

the scenes like in the background so to

speak and what is it going to do well

it's going to call a JavaScript function

that all browsers now Support called

Fetch which is a function that uses HTTP

to go fetch more data via the from the

server it's going to fetch data from a

route called SL search question mark Q

equals and whatever the value is that

the user typed in so I'm just sort of

manually creating my own mini URL and

telling JavaScript go fetch me that HTML

when it comes back via this line of text

here called response. text and let me

wave my hand at await await just means

this might not come back immediately

let's await the response and when it

does come and then let's execute this

code I'm going to do this I'm going to

search the document the whole web page

for this UL tag which is somewhere in

this page that we'll see in a moment

change its inner HTML to be that

fragment of Li Li Li of all of those

matching shows and where does this all

go well if we scroll up here you'll

notice that there's my usual HTML up at

top head tag body tag and all of that

there's the text box that we've talked

about already there's no button for

submitting cuz it all happens

automatically but there is by default

this empty UL that has nothing by

default until we start using that API

and the final flourish here is this this

is kind of a uh ugly sloppy way to get

data from a server to just get back like

a fragment of HTML like what if I'm not

using HTML I want to store these uh TV

shows in a PDF or in some other wet tag

in a table or something like that it

doesn't really make sense for the server

to be presuming that I want Li tags

surrounding each of my data better would

be to get a more generic format back and

that format is almost always nowadays

called this our final acronym Json

JavaScript object notation and let me do

this let me close these two tabs here

and open my terminal window and C cancel

the previous version of flask that was

running let me close this version and

look at our final version called shows

two and do flask run on I'm going to

hover over that URL and open it in a

browser and I'm going to manually visit

after zooming in let's do again slash uh

search question mark uh Q equals Office

enter and this is what Json looks like

now at a glance this does not seem like

an improvement like this looks crazy

that it's just this Big Blob of text but

it's just enough text for the computer

to be able to process it reliably notice

that there's a cur a square bracket here

and if I actually scroll to the Bott

botom there' be a closed square bracket

like way down there inside of that

square bracket is a curly brace then ID

colon and then a number then a title

quote unquote colon and then the title

and then the closed curly brace so what

you're seeing in JavaScript object

notation is a very standard super

popular format that's just text that

still uses square brackets for lists AKA

arrays that still uses curly braces for

dictionaries key value pairs so what you

see here is a massive list up to 50 I

think think shows that came back from

this API Each of which has a dictionary

if you will an object of key value pairs

what keys and values an ID key and a

title key Each of which has a value

respectively and this is the same data

from IMDb some of which you might be

recalling visually this is just a very

raw computer friendly way of returning a

whole bunch of data that we humans don't

need to see but I can use this data by

going back into vs code let me open

another terminal window and go into

Source

9/ shows 2 and in here let me go ahead

and open up how about uh templates

index.html which previously just used

that inner HTML trick and this is not

going to impress you're not going to be

pleased with this syntax but let me just

at least explain what we're doing it

turns out that Json is just the better

way in general the more generic

multi-purpose user agnostic language

agnostic way of returning data from a

server because it's just text so it

doesn't matter if you're using python or

C or C++ or JavaScript or Ruby or PHP or

something else like all of those

languages can process Json information

and indeed here is some JavaScript that

does just that same code as before

initially I declare a variable called

input that gives me access to the user's

text box I listen for input like key

strokes going up and down and when they

happen I call this Anonymous function uh

I fetch data from the server using the

exact same code as before search

question mark Q equals office or

anything else and then this is just now

new code that I use to convert that Json

data into my own HTML format be it an

unordered list an ordered list a table

or anything else what am I doing I've

got a variable called HTML initialized

to nothing so I've got no HTML initially

I then iterate over every ID in those

shows so every one of IMDB's unique

identifiers I iterate over them one at a

time and then I go into the show at its

ID location and I grab its title and

this

is forget this for just a moment I then

take this HTML variable concatenate or

join onto it my own Li tag plus the

title plus the close Li tag and I

skipped this because it got scary pretty

fast but it turns out that if some TV

shows have actually angled brackets in

that could break my HTML entirely so it

turns out you might recall super briefly

last week we had the copyright symbol

using an HTML entity using the Ampersand

and the hash symbol and 169 semicolon it

turns out there are other such cryptic

sequences of characters that represent

otherwise dangerous or untypable

characters like this which could confuse

the computer into thinking it's at the

beginning of a tag and an Amper sand

which could similarly trick the computer

into thinking that it's a entity which

it isn't but long story short there are

libraries thankfully that handle much of

this for you for our purposes the

takeaway is that now that you understand

a bit of

HTTP uh now that you understand a bit of

HTML CSS and JavaScript all of which

they have their roles you can use them

ultimately to start assembling your own

web applications as you will for problem

set nine um stitching together all of

those languages and building

full-fledged web applications mobile

applications or anything more and for

that I think we are all set and if the

first one up here uh can have these

cookies as well we'll see you next time

for our very last cs50

[Music]

[Music]

lecture

[Music]

[Music]

[Music]

[Music]

good afternoon my name is Sarah and my

name is Grant and we are the Harvard

crocodillos and pitches now Sarah and I

understand that today is the final

lecture of cs50 it's been a tough

semester we made it through pets four

five and even Finance now I know this is

an unpopular opinion but I particularly

enjoyed Finance I spent a lot of time

with my

flask the P set there was a p

set well um at least things are looking

up um today is our last lecture and look

how far we've come if I were an emoji

right now I'd be the face with tears of

joy sorry about that we're just trying

to work some cs50 references into the uh

intro I I mean uh boy I sure hope this

uh tide man doesn't run off my Mario

filter you could say that for Loop x

equals Open Bracket one comma 2 close

bracket boy I sure wish we had checked

to see if these jokes were funny when we

wrote

them any who we hope that you'll enjoy

this brief

[Music]

serenade your

boot

[Music]

KN House of well there's

friers Andy truck barbecue ribs Tri

[Music]

the boy

boy bo boy bo bo boy boy down at the

house the house of well your root and

you walk on down to a knock down Shack

on the edge of town There's a l

there just quit you see

fall down the house the house

[Applause]

of up your boots and you walk on down

at the

house the House of Blue

Light to your down at the of

light

[Applause]

good afternoon everyone we are the

Harvard crocodillos and it is such an

honor to be here with the Radcliff

pitches performing for cs50's final

lecture congratulations to everyone and

we hope you'll enjoy this our tribute to

cs50 one two one 1 2 3

4 C is for the language I once knew O is

for all notation I must do D is for

dynamic flask run and finance it is even

more than David May in canor so code is

all that I can give to you to youe

debugging it since

PE soon deadlines I'll make it hit

compile and please don't break it code

was made by

[Music]

foru

I for notation I must do

is

Dam even

more is

[Music]

I is for the language 0 1 0 1 0 oh is

for otation 0 1 1 D is for dynamic flas

run and finance e is even more than

David mail in canor so C is all that I

can give to

you debugging it since

PE soon deadlines I'll make it hit

compile and please don't break it code

was made by me for COD was made by me

for COD was

[Music]

by all

right

all right this is cs50 and cs50 this was

the Harvard crocodillos and the Radcliff

pitches if one more time we could thank

them for joining us

today so this is already week 10 our

last and indeed among the goals for

today are to hopefully give you all the

more of appreciation of truly just how

far you've come recall that in week zero

we began with this this visual here

whereby it was described this class as a

bit of a fire hose whereby drinking from

that fire hose or really a fire hose

from a water fountain uh is not unlike

getting an education down the road too

and so this is to say that if you're

feeling like you didn't quite get it all

down like that's actually okay and

that's to be expected and even if you

felt that with each passing week 0 1 two

all the way up now till 10 it never

really ever got easier perhaps just

consider that what was once hard like

Mario and like getting hello world to

compile is indeed the right measure of

the Delta between week Zer and now in

week 10 in fact you might recall that

again in week zero 2third of your

classmates had never taken a CS course

before now of course you all have and

indeed if you think back too to this

final sentiment from week zero that

indeed what ultimately matters in this

course is not where you end up relative

to your classmates but where you end up

relative to where you yourself began so

I would take some pride take some

satisfaction take some relief even

though a little bit more work does

remain at really just how far you've

come since that week zero and recall

that in week zero we literally started

with just zeros and ones and by now many

of you might have gleaned that these 64

zeros and ones have been spelling

something week by week in fact today is

our very last message here in binary uh

encoded on stage but then quickly we

introduced scratch and we started to

assemble some building blocks of

programming Loops conditions uh uh Loops

conditions functions and the like but

without all the distractions of

semicolons and curly braces and all of

that which admittedly we introduce the

next week when we introduce you to C but

even now that we've transitioned to

python hopefully even those kinds of

Curiosities or confusions are hopefully

starting to just get more familiar and

so you finally start to see the missing

semicolon as opposed to spending time on

that kind of struggle recall too that in

week two we started talking already

about memory and like how you can manage

things in arrays that later became of

course in Python lists uh the week after

we talked not only about uh debugging uh

bugs in code but how to debug those same

programs um thereafter we started

talking talking about algorithms and we

took a step back from code and looked at

the bubble sorts and the selection sorts

and the merge sorts and all of the

searches as well that go hand in hand

with that and indeed this ultimately is

what a lot of problem solving moving

forward is going to be about just

solving problems with some form of

algorithm and you have so many different

languages um in your toolkit now with

which to approach problems like those we

talk thereafter about pointers which are

not likely to come back in any modern

languages that you now use but hopefully

you have an all the better of a sense

underneath the hood of like what's going

on inside of the computer so that when

you're designing something you're using

something something crashes you at least

have a mental model for what's going on

and it's no longer that week zero black

box as it once was I mean you built

things like this think back to week five

when you built your own hash table and

those things are everywhere key value

pairs whether it's in python or in C or

if it's now in CSS and JavaScript and

even HTML like that principle of key

value pairs is really everywhere and so

of course now code doesn't necessarily

have to look like this it now

wonderfully looks a little something

more like this but eventually you're

probably not I'm not going to use Python

anymore something new and better is

going to come along but odds are like a

lot of the building blocks from these

past 11 weeks are still going to be

useful for wrapping your mind around

those new worlds and indeed SQL we

introduced you a little bit too and even

if you don't feel yourself yet an expert

hopefully have a sense of like what you

can do with it and what problems you can

solve uh it's of course a better

alternative to something like something

simple like a spreadsheet and now of

course like web stuff is everywhere

whether it's on your laptop or desktop

or a lot of the mobile apps that you use

on your phone even though they're native

applications like you install them from

Google Play or the Apple App Store like

they're implemented increasingly with

HTML CSS and JavaScript but they're put

in a little rectangular window so you

don't even notice that that's actually

really just an embedded browser and then

of course you can build things as you

might for your final project that to

might very well be web based if you go

that route I mean I'm still clinging to

like the very first like web app I ever

made years ago um but honestly I do that

in part because I was just so darn proud

that like I taught myself how to do

something and it actually worked and was

used by other people so whether it's

just used by you or your classmates or

your roommates or your family or your

company down the line there's a great

sense of satisfaction that comes despite

all of the the pain that might be along

the way when you just can't see or fix

that bug now of course we'll transition

as you'll see in the coming days to try

empowering you to code client side as

well up until now you've been using our

own vs code installation in the cloud

which is nice cuz you got up in running

super fast in week one focusing only on

code challenges not on technical

difficulties but among the goals now if

you so choose and want to program after

this class even if you never take

another CS course before you can use

these same real world deao standard

tools on your own Mac or PC and so

pictured here is a screenshot of like vs

code on the Mac and even though yes

you're going to have to like jump

through a couple of more Hoops to just

get python or some other language

working on your own Mac or PC like

that's what programming is ultimately

going to be about and we deliberately

transition you to this at terms end so

that now you have 11 weeks of more

Comfort under your belt with which to

solve sort of silly technical support

headaches that might have been deal

breakers so many weeks ago so there's

still more to be done uh in the coming

weeks and indeed a support structure

there for the cs50 hackathon of course

will be this 700 p.m. till 7 a.m.

opportunity to dive into your final

project well really continue diving into

your final project ideally at that point

uh alongside classmates perhaps your

project Partners if you're working

collaboratively

uh and awaiting you will be such

memories and excitement hopefully as

these even as you then turn your

attention back to your final project

there of course will be several meals

during the day culminating with 500 a.m.

uh shuttles to IHOP the local Pancake

Place uh if you are so awake at that

point or even if you get there uh this

is not an uncommon site as you might

recall from week zero and then lastly is

the cs-50 fair which is finally back

after a couple of years now of it not

being on campus and this will be an

opportunity for everyone to present

their final projects to passers by

classmates faculty and staff and really

just Delight in what it is you created

on your Mac your PC your phone in the

cloud or anywhere else and indeed it's

just going to be an opportunity to bring

your laptop to a shared space or your

phone and introduce your project to

passers by such as might appear and

ultimately celebrate what you all uh

accomplished and indeed will you be

handed at the cs50 fair your very own I

took cs50 t-shirt which I dare say I'm

still wearing all of these years later

and so you

to

we'll have that uh ahead of you as well

so for what's on the agenda today we

thought we would not only look back but

look forward but first we wanted to

thank so many of the team members that

have been helping both on stage and off

who've made this course and these

sections and so much more about cs50

possible of course um the building that

we are now in there's a whole team

downstairs in Memorial Hall who helps us

get set up and organized each day our

thanks to them there's the education

Support Services team who makes

everything look and sound so well down

here especially when we have all of the

more microphones as well our friends the

harbard crocodillos and the redcliff

pitches most recently and then of course

cs50's own team uh but butter cs50's own

favorite restaurant Chang show down the

road indeed if you find yourself in

Cambridge for the next one two 3 four

years or visiting from out of town uh do

pay a visit to our friends just down the

road and in fact we'll have our very

last cs50 lunch this Friday if you're

able locally to partake and then there

cs50's own team um both on stage and off

in thanks truly because not only do they

make the everything run so smoothly they

capture it for students here who might

not be physically present here for our

friends down in New Haven at Yale and

certainly for anyone online who might be

tuning in as well and then lastly wanted

to thank of course the huge team of your

classmates your peers that make cs50

possible in sections and office hours

tutorials and more allow me to share

with you the outtakes so that even we

the teaching staff sometimes struggle

with computer science here are some of

the clips that we captured when just

passing packets via tcpip a while back

you saw the finally the the nicely

polished version but here are if I may

if we could dim the lights are some of

the

outakes nothing

go buffering

okay Josh

nice

[Music]

Helen no oh wait

that was amazing

Josh uh um

[Music]

Sophie

amazing that was

perfect

B I think

I what

you oh nice

[Music]

guy that was amazing thank you all so

good indeed and that moment if we could

just one round of applause for everyone

who's helped out this

semester so back in week zero uh we

introduced you of course to this idea of

computational thinking which is to think

a a little more methodically a little

more algorithmically and by way of these

various languages hopefully that is

something you notice maybe not in the

moment but in the months and the years

to come that you do find that your

thoughts are indeed a little more

cleaned up and you're just able to

express yourself a little more precisely

and even spot illogic in someone else's

document or statements as well but at

the end of the day really a course like

this is also about critical thinking and

indeed rewind again to week zero when we

frame the entirety of computer science

is really just this like problem solving

and any problem in the world be it CS or

otherwise has some input and we decided

how to represent those inputs it needs

some output the solution there too and

then all of what you focused on doing

and learning and applying these past

several weeks is in that proverbial

black box which hopefully is not such an

abstraction anymore but is indeed

something that you know how to harness

and know what could be going on

underneath the hood even if it's in some

technology or some language that maybe

we ourselves didn't cover because a lot

of those first principles remain the

same now along the way we talked about

the quality of solutions to those

problems we happen to focus on

correctness just does it work design

which is a bit more qualitative and

subjective and then style the Aesthetics

of it all and these two are

characteristic maybe not with these same

words of just how you might write or

evaluate other creations in life be it

physical or written or the like so think

about too as you solve problems just how

you can sort of frame for yourself like

am I doing a good job or not by

quantizing it along these or perhaps

other axes as well and we thought we'd

highlight just two topics from that week

zero that have really been manifest for

the past several weeks namely

abstraction like taking complicated

things and ideas and trying to simplify

them so that we can sort of operate at

this level and like solve problems we

care about without getting into the

weeds of implementation detail so to

speak but there's this tension because

you know now from all of these different

languages that code is fairly

unforgiving I mean even omitting a

stupid semicolon sometimes breaks

everything and so Precision is sort of

at odd sometimes with this idea of

leveraging abstraction and so we thought

we would try to tease this apart

especially all these Weeks Later here uh

with a bit of a uh live demonstration so

on the way in you probably all received

a sheet of paper if not but someone near

you did just tear theirs in half and

borrow half a sheet if you can or any

piece of loose leaf paper or the like

will suffice so long as you have a pen

or pencil and for this allow me to

propose that we invite up maybe two

final cs50 volunteers this semester and

like a lot of hands are going up in this

okay a lot of hands how about I saw the

first hand there uh yes yes who's yes

who's pointing at herself now come on

down we just need the one hand for now

but oh wait oh wait uh you'll be our

number two well okay we have way too

many volunteers now no no please please

come down yes in the black shirt and if

you guys we will okay we'll do pair

programming in just a bit if you want to

hang out in the wings here we'll have

our second demonstration as well so okay

now maybe a round of applause for our

three

volunteers oh come on up first oh second

and third okay you come first we'll do

order no uh this is a q okay Q here okay

what's your name I'm Danny Danny okay

take this mic okay so we will DQ you

momentarily all right so Danny come on

over to the middle here and in a moment

I'm going to hand to Danny a sheet of

paper that has a picture on it and this

picture is going to be something that

I'd like you to verbally program the

audience to draw you can use any words

any abstractions any PR level of

precision that you want but you just

can't make hand gestures or sort of show

them what to draw But first you want to

tell us a little something about

yourself including everyone here I'm

daddy and I took ts50 okay wonderful

wonderful so I'm going to reveal the

picture only to Danny and if each of you

would like to take out that sheet of

paper and just make sure that no one

else can see this if you want to hold it

up this way everyone here is now holding

their pen or pencil and in some number

of steps give them a verbal algorithm

for drawing what you see and you can say

anything you want but no gestures okay

so you're going to want to

draw a square in the center of the paper

with the

diagonal pointing to

the center of the

edge wait no actually I scratched that

draw a rbus in the center of your

paper and for those who forget what a

rhombus is for um a d

Diamond a a square that's on its

side and then from the bottom vertex

draw a straight line down but not all

the way to the edge of the

paper okay and then keep your pencil or

pen at that point and you're going to

want to draw a line that's parallel to

the line of the original rumus to the

right and then keep your pencil or pen

at that point and draw a line straight

up connecting to the side

vertex yes and then go back to the line

that you drew from the bottom vertex to

the bottom of the paper and then draw a

line parallel to the left edge of the

rumus and then then keep your L your

paper your pencil at that point and draw

a line up to the vertex of the rumus

again the end the end all right well

thank you to Danny hang on to your paper

thank you so much and if you want to

step off to the stage there we will

reveal thank you a round of applause if

we could for Danny that is not an easy

task I'm sure and if Carter wouldn't

mind just grabbing a few samples here

let's actually take a look on the

overhead if we could I'm going to pop

down over here real fast we don't need

to collect them all but if you're

feeling either good or bad with what you

drew happy to collect a few of them

okay okay thank you thank

you okay hope you won't mind if I can't

reach everyone just a couple more okay

over here okay all right that's that's

okay this one's really funny okay I'm

going to go with this one if I may and

Carter has some

too

okay thank you so much okay so just a

random assortment here let me turn on a

camera so I can show you what I see here

for instance is one classmate drawing

which might resemble perhaps what you

drew here uh here is the beginnings of a

house it

seems nice this

one okay this one is larger

and how about a couple of others that

were getting closer I think okay so more

edges and vertices there this one seems

a little similar in

spirit if not

proportional this is uh Zach's the best

one but it turns out if I may Zach

you're not all that far off here Denny

is what you were reciting to everyone

algorithmically indeed it was this here

Cube and so Danny can you come on back

up for a

moment so if you'd like to share for

just a moment like what were some of the

thoughts going through your head and why

did you choose the words that you did

okay so what was going through my head

when I saw the cube um I didn't know if

I could say draw Cube so I decided to

start with the top and so draw a Remus

in the center of your paper and then

draw a line down and just like do the

first part then the second part then the

third part and then you would get a cube

like Zach yeah and so had you said and

you could have said draw a cube which

would be more of an abstraction even

that's not necessarily sufficiently

precise right because you don't

necessarily know what the orientation of

that cube is the size of it the

positioning on the paper so you instead

took a lower level approach which is not

unlike scratch if you think about

scratch being able to move up down left

right turn 90 degrees turn 15 degrees

and the like I mean that is ultimately

how a lot of graphical programs and

games even might be implemented by

really focusing at the level you do but

of course there's this tension with us

humans whereby we prefer often to think

at this level but even that might not be

sufficient which is to say here in week

10 like these are still going to be in

our hard problems uh but a hand if we

could one more time for Danny for

getting us that far along thank you so

much let me give you a stress ball here

all right and if we could have both of

our volunteers come up here we're going

to have yes come on up come on up and

let me uh have you guys introduce

yourselves in the middle

here hi I'm

hey there I'm sadik from Turkey nice to

meet you all wonderful welcome and this

time we're going to flip it around so as

to have the audience do what Danny just

did for us the only catch here is that

the only means we have for showing the

audience what they need to tell you to

draw is like literally right above the

chalkboard so on our system here that

your eyes must stay on the chalkboard

and not look up and in just a moment if

you guys want to both stand in front of

the chalkboard back to the audience and

as you're talking with each other other

verbalize it through the microphone if

you will I'm going to show everyone else

in the room a second and final drawing

and we'll just go rapid fire around the

room give us one step at a time

collectively and we'll see if these guys

can't draw exactly that same outcome so

is there another chunk what's that is

there another chunk or just just the one

so you'll have to collaborate and let's

give you a clean slate here

literally all right so no looking up

that's the only rule for you guys here

we go for the audience here is what we'd

like them ironically to

draw step one from anyone in the

audience yes draw a circle draw a circle

anywhere not anywhere not

anywhere okay that's step one step two

someone else yeah in the

middle

draw a line down from the bottom of the

circle about halfway down I think there

was a hand in front of you too number

three

[Music]

okay okay the overarching goal here for

those unable here is to draw a person

fig okay it may be a SI figure draw the

left leg of the person of this person

okay good job all right next step four

yeah

of line up Circle

okay the left oh sorry to the right draw

a v okay to the right of the vertex at

the bottom of the circle draw a v draw a

v um like what V nope not interactive

draw a v um no no

well yeah it seems Weir

weird let's get ready for maybe

something like step

five okay we'll go with that step five

someone else step five someone else

someone else

yeah draw the right side of the leg okay

nice step six step six happy face six

six yes erase the line

erase the line that you have on the left

on the

okay okay step seven yes instead of like

that line that was before going up make

it go down instead of that line before

going up make it go down mhm okay step

eight step eight step eight yes connect

that line to the hip connect that line

to the hip not like not touching not

touching something like this

maybe okay compromise not touching okay

not touching okay all right step nine

almost there I think step

nine step nine step nine yes in

back uh write the word high on the top

left of the circle

here okay

and step 10 almost there line draw a

line pointing to high so like a spee

bubble basically

yeah okay and step uh 10

11

yeah erase the exclamation point nice 12

do we want to give them one

more 12 or we good yeah last one erase

the erase erace the right arm okay I

think we're going to need a 13

then and

then yeah

repat repeat the left arm but rotate it

by

90° that feels

wrong wait how would you like as an

organic human being how put your

arms like would you put your would you

ever structure your arm like that that

would not be a stick

figure CU would you do this or would you

do that or a little hint maybe get a get

a give me a step 14 step 14 and final

step

14 I think we just got to tell them what

to do step 14

yes think of a walking man

and and have the left the right hand

walking to your right all right so like

it's like where could the hand go where

should the hand go on that

arm but yeah yeah okay yes

no yeah I mean look look look like right

here look right here look right I yes

sorry thank you 14

St that's pretty close so

congratulations to you guys and thank

you as well all right so I mean these

these things too are not yes Round of

Applause then sure

so so this is to say that these ideas of

abstraction and precision and really

every other term of art that we explored

this term are sort of omnipresent and

can be easier or harder to implement

depending on exactly what the problem is

but what we thought we' do now in our uh

final day is try to now similarly

prepare prepare you for life after cs50

and this is really going to be a list of

really potential to-dos so that you can

stand on your own after the class after

the class's infrastructure write actual

code and then we'll come full circle one

final time with our friend Jennifer 8

Lee to look at the world of emojis and

how they relate to all forms of

representation that we've talked to uh

talked about up until now so one how can

you go about programming after cs50 so

one you can actually install command

line tools on your own Mac or PC perhaps

unbeknownst to you Windows has what's

generally called a command prompt Mac OS

literally comes with a terminal program

in your applications utilities folder

and so even if you've never run those

programs you've actually had a sort of

blinking cursor black and white prompt

available to you might not have all of

the same software installed as your code

space in the cloud but you have that

command line interface even within

today's graphical tools and among the

tools you can install within that

command line interface would be

something called xcode on the Mac which

comes not only with a guey IDE

integrated development environment but

also those command line tools and

Microsoft for Windows has something

similar as well learning git so we've

used git only unbeknownst to you

underneath the hood for the most part

but git is a very very popular tool if

challenging to pick up for the first

time that makes it easy to push code to

a website called GitHub or any

equivalent and then collaborate more

effectively with classmates there's

definitely a bit of a learning curve but

thanks to cs50's own Brian youu you can

start for instance with a video like

this and this indeed is going to be one

of these deao standards in the real

world at least for the next several

years that you'll probably encounter if

you work in Tech or really any company

where you're doing some programming vs

code itself will walk you through this

process in the coming days but you can

indeed install it on your own Mac or PC

and what can you do when you write code

well you can certainly write software

for your Mac for your PC for your phone

or of course per week 10 uh week nine

you can host uh your own website be it

static as in week uh week eight um

hosting it at websites like these which

gener generally have free or

student-friendly accounts via which you

can put something statically on the web

at a real domain name that you might

choose or you can host a full-fledged

web app and using uh student tiers on

Amazon and Microsoft and Google's cloud

services or others you can sign up for

being a student certainly a whole lot of

free software free hosting so as to if

nothing else um experiment and uh

perhaps maximally get your own app or

website up and running so know that

those are resources available to you and

this is by uh certainly a non-exhaustive

list if you'd like to geek out in the

coming months in the coming years these

are just some of the places that people

who take computer science classes who

write code might tend to hang out and

ask and answer questions of each other

um so keep an eye for instance on these

here and then cs50 has its own

communities as you'll see if you go to

this URL here via the open coreware

version of cs50 which is open to the

world do uh is there a Vibrant Community

uh thanks to time zones that's pretty

much active 247 365 talking about not

only cs50 going on in problem sets and

projects but really technology more

generally as well so certainly feel

welcome to partake either asking or

answering questions now in speaking of

asking and answering questions a couple

of weeks ago you kindly gave us a whole

bunch of review questions which we

called through and picked out our

favorite 20 of them these of course were

multiple choice questions and in

preparation for this week uh in

preparation for life ahead we thought we

would choreograph a bit of a a quiz show

here and ceed as you came in at the

start of class you might recall being

invited to go to this URL here

cs50. either here in person or if you're

watching live from home at this URL here

you can use a phone or a laptop and if

it's easier on a phone you can point

your camera at this 2D barcode here

we'll give folks a moment to pull that

up and again that URL was cs50.

l/p

and once it looks like most folks have

it up and running our friend Carter here

will help us dive into this uh review

session if you will with a bit of fun

along the way all right Carter if you'd

like to take it away what do we have as

our first question you should see on

your phone or laptop this same question

being asked the first question is how do

you print quote un quote hello world in

Python so among the possible answers are

these here

buzz in on your phone or your

laptop we've got a few hundred responses

are

ready 7 Seconds to make your

decision this is question one of 20 go

to it with some confidence I think we're

down to zero on the clock and Carter it

looks like 98% of you uh indeed said

hello world and Carter per the check

mark That's indeed the correct answer

here now to make things interesting in

know that you'll see some number of

points and we've deliberately anonymized

it so only you know what number you are

so a whole lot of guests have a perfect

score of 1,000 at the moment hopefully

we'll see over the next several

questions things start to bridge out uh

but know that the speed with which you

buzz in will also factor into how many

points you now get So the faster you

move the more points you get question

two if we could what does DNS stand for

from just a couple of weeks back domain

number system domain name system data

numbering structure or there's no such

thing as

DNS few hundred responses are

in 8 seconds

remain fewer points now but still a

chance to buzz in and now as we hit zero

the responses are these domain name

system which is indeed correct and 84%

of you got that one correctly and indeed

exists we talked about it a couple of

weeks ago so we're still seeing a whole

lot lot of ties at 2,000 we'll see if

someone starts to pull away before long

question three what is the upper bound

of merge sorts

runtime so that escalated

quickly Big O of n log n Big O of log n

Theta or Omega of log n or Big O of

one what is the upper bound of merge

sorts

runtime that was the last of the

algorithms we solve for sorting and in 1

second we'll see that the correct answer

is is just edging out everyone else

indeed 46% it is n I know it's n log now

if I may as the teacher it can't be

login because login is strictly less

than n and you can't possibly sort n

elements unless you minimally look at or

touch each of them so it's got to be at

least greater than n intuitively we

still have a whole bunch of ties let's

move on to number four what is stored in

ARG

C back to the language C is it in Array

of arguments the maximum size of an

array the count of arguments given to a

program when first run or how much

memory is allocated to a

function again you wrote all of these

questions and we have 5 seconds for the

reveal ARG C

is indeed the count of arguments given

to a program when first run think back

to C when we did command line arguments

there was argc and arv argv was the

array but Arc was indeed the count the

CN AR save all right we still have a

whole bunch of ties at the top here but

let's move on then to number five what

is the duck debugger favorite hobby

according to one of your classmates

dressing up like Dracula swimming across

the stage filling up the entire bathroom

of a guy's house and sitting quietly on

stage the third of course is a reference

to a YouTube video that was on the

course's website that week but according

to your class mate number two seconds

remaining sitting quietly on stage is

its favorite pastime so a little harder

perhaps than the

others 5,000 now we're starting to see

some spread so we only have six six

guests in contention for first place and

the next question now is six what is the

function used to open a file in

C F open open file open

file

what is the function used to open a file

in

C 7

Seconds there's some differences between

C and python here and the reveal it is

indeed fop it's 77% correct too all

right let's see the rankings now if you

are guest 15 9715 6171 3753 or 3273

you're now in the lead as we move on on

to se question seven how does Sterling

compute the average sorry how does

Sterling compute the length of a string

in C it looks at how much memory the

string uses it counts the number of

characters until it reaches back0 it

counts the number of bits in the string

or it creates pointers for each

character and counts

them 10 seconds Sterling in

C recall that we implemented this

ourselves in class but then we used the

library thereafter and in indeed with

85% it simply counts the number of

characters until it reaches that

Sentinel back sl0 AKA null and in this

case we

have five four four of you tied now for

first all right question eight where

does Malo allocate memory

from the stack the Heap the pointers or

the

temp where does malok allocate memory

from responses are coming in 8

seconds a good review question at that

in two seconds we'll see that malok

allocates memory from woo close one the

Heap is correct the Heap is correct the

stack recall is where functions store

their local variables and their

arguments and that just happens

automatically the Heap represented in

our pictures up top is where malok draws

from now we have guest

15 has

made its way to the top here but others

can catch up if they don't buzz in fast

enough so number nine how many people

flew from 50v to New York on the day of

the

crime 16 29 8 or

three anyone with a laptop perhaps has

an advantage

here 5 Seconds

and the answers are but the answer is

16 let's see if guest 15 got

this they did not goodbye to guest 15 at

the top all right question 10 we're

about halfway there what are meta tags

used for in

HTML to describe a web page to Define

parameters for an element to group

elements together to translate content

into machine readable format

at what are meta tags used for in HTML

we saw a few of

them for different use

cases and with one second we see that

indeed the number one answer was to

describe the web page be it for a mobile

device be it for screen scrapers like

Facebook and Twitter and other such apps

that grab images and descriptions

thereof all right we're in the second

half now guest nine guest four 669 has

edged ahead guess 15 is now in sixth

place all right number 11 is how do you

find the address of a variable in C

think back a few weeks star dollar sign

Ampersand or

ask from one of your own classmates how

do you find the address of a variable in

C and the number one answer is ersan

which is indeed the address of operator

at 62% nicely done let's see who's the

top of the list now guess 4669 has

retained their lead so we move on to 12

what does the arrow operator mean in C a

hyphen and a greater than sign nothing

starts a comment replaces a star and Dot

operator declares a

pointer what does this Arrow operator

mean in

C

again from a few weeks back 3 seconds

harder assortment perhaps and it's oh

replaces a star and Dot operator the

number two answer was indeed correct

this was just a cleaner way syntactic

sugar for collapsing what would be a

star and then some parentheses and then

a DOT into quite simply something that

looks like an arrow itself all right

Carter who's in the lead now still that

same guest and let's see what 13 has for

us which of these is not a data type in

SQL light blob string integer

text we used a few of these more

commonly than others but not all of

these are for

real 5 Seconds to make your

decision and the results are blob is a

thing string is not in SQL light it's of

course called text as we've seen it blob

as goofy as it sounds is just binary

large object but indeed it's how you

might store a binary file in your

database all right the rankings now oh

guest 8444 has Eed ahead so we move on

to 14 which of the following is a valid

way to print Exclamation point

Exclamation point Exclamation point

Exclamation point in

Python I'll let you read these

yourselves

which is a valid

way everyone got quiet in is

thinking all right 6 seconds few hundred

responses in so

far all right and yes nicely done 78% is

correct you can use the star operator to

essentially multiply the character at

left all right who's in the lead Carter

we now have still G guest

8444 and so we move on now to 15 what

does the free function do D allocates

memory from A Primitive deallocates

memory at the given pointer terminates a

loop or returns a value from the

function what does the free function

do all right 5 Seconds what does the

free function do call it's the opposite

essentially of malok and it deallocates

the memory at the given pointer as well

and in C that's on you in Python you

don't need to worry about allocating or

freeing aik all right we now have at the

top still guest 8444 and seeing more and

more spread 16 which is not a step of

compiling think back now to week two

compiling pre-processing linking or

threading which is not a step of

compiling we use it as a catchall but it

technically means a few different things

collectively all right and threading is

indeed not on the list that is a

technical thing generally meaning a

program can do multiple things at once

but that is not related here to

compiling all right guest 8444 is still

at the top we have just a few questions

left and so 17 what was the surprise at

the beginning of the Halloween lecture

according to your classmates someone

scared me someone hid candy under every

chair someone dressed up as me the

entire staff dressed up as

Carter interesting litmus test of who

came to or watch lecture

perhaps let's see which guests got this

correct someone someone's really dressed

up as indeed me at 64% so I think we

have attendance here essentially from

that lecture so let's move on now to the

final few questions after seeing that

guest a 8444 8444 is still doing well

can anyone Dethrone them why is it

incorrect to use the equals equals

operator in C to compare

strings it's computationally inefficient

you comparing the locations of the

strings strings don't exist using equal

equals will lead to buffer

overflow why is it incorrect to use

equal equals in C to compare strings 3

seconds we saw this live and it

motivated like an entire week because

you're comparing the locations that is

the addresses in memory of those same

strings all right let's see if this

leaves the rankings the same guest 8444

is still at the top and pulling ahead

two final questions 19 what is the

difference between null one L and null 2

L's they null and null mean the same

thing nulls refers to back sl0 whereas

null to L's is the zero address null is

the zero address whereas null to L's

refers to back sl0 null is null but

lazier 5

Seconds subtle not the best design

perhaps to have in technical terms but

indeed 62% of you got that NL is the

first thing we talked about when we

talked about back sl0 and N is a pointer

it's the zero pointer same thing same

number but different context all right

Carter guest 8444 is the person to beat

with our final 20th questions what do

the binary bulbs on stage spell

today and these are your four choices

different from usual we usually use 8

bit asky today we are using utf8 which

is a form of Unicode which is the larger

subset that uses one or two or three or

even four bites to spell a single

character and the answer wow close close

is indeed a cupcake indeed a cupcake

well done and let's see the final

results 8444 is the winner are they here

in person perhaps

8444 you're 844 come on

[Applause]

down thank you here you go

congratulations oh you're all right so

today if we

may give me just one

moment all

right all right so today we are so

pleased to be joined by uh Jennifer 8

Lee who's an of the college a dear

friend and is actually really the reason

why there's evidence of Muppets in cs50

in fact some years ago I was visiting

her and she had on her shelf like this

custom Muppet it wasn't one that appears

on TV but she had somehow gone on a

website former toy store called FAO

Schwarz at the time and you're allowed

to configure your own Muppet whatnot

choose the eyes the nose the face and

the Torso and I just thought this was

the coolest thing and so in the taxi on

the way home I was like going on the

website trying to purchase our very

first Muppet I then woke up the next

morning thinking why did I just buy a

puppet in the back of a taxi and so it

sat on the shelf for really 2 years and

then a colleague of mine within cs50's

team decided after I brought it into the

office to sit on a shelf there to

actually bring it to life and indeed if

you Google around cs50 Muppet and

puppetry online you'll see in fact these

as characters not only over the past

couple of years in coid times when

really there was next to no one actually

here and so they were instead um but

indeed she's brought not only this this

educational element this pedagogical

element this playful element El to cs50

and we have her here today to speak to

exactly the sorts of encodings that are

here on stage Jenny is the former Vice

chair of the Unicode subcommittee on

emoji which is to say that she and her

colleagues have been influential in

taking emoji from what was a very

limited character set early on and by

far unrepresentative of much uh human

emotion in speech into really an

initiative now to capture digitally all

of the world's languages past present

and future as well as the range of

emotions that might see here in the form

of that pillow or even in the cake that

awaits so allow me to introduce Jennifer

8 Lee thank

[Music]

you I much drink okay clicker hi all

right hold on I have to hide my drinks I

might need more water all right um I'm

really excited to speak to speak here

last time last year I was here uh one I

was wearing a mask which is like a real

bummer if you're lecturing and then the

entire like front part was all Muppet so

I'm really happy to see humans actually

um and it's always an honor to speak at

Sanders and and then Dave and I were

actually classmates way back when so I

do remember him when he was an undergrad

much like you so I um I'm going to give

a talk on the world of emoji and how I

kind of became an emoji activist so it

kind of all starts um with my friend e

inl who is a designer who's well known

for doing the Twitter fail whale which

was kind of laid to rest except now this

week it seems like it may be like may be

necessary again um so she and I one day

were texting this is back in 2015 we're

texting about dumplings because we are

Chinese is women and we like like to

text about food and so I sent her this

picture of dumplings she was like yum

yum yum yum yum yum yum yum um and then

she was like oh Apple doesn't have a

dumpling emoji and I was like oh that's

kind of interesting and didn't really

think anything about it because like you

know people Point things out to you all

the time and then you just like forget

you just like move on but then half an

hour later um on my phone appears this

like dumpling with hard eyes and you

don't see it in in because it's a still

shot but it actually had like blinking

eyes so she liked to call it like bling

bling dumpling so she as a designer had

decided to like go in and like make her

own dumpling Emoji um because she was

like I'm a designer I can fix it but

that actually got me thinking I was like

where do Emoji come from and like how is

there not a dumpling Emoji cuz from my

perspective dumplings are this kind of

universal food right so and there are a

lot of Japanese Foods

on the emoji keyboard and I I was not

this was like back in 2015 I was like

not a big Emoji user like at all so I

mean you have things like Ramen you have

Bento boxes you have Curry you have

tempura you even have like kind of

obscure um kind of foods like this thing

things on a stick turns out to be fish

fish things on a stick then this pink

and white white swirly thing is also a

fish thing and there's even like that

triangle rice ball that looks like it's

had a bikini wax all well represented on

the Emoji Keyboard but no dumplings and

it's very strange cuz like all cultures

kind of have their dumpling right

whether or not it's um kinali or ravioli

or aanas like essentially everyone sort

of like discover the idea of like um

yummy goodness inside a carbo hydrate

carbohydrate shell whether not baked or

fried or um steamed so I was like okay I

literally Google I was like who controls

emoji and you discover that they're

actually regulated by a nonprofit called

the Unicode Consortium and um it is you

know and I just like went on their

website and I discovered that they had

12 full voting members as of 2015 so

this is 2015 and they were like mostly

us multinational tech companies it was

Oracle it was IBM Microsoft Adobe Google

Apple Facebook and Yahoo and of the

three that were not multinational US

tech companies they were let's see a

German company called sap a Chinese

Telecom company called Huawei and then

like the government of Oman like those

were basically the 12 full voting

members of the US multi-national tech

companies so they at that point paid um

$118,000 a year to have full voting

power on the you know Unicode committee

and I was like oh that's a that's a lot

of money and I kind of felt indignant

about this and uh but then like if you

kind of keep on digging on their website

you found like there was this kind of

interesting loophole which is you could

join as an individual for

$75 um you don't get voting power but it

gave you the right to put yourself on

the email list and also to like attend

the quarterly Unicode meeting so I was

like I'll do that I had no idea what I

was doing but I'm like I'm going to go

fight um for this dumpling Emoji because

from my perspective dumplings are

Universal Emoji are kind of universal so

uh the fact there was no dumpling Emoji

meant like something was wrong in the

universe and I was I was determined to

fix this so um you know I was on this

email list and then a couple of like

maybe even like a couple weeks later I

got you know they they they kind of sent

out this note that's like hey who's

coming to the quarterly meeting and I

was like um I looked at the calendar I

looked at my schedule I was like oh I'll

be in like you know Silicon Valley that

time so I basically like rsvpd and I was

like I will be there and took um cow

train to an Apple building it's a legal

building in I think it was sunny Veil so

I just like show up and I don't know

what I was sort of expecting like with

like you know the Unicode I think maybe

thought it was going to be like a baby

Congress like you know like with a

little you know very formal seats people

with gabbles um that is not what I found

uh basically it is a it was a conference

room full of people who skewed wider

skewed older skewed Mal skewed engineers

and this is basically the room where it

happen so this is 2015 these were the

people who decided your Emoji um all

very nice and um there was you know one

one even had a daughter who had a sense

of humor and made him a shirt that said

shadowy Emoji Overlord um so I just kind

of listen to them debate things like

milk emoji and beans emoji and it just

seemed like not quite right to me that

like it would be uh This Global visual

language that were basically decided

decided by like a a small group of

people inside a conference room in

Silicon Valley so I decided to former

group called Emoji nation whose motto is

like Emoji by the people for the people

and it basically advocates for more kind

of Representative inclusive Emoji um you

know we we we we start with a

Kickstarter campaign uh dumpling Emoji

process trying to like you know write

the wrong in this world and uh made this

little cute video sort of advocating

dink for one of the most universal

crosscultural Foods in the world Georgia

has Kali Japan has giosa Korea has Mandu

Italy has ravioli Poland has barog

Russia has pelman Argentina has

empanadas Jewish people have kler China

has pot stickers Nepal and Tibet have

Momos yet somehow despite their

popularity there is no dumpling Emoji in

the standard set why is that emoji

exists for pizza tempora Sushi spaghetti

hot dog and now tacos which Taco Bell

takes credit for we need to write this

disparity dumplings are Global Emoji a

global isn't it time we brought them

together

oh yeah and while we're at it how about

an emoji for Chinese

takeout so uh I did put together a

dumpling Emoji proposal I wrote this uh

I remember Thanksgiving Day 2015 on a

plane and uh actually and we got it past

basically dumpling takeout box

Chopsticks and uh fortune cookie I have

to say I don't think fortune cookie

would have made it on its own merits but

it kind of like slid in on the clo tails

of the other ones um and so these were

the proposals as we submitted them and

then these are the ones that kind of uh

exist now on the Apple keyboard and I

have to say the dumpling looks really

really realistic um oddly realistic and

whereas like the fortune cookie is think

it's like a big fail because first of

all there's like it has no it has no Gap

it looks like a dead 3D Pac-Man so I'm

very

disappointed in uh the manifestation of

that but that's okay that's okay um and

so it's kind of interesting like what is

the process of getting an emoji um ped

and I will sort of walk you through it

so first of all you come up with your

idea right and then you kind of write

this proposal and then you submit it to

the Unicode Emoji subcommittee who then

kind of gives you comments and then

sends it back to you and you kind of go

around and around in the circle um and

when so these are things that we

consider so somewhere in there I also

like fought my way on to the Emoji

committee and then also became a vice

chair became a vice chair sort of a

extracurricular that's like completely

run a muck in my life um um so things

that matter uh popular demand is a

frequently requested Emoji um multiple

usages usages and meaning so that's

actually kind of very important for

something like you know certain animals

have meaning so we did you know sloth a

while ago and that also has not only the

literal meaning but sort of like um like

um connotations there are visually

distinctiveness it can be recognized so

this was a blocker for one of the uh

emojis that I have worked on before

which is cave which is kind of hard to

do I think you know meaning wise is

pretty good but it's really hard to get

a cave down in emoji sizes and then it

filling a gap completeness so a good

example for that is like for some reason

for many years we had Red Heart yellow

heart green heart blue heart purple

heart and uh no orange heart so so

somewhat so you know you would do the

rainbow but people would substitute the

pumpkin in so You' like you know have a

have a rainbow heart thing with a little

pumpkin stuck in the middle so orange

heart obviously should be added and give

a sense of completeness um and then

something else is existing vendor cap

cap compatibility and so a good example

for that was um many years ago what's

app decided to uh add the gender

non-binary emoji and then once it did

that then all the other kind of vendors

um jumped on um so what kind of knocks

out an emoji so too specific or narrow

so we'll often see that with like very

specific animals or a very specific

group it's redundant so one

year oh my God who makes that Butterball

Butterball makes the turkeys Butterball

submitted um a an emoji proposal that

was like a cooked turkey but we already

had a live turkey so it seem kind of

redundant have both like a cooked turkey

for Thanksgiving and a live turkey so so

not visually discernible um this is a

struggle for things like I don't my

friends have kind of proposed

kimchi kimchi is is really hard on emoji

sizes for many reasons and part of that

but part of that tension is because it's

not visually discernable then there are

no logos Brands deities or celebrities

so no Nike swish no McDonald's M and

then this is one that we kind of decided

in the last uh year or so which is no

more flags flags are a very complicated

thing um and as a result Unico does not

want to be in the business of deciding

what is a country or not a country uh so

like you know when you get a proposal

from like Kurdistan you're like yeah so

right now the way that the Emoji flags

are decided is they kind of depend on

what the UN recognizes and then those

get passed down to the international

standards organization and then and then

Uno just does that like it does not want

to be in the business of kind of you

know geopolitical

Affairs um so once it comes out of the

subcommittee it goes to the full Unicode

technical committee UTC those were the

people that were in the room that I

showed you um and they vote once a year

basically to pass all the Emoji and

takes a while there's a lot of like

coordinating with the international

standards organization and like a lot of

working with the companies and

eventually it takes a long time but it

ends up on your on your devices and that

historically is taken about 18 to 24

months from when you put in the propose

to when you get it in it's going to be a

little bit longer um going forward

probably is my sense as we were're going

to Unicode has decided to sort of space

out some of the Unicode releases so yeah

Emoji Nation you know kind of has done

its thing so of the weird things is like

how did Unicode this like kind of

nonprofit organization based in Mountain

View California end up controlling this

like Global visual language um so a lot

of it has to start with has has to do

with the fact that emoji started in

Japan uh back in the late 1990s one of

the this set from dok 1999 is considered

is widely considered sort of like the

first like color Emoji set it has been

collected by the Museum of Modern Art

and um so these the Japanese telecom

companies would use

basically would have their own sets of

emoji and then they were different

companies so they would have different

sets so you could basically only send

these like visual characters with

someone who is on your same carrier so

it's like it's like basically equivalent

of if you were on like Verizon you can

only text people on Verizon with like

Emoji or if you were on um T-Mobile you

could only do that so at a certain point

they um decided they were they the Apple

and Google came into Japan they wanted

to start selling smartphones and they

realized that it was a hodgepodge of

systems and they wanted to unify it and

so in 2007 they went to unic code and

they're like okay help us unify the

Emoji um like kind of like basically all

the Emojis so that we have one standard

system and part of the reason is why

Unicode because Unicode basically has

this mission is to enable everyone

speaking every language on Earth to be

able to use your languages on computer

and smartphone so it basically unifies

um all written languages into one

ginormous set and that was not the case

actually when I was uh growing up there

was a point where like if you you were

Japanese on Apple that would be

different than Japanese on um dos or

like Chinese or Arabic so it drove

everyone crazy and they basically

decided um around the late 80s early

1990s that they were going to come up

with one standardized system that sort

of encoded all characters in in one

ginormous set so um there's three main

projects for Unicode if you care so one

is encoding characters including Emoji

now they're about 100,000 characters

aside assigned so that includes like

like Chinese Japanese Korean uh Arabic

cilic um actually all the hieroglyphics

all of the Emoji um a lot of things like

the Bitcoin symbol or like copy left or

whatever those all assigned were about

100,000 uh characters even those

languages that are basically out of use

so the other thing it does it creates

um localization resources so things

things so that you know like oh if

you're in this country this is this is

uh you're using the Euro or you're using

the pesos or something so there's a lot

of localization that is that data that

is needed depending on which um

geography you're using your device from

or like that you know you you know that

the time is used this way or the dates

are shown that way so that is called um

the common local data repository or cldr

as they call it and the other thing they

do is they kind of maintain libraries

for developers in order to do all um to

kind of so they are not building

everything from scratch and that is

called ICU so it's very funny cuz like

it's called cldr and then one day one of

our friends girlfriends made him this

thing which she called a seal deer

because whenever he talked about

cldr he thought he thought she thought

he was talking about seal deers so now

this is his little like mascot for like

Unicode related things so what is um how

does Unicode do this so it basically

asss code points and each code point is

a unique number assigned to each Unicode

character so uh face with tears of joy

can be written like this with a code

point or it can be written like this in

terms of decimal numbers and it can also

be written this in binary numbers so

these are all basically the same and the

key thing to know at least about Emoji

is that when your device if you're

talking your iPhone with um someone

who's on a laptop or an iPad or like um

an Android device it's not sending the

images back and forth it's sending just

a number back and forth and then locally

your phone or like your laptop decides

like oh this number correlates with

which image um in terms of our emoji

font and then pulls it up so this is

really key to know why different Emoji

look different on different

platforms so 2007 to 2010 it took about

three years but Unicode 6.0 came out

with a first our first little baby set

of emoji um and it just kind of hung out

there for a year like no one it wasn't

doing anything so 2011 though Apple

starts adding the emoji keyboard and it

it just like explodes like I feel like

in some ways Emoji are were not like

invented they were discovered they like

obviously touch something very very

Primal to um to like our human desire to

like communicate in like little colorful

glyphs on electronic devices and you

kind of have what's kind of really

interesting is like the ambiguity that

comes with what emoji kind of mean and

so one of the you know this one my

favorite emoji is sort of like an upside

down smiley face very very ambiguous

clearly very ambiguous because if you

start typing into Google like the top

hits are like what does it mean from a

guy what does it mean from a girl like

it's clearly something that a lot of

people are are using in like complex

situationships between each other so um

so so one of the fun things is who can

propose Emoji technically anyone can

anyone here can normal normal humans can

we have basically Google doc uh or

Google form that we throw up uh

historically between August sorry April

and August in the last two years um and

uh so this is one of our my our favorite

examples this is Ru aumed she was a

15-year-old Saudi Arabian girl who was

living in Vienna at the time that she

proposed the um hijab emoji and then you

know then she was like Time Magazine

like you know coolest teens she got like

a whole bunch of different things she

got into Harvard and Stanford and she

went to Stanford um and uh so this is a

PR we got then there was a group of

folks from Argentina who got the mate

Emoji kind of they you know similar

their national drink then we worked with

um there was a a nonprofit that really

wanted to like get a menstruation emoji

and so what they proposed to begin with

was like bloody underwear and I was just

like no no no so many different reasons

why this is a terrible Emoji uh but so

we did get blood drops so then you can

do Moon and blood drop or underwear and

blood drop there's a lot of different

ways actually it's really funny cuz I

was ping my friends beforehand what they

would use to indicate menstruation

before there you know there was a blood

drop so there was like red wine there

was like kind of that Rose um with the

like falling petal and then my favorite

is actually um my friend who used a

Japanese flag as a way to indicate that

she was having her

period so um one of the biggest

contributors this uh skin tone Emoji

appeared I think in 2015 it was amazing

it is and it was proposed by a mom

Katrina parrot she is a entrepreneur and

a mom who is just like at home one day

and her daughter comes home and is

like you know I wish there were emoji

that looked like me and her mom was like

that's great honey what's an emoji and

so she like me I guess Googled and just

figured out that uh Unicode controlled

emoji and she just came up with a

proposal saying we should not only have

the yellow you know skin tones at that

time everything was sort of Simpsons

yellow um it's really interesting to see

how race and like nationality are like

depicted in different parts of the world

so originally in Japan everyone was

yellow um but this these were the non

you know by default everyone was just

like like you know human or Japanese but

they had like a couple things that were

like not like one was you had a blonde

person so there's an emoji called like

blonde blonde woman or whatever that

represents all westerners okay so that

was one and then they have one that's

like an Indian guy with a turban so

that's supposed to represent Indian

people and then there's like a like a

guy with a little like um little hat

that's supposed to represent like

Chinese people so that was like that was

the view Japanese view of race which was

like default then you were I then you

were like blonde Western Chinese or

Indian and and that is all there was and

um obviously in the United States we

care a lot about race and then so she

came up with this system with five skin

tones um like just like normal people

some guy in Germany decided that he

wanted to do a a a face with one eyebrow

raised or as we call it the coar Emoji

um and oh this one's fun so woman's flat

shoe I have to say not highly used

statistically at this point but I really

kind of appreciate it because it was a

mom who was very offended that all

women's shoes had heels even the sandals

so this is her she had like three kids

at the time now is four she was very

fertile um and she also did um the women

women's flat okay she also did um um one

piece bathing suit because she was also

um offended by the fact that the only

kind of bathing suit you had was like

this like little itsy-bitsy polka dot

bikini thing which is not great if

you're like taking your six-year-old so

I have to say that got passed but like

it didn't go over like super well with

everyone um so you know unic Cod because

it's very public submits things for

comments and um we got this comment back

so one piece bathing suit why a person

want to indicate the use of swimwear

can't use existing bikini is this really

necessary what about a Victorian bathing

costume or a wet suit or water rings

this is like literally in in like uh the

records and like do not encode um and so

the person who did it's actually very

impressive he's actually the person who

created the middle finger emoji and

actually you ever seen the Vulcan Emoji

he he the Vulcan hand emoji he's

actually very active and I have to say

this is actually I think one of the more

impressive Emoji so so obviously

obviously we have a lot of active debate

um sometimes you get like whole

countries submitting so literally the

government of Finland as in like their

equivalent in the state department their

diplomacy kind of thing uh submitted a

proposal for SAA and um these were so

creepy and so

weird so much is wrong with this and but

I I I felt where they were coming from

first of all like they're naked they

have no they have club feet and so we

decided to help them we're like okay we

see where you're going with this let's

see if we can like help you like come up

you know like we had the spoon like

should there be like steam around them

like should they be naked or wearing a

tow towel it's like super like dicey but

we wanted to help them because it was

like literally a foreign government

coming from an Unicode advocating for

the sauna emoji on behalf of their ENT

country so then this is um is sort of

evolved into just basically person in

stey room which is which is the most

sort of like the PG version of sauna no

and there's no spoon they're all dressed

it's very odd but um so you can see the

evolution of what it started out what we

submitted and what it ended up so

there's a lot of like Evolution

throughout the entire process um and

like companies can submit Emoji

proposals too so Google actually worked

on this one I love this one okay so just

to give you some context as of 2015

there are many ways you could be or have

an occupation as a male on the Emoji

Keyboard right like like for example you

could be a police officer you could be a

detective you could be a Buckingham

Palace guard you could even be Santa

Claus like these are so many jobs that

you could have but if you were a woman

as of 2015 there were four things that

you could be you could be a princess you

could be a bride you could be a dancer

or you could be a Playboy Bunny these

were the sum total of all the

occupations that we can have so so we

there was sort of this movement at that

time there was like this like video on

YouTube that like went viral there was

like a New York Times op that was like

where are the women with professions so

um basically they came up with a set of

emoji for professions and what's nice is

not only did women have these

professions now men have them too so of

emoji Nation Emoji these are some of the

ones that we've worked on I think about

130 of the Emoji on your keyboard

probably came through touched our system

in in some way including I I have to say

uh microbe or virus I think I have the

opinion that every Emoji has its day

right like it might not be like today it

might not be next year but I have to say

virus was not doing anything then came

2020 and that was like such a good

moment for it um along with soap we had

also done soap so um you know among the

other emoji that we have worked on are

sari moon cake llama like uh teddy bear

there were no toys I felt really sad for

toys we have like giraffe um there was

Hut bubble te bubble tea was very

controversial actually I have to say it

kind we tried to slide it in originally

with the um takeout box and the

dumplings and people were not not having

it I understand that CU there's not a

lot like compared to like beer or wine

like bubble tea does not is not long

does not have a long history on this

planet um but I will say that um they

submitted again actually kind of

originally proposing that it was not

just bubble tea but but like a black

ball and milk and tea it was It was kind

of cool and I I have to say there was

definitely a generational divide between

like the Asian women who sit in that

room and are like this absolutely is a

thing that we consume like almost like

every week of our life and people who

are a little bit older who are like that

looks like a parfait how do you not know

that's a parfet and we're like we

absolutely know it is uh not a parfait

and so so it got in eventually so it

does sort of influence it kind of shows

like who is in the room influences you

know the decisions um that get made or

sometimes in the room now sometimes more

more likely in the zoom um I actually I

just say beaver Emoji if you see Beaver

Emoji that's one of one of the ones I'm

most proud of so that is actually

co-authored by a professor here at

Harvard um who is both lesbian and was

married to a woman from Canada so it was

very important to her to get um the

beaver Emoji passed and she promised me

it would always be the first line of her

bio so and indeed if you go to her

Twitter handle uh it's like Joan Donovan

creator of the Beaver Emoji comma is

headed research at the shoren scene

Center at the Harvard Kennedy School

It's Kind it's pretty impressive um and

then we did greens actually greens was

really interesting because people this

was also like a generational cultural

thing people were like why do we need

greens we have salad and I was like

we're Chinese we don't eat raw greens

cuz like you don't know where it's been

or if it's clean so we cook our greens

so salad is not something that we have

So eventually I got my greens and um so

that was kind of fun and then so these

are some of the people who sort of have

contributed to our little Emoji Nation

things including a number of Native

Americans who help get feather so why do

I care so part of it is because I'm

Chinese I grew up speaking Chinese and

English at the same time and it's really

interesting to see like Chinese and

English characters in terms of emoji and

Chinese together right so this is fire

you have like Fire characters and now

you have the Emoji things and it's kind

of It kind of shows that there is a

longevity in The Human Experience of

something that was designed 4,000 years

ago as the same visual cues in the human

experiences as now so mouth Tre tree

Moon Sun you can mix and match them

which is super fun um so you know two

trees kind of make a forest oops sorry

oh well okay then the um Moon and Sun

together means bright which I like um I

like this one so if you stop and you

think about this so this is a basically

a pig under a roof and what does that

mean

it does not mean Farm as you might think

it um it actually means home or family

so in the Chinese kind of structure and

outlook on The View it's like where you

keep your pigs is actually where your

home is and what your farm is um so it

gets kind of weird in all kinds of ways

so one of my favorite radicals so this

character means woman uh KN

and as I was learning Chinese you kind

of notice like how it shows up so so

this is a woman underneath a roof and

you're like oh it means mom or wife or

or like whatever like home um it does

not it means peace because things are at

home when the woman is or things are at

peace when the woman is at home

underneath a roof which I always thought

was a little bit odd um then there is

also uh woman plus child so you're like

oh and actually specifically Boy Child

the connotation there is a little

unclear so you're like a woman plus

child family mom you know whatever um it

is not it means good so the standard for

goodness in ancient China was a woman

who had a male child child so that kind

of kind of just like kind of irked me um

growing up and then you know three women

together means evil which is like very

like

MCB this character means greed this

character means slave this marriage

let's see I think this one is jealousy

and this means means adultery or

betrayal so like definitely not loving

the way women were P you know portrayed

um on the emoji keyboard so uh in case

you're wondering we just came out with a

kids book called had emoji that kind of

Compares emoji and uh Chinese and I

think I I think they sent a bunch of

books so that you guys can do some kind

of contest some like later on with cs50

um so but the mixing and matching is

really interesting right for example the

skin tones are actually the same yellow

character plus a layer of skin tone on

top of this so I kind of took my lessons

from Chinese in terms of seeing how

things can be binded so there's

something you should know about which is

zge this is also an invisible emoji

character it stands for zero with Joiner

and it was actually originally created

mostly for I think Arabic where you

would you would basically kind of force

something to have like be in the

beginning of of a word or an end of the

word by kind of having this like

invisible character so the rainbow flag

for example is actually a rainbow plus

the white flag

um and we we could have all kinds of fun

combinations if you look at polar bear

it actually is if you have an older

device or it breaks apart it is bear

plus snow which is really cute it was

originally I had bear plus white and

then we decided that bear plus snow made

a lot more sense so uh another one this

is new I I think if you guys should have

it if you've updated your phones in the

last year or so so mending heart is

heart plus

Band-Aid um um what is this one oh this

is interesting um there was a

breastfeeding woman for a long time and

people felt like there was not gender

par was actually really interesting all

the people who wrote it and were like I

want to I want to be a dad showing that

I'm holding my baby why is there only a

woman holding my baby so so we kind of

created a whole set so this is man plus

bottle Tada um and so this is and all of

these occupations are actually often

times a woman plus like you know the

fried egg or like um a school or a

tractor and that's how you got the

occupations if you if you if you send an

emoji over into an older system

sometimes you'll it'll break apart so

one of my kind of favorite kind of

contributions in this in this world is

interracial couple Emoji which we did

with um Tinder which is super fun

because then you could have you know

different couples that are and and like

so many combin this is like this is a

fun if you guys ever have to do a

combinatoric test um this is really fun

because you have two genders plus a

third like gender plus two people plus

five skin tones plus yellow like how

many Emoji couples can you come come up

with when you introduce this Factor so

this is um and underneath it it's just a

wige sequence it's like two people

standing together that are like glued

together now this actually gets

interesting from a cs-50 perspective

because in many cases even though you

only see one character underneath the

hood depending on how your system works

they're counting each one of these as an

individual character so your string

length actually might be five and

instead of one and this this kind of

became a problem with things like

Twitter where things had a hard skin um

length so gender inclusivity um is

actually one of the things that we've

been dealing most with um in the last

couple years so it's kind of interesting

if you think about both what a pictorial

language looks like versus the

abstractness of a of a spoken language

so because you know we had boy and we

had

girl but there was no way to say a

generic child right like if you were on

you wanted to say child you had to pick

a boy or a girl but not a way to say

like just some little person and that's

really key cuz in English at least there

is no gender implied by

child um so how do we mimic that and it

also is key for something like doctor

right doctor and teacher those are those

have don't have gender implied but when

we have them on on the Emoji Keyboard

you to pick a male teacher or um you

know a a a female doctor or whatnot so

there was actually uh a guy at Adobe who

can considers himself non-gender binary

also The Man Behind the orange um the

orange heart he fought and got basically

uh the first three non-gender uh

non-gender binary emojis so child adult

and old older adults so those are

creative and then we started having to

propagate them through actually all the

occupations so these are the gender

neutral versions of many of those um but

then uh we got into this whole thing

where every emoji that had a gender

originally had to be mirrored so

originally we had bearded man and then

we're like okay well we actually have to

get bearded woman so that is on your

keyboard there is pregnant woman there

is now pregnant man which is interesting

um there is you know woman in a bridal

gown there's now man in a bridal gown um

and then there were ones that actually

had to be created that were neither man

or woman so this is a mer person so

there was merman there was mermaid and

there was M person it was really

interesting to figure out like how do

you draw a gender neutral M person like

a bunch of them in the beginning

actually had the arms crossed around

sort of the chest um Monarch so there

was prince and princess and now there is

monarch and one of my favorite actually

is there was Santa Claus and there was

Mrs Claus and now there's MX Claus like

the name of this character literally in

Unicode is MX claw so I feel like it's

sort of like a very official enshrining

of gender non biner in like the world um

not everyone loved it New York Post did

not love this that they like you know

we're we're cing into like Emoji woke

Wars um so some Emoji stats for you this

is very fascinating this is sort of like

the Gen the General Distribution by far

the single emoji that used more than

anything else is the face with tears of

joy about 10% of all Emoji scent is that

one character and then number two is

heart red heart and then it kind of goes

down so um there's a frequency of emoji

use these this is sort of done by um

order magnitude so one is half of two

two is half of one so it's really

interesting it's a very very steep drop

off after the first couple um in case

you ever want to go onto the the Emoji

kind of Unicode website you can you too

can see all the frequency things so I

think it's really funny so basically

it's green going this way it increased

in usage between 2019 and uh 2021 and

it's red going this way it drop and so

pleading face which is a relatively new

emoe you just sort of shut up on the

charts um and whereas actually like

smiling face with heart eyes

like kind of kind of slipped which is

interesting so we just closed our Emoji

proposal round for 2022 these were sort

of the breakdowns people love submitting

Smileys and food and beverages animals

and nature da da da I don't I mean these

are very googly colors um so what is the

future of emoji I will um I will tell

you because we just had a meeting two

weeks ago so I can now publicly talk

about it so historically there was this

whole idea like Unico doesn't want to be

in the world of like encoding glyphs for

like devices everywhere like there was

very controversial when it started doing

that because mostly what Unicode used to

do was you take an existing language

could be dead you know and then it would

just take it and digitize it right it

took languages that existed and just

digitize them and uh then when it kind

of wandered into Emoji World Suddenly

It's like deciding what deserves to be

like an emoji decides to be digitized so

um trying to get kind of get out of it

and they have proposals over time where

it's like oh maybe we should like come

up with a a way to just send pictures

back and forth where you you it's a

fixed picture and you like use a hash so

that you know like we would look at the

picture and then like go do a lookup

somewhere like that did not go over well

then there was actually a really

interesting proposal I kind of like uh

didn't go over well which is using

something called the qid which is in

Wiki in the Wikipedia world so in

Wikipedia World items all have uh

numbers across the different language

Wikipedia so Obama human Earth they will

have an ID number so that the page in

English and the page in German the page

in Chinese all know that they're

pointing to the same thing so the

question so one idea came up like why

don't we use the numbering system so we

can use like Eiffel Tower in you know

see the number and then like oh people

know like oh you're trying to say Eiffel

Tower that did not go over well so those

are both both of those proposals seem

dead as of yet um and it's too bad cuz

and you'll see kind of what's happening

okay so what's coming in 22 so these are

the Emoji that I actually sort of

thought they would be on your phones by

now but cuz we're in mid November and

they usually update early November so

three more Hearts people love hearts

like uh wing blackbird Goose Birds also

purple flowers jellyfish moose face

donkey donkey was a little bit late for

the kind of Elections um Ginger peep pod

Wireless cond shaking face um folding

hand pan that one was interesting cuz

when people first proposed it they

proposed it as like an electric fan and

that didn't like who knows what electric

fans will look like in 100 years because

the thing is Once An Emoji always an

emoji they never retire so they're

always looking for things that have a

long visual longevity floppy disc did

not actually do that so there's always

like we don't want another floppy disc

um and then hairpick is interesting so

there was a whole debate about how to

convey um like afro African hair like

the the curly hair that they introduced

a couple years ago was supposed to do

that and most of the vendors actually

have it in a sort of Afro way except for

Apple so there's a lot of complaints but

um hairpick was sort of a an interesting

way it means both comb but also has sort

of an interesting historic connotation

and it's been around for about 2,000

years uh a couple music things maracus

and flute uh Beyond 2022 one of the

things that's going to die oops can I go

back no I can't oh well uh we're going

to retire the the family Emoji um they

didn't go over so well there were so

many of them combinatorically if you had

all the everything in in uh you know all

the race all the races all because you

wanted to have skin tones because you

didn't want to imply that families can

only be one race it was such an ordeal

uh essentially we're all like no one

uses them and there's so many and it's

like the fonts like in terms of the load

is like too large so they're just going

to make them all into like basically

little like bathroom symbol Type U folks

um so I think that is those will

disappear what what's actually really

interesting about the family Emoji is

they had gay when they introduced the

family they had gay family emoji and the

Russian government went berserk and

actually you can Google this in 2015

you'll see a bunch of articles about

like the Russian government considering

it homosexual propaganda to Youth and

there was a big debate about whether or

not they were going to ban Apple devices

and what so you can see a lot of the

media coverage from that time but I

thought it was really interesting how

upset a national government can get

about little pictures on your phone um

another thing that's going to that's on

the agenda as of you know a couple of

weeks ago is directionality in terms of

emoji so as you know most emojis kind of

just flip one way or another and the

reason why it matters is because not all

languages run in the same direction so

for example Arabic so we are used to

left to right but a lot of languages go

right to left um so and this kind of It

kind of changes the meaning of emoji for

example right to left I send this a lot

to my friends when I'm going flying from

the Bay Area to New York um if you do it

from left to right however that is what

it looks like so it looks like um you

know you're in NE Bay but the plane is

still going that you know kind of up and

to the right and then now it looks like

you're going from New York to the Bay

Area the other place is like oh it's

it's a girl and she's running really

fast right uh that is left to right to

left in our world wait left to right

sorry sorry that was that's that's

supposed to be left to right and in here

it would be she's like behind like

pollution or something like that so

sorry about so so an example this is

actually in The Proposal like in one

case if it's left or right you're

running away from a line of cars and the

other one it's a warning to not run

behind car fumes so they are trying to

figure out like how do we mirror a bunch

of the Emoji um but the LA the main

thing that I think sort of I don't know

when it's going to happen I'm really

hopeful is going to happen is trying to

come up with a system that supports

little stickers in line that don't need

Unicode so this is like slack or on um

twitch you can embed little pictures on

in line and all all all the vendors have

to get together and agree to come up

with a standard way to do that they have

not yet come up with that but that is

sort of one of the ways that unic coded

like want it wants to back away from

actually being like a global regulator

for like little colorful glyphs and so

if you ever need to reach me um on in my

emoji world you can find me um Jenny

EmojiNation org there will

be um the it will be actually a while I

think before we see a next generation of

emoji showing up in it used to be like

every year they would get new code

points it might be a little bit less

than every year now as they work on

things like directionality over time so

that uh if anyone has questions you can

ask questions you can find me afterwards

I think I've I feel like there's

supposed to be some some hubub right now

about maybe maybe

um microphones but maybe not but maybe

I'm just done and if if there are

questions or if David is around I'm

happy you know he can I'm I'm happy to

to answer any questions that folks have

yes hi yes I was wondering what were

your thoughts on the

emo so the question is what are my

thoughts on the Emoji Movie you're

talking about the Sony animated one yes

okay my thought on that is it is better

than a 6% rating on um Rotten Tomatoes

would lead you to believe so that's my

one thought and my my next thought is

that um that was a rush job from an

animation perspective that was that was

was about 18 months of whereas a typical

animated movie takes four years so in my

spare time I also produce movies and

documentaries so one thing that is key

to know

about about movies and animated movies

and this is very important they take a

very long time but you can always fix it

because you haven't shot anything and a

very good example of that is I assume

you guys have seen Frozen if you haven't

seen you seen of the age you would have

seen Frozen um I do not understand like

how huge and a phenomenon or why it was

such a huge phenomenon but

um they actually did a original cut of

Frozen and it did so I don't know you

guys know the the Eye the the sort of

Snow Queen thing but she's like super

dark and like not fun and like kind of

evil and like not someone you want to

like get behind as a character so they

actually did sort of a rough cut of that

of Frozen and they came out of that with

um it's just storyboarding and they're

like that is not good and they killed it

so they were like we can't go with this

and then started from scratch more or

less again starting with the song um Let

It Go which is actually written by a kid

from my elementary school Bobby Lopez or

co-written by Bobby Lopez um I also

actually fun fact I also went I would

took the school bus with uh Lyn mmel

Miranda so I was a fourth grader when he

was like a kindergartener so we had a

very musical Elementary School in New

York City but the thing is they could

fix it because they had enough time and

have enough money not like movies where

you shoot humans much harder to fix so

you have the footage that you have and

you can do little pickups but you can't

fix it so essentially what happened in

that case I think um it's 18 months and

it could have gotten better and a lot of

the movies that you see with Pixar like

it's very it's actually sort emotionally

similar to the movie called inside out

and uh but inside they just have more

time and so it's better so as opposed to

18 months which is not long enough to

make a animated movie good but uh the

other fun thing is is it was the it it

was so weird cuz they sold sponsorships

it was like like oh my God here comes

the Bots and the malware let's go into

Dropbox and protect ourselves and was so

I think that it got a lot of like bad

kind of um kind of uh vibes from the

from the press for doing things like

that but from a kids perspective it's

it's fine I think if um I don't know

that I would like put into my top 10 of

animated pictures but it's better than

6% on Rotten Tomatoes

and then actually if you guys ever care

we we have done uh I did a documentary

about Emoji so and all the people that

create helped create emoji and uh we did

a cs50x movie night I think during the

pandemic was it during the pandemic

everything sort of like blurred together

but it was during the pandemic yeah Are

we more questions yes um I wanted to

know you mentioned that one of the

criteria for

AC do or there's demand

yeah yeah that's a very good question

yeah so the question is one of the

propos one of our criterias of of um

getting an emoji accepted is to sort of

to demonstrate demand and how do we

demonstrate demand and I would say in a

in a pretty um clumsy way actually at

this point so the main thing that you

have in our in our current proposal

process is we have um a median Emoji

which is elephant so elephant is like if

you stack ranked all the emoji for

popularity elephant is like right there

in the middle and it's also a concept

that's like universally understood

across all

languages so um elephant shows up

somewhere between 500 million and 700

million in Google search results like if

you type it into a laptop you'll see you

know elephant 500,000 uh 500 million

search results and generally you're

you're trying to when you're comparing

your term to elephant you want to see

very roughly how many Google search

results B search results sometimes

Instagram so actually something that was

really surprising to me was someone

proposed hummingbird I think hummingbird

is uh a good proposal and um but if you

look at Hummingbird it's only like 21

million in terms of the stat so which I

thought was like very surprisingly low

so that's one of the main ways that we

kind of we kind of see like is it also

visually used and all of that yeah any

other questions are we good I didn't

even need my water or anything oh can I

take I'm going to take a picture I'm

going to take a picture for because now

you guys are actually human and not

Muppets so I'm very very excited about

this so I will send this to like my

block mates and be like I just lectured

at CS you know in Sanders Theater my

thanks to Jenny Lee thank

you yeah you can stay up here for give

me one

second

so if up until now thought it would be

appropriate to toss this up on the board

if up until now you've not yet gotten

cs50 stress ball on the way out please

do grab one we got some extras as well

off to the side but I would also keep in

mind back in week zero where again we

began we asked you to categorize

yourselves as to whether you are among

those less comfortable those more

comfortable or those somewhere in

between uh please know now that you are

officially all some uh please know now

that you are all officially among those

more comfortable and indeed even though

a couple of more Milestones await we

cannot wait to see what you accomplish

with your final projects in the meantime

as always this is and now this was

cs50

[Laughter]

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Mr when

I in CS I'll say truth be told I thought

i' be real coding hero because I did

so it picked up so fast turn out that

harder scratch hell world and

goodby

[Applause]

[Music]

because

Mr I was on

the trying so hard not fall behind

attending section and wanting a stuck in

the in void of my

computer to pyth in why on because this

is

[Music]

s Mr s we completed

JavaScript sequ CSS

and every language our to reset don't

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get again I repli

obviously because

cie

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because

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[Music]

all right so this is cs50 my name is

David May and this is Harvard

University's introduction to the

intellectual Enterprises of computer

science and the Art of programming and

this of course is our special family

weekend wherein not only are cs50's own

students here in the audience but also

some family members as well now you're

showing up in the semester a little bit

late we've just tackled week eight which

is really our ninth week since computer

scientists start counting from zero so

we've done a whole lot of work over the

past few weeks as you might have heard

via emails or text messages home

including a language known here as binar

so on the screen here of course is a lot

of zeros and ones and suffice it to say

let me sum up the past nine weeks with

this is what's going on underneath the

hood but of course today we thought we'd

make things a little more accessible a

little more broadly applicable and

indeed our Focus today will not be on

what these patterns of zeros and ones

represent which in astute eye might

notice are replicated visually with

these light bulbs being in a pattern on

and off and as your child might have

hinted uh before class or perhaps now

this might very well spell a word up to

eight characters long because you can

encode even in the real world things

digital too but today we'll focus on

things much more high level this notion

of cyber security like our the security

of our data our privacy of our systems

particularly on the internet nowadays

because presumably all of us are

carrying Technologies around in our

pocket using laptops and desktops every

day and so the goal today is to

stipulate that this is what's going on

underneath the hood but let's solve some

problems at a higher level so that your

homework when you go back to wherever

you're visiting from can actually be to

apply some of today's Lessons Learned so

with that said perhaps the most common

familiar defense uh of one's systems and

data phone and laptops and desktops

would just be these simple passwords

unfortunately you and I are frankly as

humans not all that good at choosing

passwords and this is in itself a

relatively weak form of Defense even

though each of us has dozens hundreds of

passwords nowadays or at least dozens or

hundreds of accounts Maybe fives or tens

of dozens of passwords indeed if you're

in the habit of reusing passwords we'll

see today probably among our first

Lessons Learned so for instance if we

look back at the past year 2021 thanks

to security researchers who take a look

at data that has been hacked or leaked

online by way of public databases we

have a sense as computer scientists of

what the most popular or equivalently

what some of the worst passwords are

that you and I are choosing for our

system so as of this past year according

to one measure the most commonly used

password in systems everywhere was 1 2 3

4 5 6 all right number two password in

our top 10 here list was only slightly

longer 1 2 3 4 5 6 7 8 9 after that we

took a turn in the other direction 1 2 3

4 five alone after that got a little

more interesting quiry which might sound

pretty cryptic but not if you look down

at your US keyboard and it's the top

leftand row of the keys on a an American

keyboard so also not all that hard uh

perhaps not surprisingly uh a little

disconcertingly number five was

password meanwhile number six returns us

to digits 1 2 3 4 5 6 7 8 after that

really less effort 111 111 uh after that

a little more variation but not all that

much 1 two 3 one two 3 after that it's

getting even less interesting 1 2 3 4 5

6 7

890 and then lastly topping the list is

just 1 2 3 4 5 6 7 so this is not a good

top 10 list to be on so among today's

first takeaways is if you see your

password on the screen like you didn't

make the list in a good way this means

hundreds thousands millions of other

people probably have that password of

yours now in of itself that's not

necessarily worrisome because I don't

know who has these passwords in a room

as large as this but just intuitively

why is this a bad thing either parent or

child welcome to raise a hand here why

might this be

intuitively yeah access to it so access

to it like I mean we literally as

computer scientists now have a database

of really common passwords and your your

thoughts password and can just find it

out quickly yeah you can just find it

out quickly I mean you could imagine

trying to guess someone's password by

just typing in random letters random

numbers random words but not if you have

a top 10 list like the the adversaries

in the world might as well just start

with this list now you'll notice that

even absent from this are slight

variants like some of you might be

thinking I'm not on the list cuz I do

something clever like I use an

exclamation point for the number one or

a three for an e or a five for an S and

based on the smiles in the room right

now you're not all that clever it turns

out because other people are smiling too

which is to say that an adversary can

take those same fistic that you might

think are making things more secure by

just tweaking some letters to numbers or

vice versa but if you're doing it and

other people are doing it the bad guys

so to speak are going to be doing it as

well so unfortunately when it comes to

passwords better is longer and random

and really unguessable but that's not

what most of us have in fact case in

point on our phones whether you have an

Android device or an iPhone nowadays you

know odds are you have something

relatively simplistic protecting it if

you have anything at all but at least

Apple and Google are pretty good at

least nudging us to choose these kinds

of passcodes now and a four-digit

passcode is quite common nowadays and so

here's where we have an opportunity

thanks to uh the URL that you saw on the

screen earlier to conjecture as a group

just how long might it take an adversary

someone out there who's out to get us or

get one of us uh how long might it take

an adversary to figure out your phone's

4digit passcode this is a cs50 is on

Carter Carter if we could switch over

and poll the audience here if you take

out your phone or laptop whatever device

you might have used a few minutes ago to

scan that QR code or to visit that same

URL you can see these questions on your

browser and if you can't that's fine

we'll share some aggregate data

nonetheless but you should have an

opportunity to tap one of your answers

and we'll give folks a few more

seconds if you'd like to play along at

home and here in just a

moment probably have many people

reporting but why don't we go ahead and

take a look at some percentages it looks

like most of you 67% are proposing just

a few seconds so that's not all that

good news if it's a four-digit passcode

some of you are hoping it's a few

minutes uh 8% are hoping a few hours

four more than 4% of you are really

hoping perhaps it's a few days well

let's actually consider how we can

answer this question and make today not

just conceptual but a little

quantitative too and see if we can't

slap some numbers on questions like

these so ultimately you can make more

informed decisions with your system

security so for instance when it comes

to four digigit passcodes rather than

just consider how secure it is well

let's make it a more precise question

like what are the forms of attack well

the simplest attack might be just

someone grabbing your phone be it in

your family or maybe at Starbucks or the

airport or the like and just starting

all possible combinations maybe 00000000

then 00001 and 00002 we could maybe

automate this a little bit so for

instance I might potentially be able to

do something like uh roboticize this

here let me go ahead and full screen a

quick video here that's just going to

paint a picture in just a moment on the

screen of how if we're a really clever

adversary and know how to build things

well at least maybe we could automate

some of that process so here's an

Android phone on a counter here's a very

simple tripod and a little touch device

robotically doing all of that hacking

for you starting at 0000 Z probably all

the way up to 9999 now that too wasn't

necessarily all of that all that fast

but at least you the adversary can step

away and doesn't actually have to be

bothered with the time involved the cost

involved in actually hacking that

particular device well let's go one

level deeper a little more interestingly

and consider here um the how much much

time really this so-called Brute Force

attack would take and that's actually a

term of art much like in yester year

when maybe there was a battering ram

trying to brute force their way into a

castle or something like that a Brute

Force attack digitally is just someone

trying manually all possible codes or

maybe robotically trying all possible

codes but generally automating the

process in some way to go through all

possibilities well if you've got for

instance um a 4digit passcode let's ask

maybe a follow-up question here not how

long will take but how many possible

4digit passcodes are there because then

maybe we can do some quick math and if

every passcode takes me a second or a

few milliseconds or the like then I

think we can try to extrapolate from

that whether the first answer was

seconds or minutes or days or hours or

something else so how many four-digit

passcodes are possible if you take out

your same device it should have just

changed automatically if it doesn't seem

to have maybe reload your browser with

some menu option and then tap in here

how many four digigit passcodes are

possible four total 40 99,999 10,000 or

unsure is okay

too so let's see we'll give you a few

more moments how many four digigit

passcodes are possible and shall we

reveal the results so now it looks like

uh a a few of you uh 2% of you are

saying just four passcodes 40 99,999

there's definitely some contention here

and 6% are un sure well how do we wrap

our minds around this well let's just

kind of do this real simply here let me

switch back over to doing a bit of math

and if we have here 10 possibilities for

each digit if there's four digits each

digit can be 0 1 2 3 4 5 6 7 8 nine so

that's 10 possibilities so if you think

about the number of permutations that's

10 possibilities for the first digit

times 10 for the next time 10 to the for

the next times 10 for the next and so if

we do that out 10 * 10 * 10 * 10 or 10

the 4th there are indeed as 66% of you

uh found uh 10,000 possibilities and so

now we can kind of work backwards and

decide how long is it going to take for

an adversary to hack into this phone

because if it's one attack one guess per

second well that's going to map out to

10,000 seconds but maybe not if the

adversary isn't a roboticist or a human

what if they're like at a software

programmer someone who's taken even a

class introductory like cs50 and learned

a little bit of programming well a

little bit frighteningly it's not all

that hard to hack into systems if you

just know how to code to and really have

the computer do your work for you so in

fact let me go ahead and change over to

another screen on my computer here um

this is different for students in the

group from vs code this is just a black

and white version of it that we've used

briefly in the past and I'm just going

to go ahead and create a program called

crack.pie to crack something just

technically means to figure out what it

is figure out a password in this case

and dop means I'm going to use a

programming language that we here in

cs50 have been dabbling in in the past

couple of weeks with more to come next

week as well so it turns out and you

need not understand each of these lines

of code if I want to try maybe

generating all 10,000 possible codes I'm

not going to bother with a robot I've

got all these cables coming out of my

computer and odds are one of them is a

USB cable or a lightning cable surely we

could figure out how to connect like

laptop or desktop to phone and just like

automate the process nowadays by just

sending all of the numbers into the

phone until one uh unlocks the trick

just like in the movies or TV well in

Python I could write a program that does

this as follows I can import so to speak

all of the decimal digits 0 through n

and this for students in the room is

just a slightly better version of typing

out 10 different numbers manually I can

also import from a library so to speak

called iter tools for iteration tools

which means to do something again and

again I can import a function called

Product which means the cross product

like combine this with this some number

of times and then it's just two more

lines of code I can use what's called a

loop and programming so for every

passcode in the cross product of all 10

of those digits repeated a total of four

times let me go ahead and rather than

bother connecting my phone and hacking

my own phone let me just print out every

one of those 10,000 codes on the screen

and we'll see how fast the hacker could

do this let me go ahead and print and

with an asterisk which is a little trick

to format it nicely I'm going to print

out each of those passcodes and that's

it four lines of code maybe 40 seconds

of talking but maybe really four seconds

of code if I actually did this without

the audience and now let me go ahead and

save the file and I'm going to run as we

do every in class of late python of

crack.pie and when I hit enter I should

see on the screen all 10,000

possibilities from 000000 to 9999 so

let's see is it a few seconds minutes

hours or days done so barely even

seconds plural if that so that should be

a little disconcerting because all that

adversary needs to do is grab your phone

off the counter plug in a cable and boom

they're done like there's no ticking

clock or worries as in the movies or TV

that maybe you're going to come into the

room you don't need that much of a

window of time so what would be better

than this well let's consider what our

options might be if we don't want to

just use four-digit passcode some of you

indeed might have better passcodes than

that and maybe you use four letter

passcodes instead so A through Z maybe

uppercase and lowercase that starts to

make things a little more interesting so

should we pull this question too if we

upgrade from four digits to just four

letters English letters a through z

uppercase and lowercase why don't we go

ahead and pull the grip here and ask how

many four letter passcodes are there

instead so this time the range starts at

four still not the right answer though

this time how many four-letter passcodes

are

possible not just just take a couple

more

seconds

all right almost a couple hundred

responses in already few more

seconds

and why don't we go ahead and reveal now

the answers which are Okay so we've

solved a couple problems at least so we

well we well okay someone's just messing

with us now all right so it looks like

most of you 76% of you have claimed it's

7 million plus possibilities so that's

encouraging cuz that's a whole order of

magnitude more than before well let's

figure out how we might do this

mathematically so if we've got 26

lowercase 26 uppercase that's 52

possibilities now for each of those four

digits so that's 52 times itself four

times which indeed either off the top of

your head a good guess a calculator on

the same device you're using right now

indeed gives us 7 million instead well

what might be slightly better than that

well maybe four characters and this

indeed is what your Macs PCS and phones

are urging us to do nowadays not just

numbers not just letters but like really

annoying punctuation so it really looks

cryptic not just to the adversary but

also to you and me unfortunately and

that's the downside but here now we have

a mental model and really a

computational framework via which we can

evaluate the security of these and I'll

go ahead and spoil some of the math here

if we've got 52 uh letters of the

alphabet uppercase and lowercase 10

digits and if I count them out on my

keyboard about 32 punctuation symbols in

typical English grammar that actually

gives us 94 possibilities now which is

up from 52 which is up from 10 so now

we're really moving and now that would

give us 78 million possibility so

another order of magnitude now it's

still going to be relatively fast

because you know what I can actually do

this let me go back into my code here

let me reopen the same program and I can

point out just how easy it is to make

these changes instead of importing

digits as before I can import as your uh

child might know asky letters which are

a through z uppercase lowercase and I

can just change this here asky letters

and so this was that first version where

we just changed to letters let me now

rerun the code and instead of seeing

numbers we'll see letters flying across

the screen and if I walk over here to

the screen we'll see that by the time I

get here we're halfway through the

entire alphabet lowercase if I now start

walking away I think yep we're already

done now with uppercase as well if I

upgrade this slightly further let's go

ahead and take it one more level and

perhaps do let's say ask letters and

digits and punctuation and this would be

the pythonic way to say that and I'm

going to add to those letters those same

digits those same punctuation symbols

let me shrink my font just so the code

still fits on the screen and what we now

have is with a two seconds of changes a

program that if I run this version

whoops without the typographical error

this is what we call in cs50 a bug so

now we run the same this is what we call

in cs50 a second

bug

punctuation okay this is where I cross

my fingers okay so now it's going to be

a little hard to see as it flies across

the screen but you probably are seeing

glimpses of some weird punctuation

characters as well and I won't waste our

time trying to talk through this because

this is going to take longer we're still

in the lower case I'm still over here

already we've not even gotten to n now O

then P so this is going to run longer

but let's end with one final question on

the security of all these systems I'm

going to cancel that by hitting contrl C

on my keyboard and let's ask the

question instead if we use eight

character passwords so twice as many

characters but even that is not terribly

long right this is eight characters

alone on the stage eight characters

using letters numbers and punctuation

might be better let's do one final vote

here if we could on your same device how

many eight character possibilities are

there now

for these

passcodes and now for didn't even make

the list this

time all right few more seconds about

100 responses so

far how about we go ahead and Carter if

you would't mind let's reveal the

results based on the vote a pretty

decent spread here although the

quadrillions are quickly buzzing in and

they're contending with the others here

looks like 44% of you said quintilian

34% said quadrillion and this time for

the first time uh you overbid so indeed

if we go back to the math here at least

the majority overbid if we have eight

character passcodes that gives us 94

times itself 8 times or 94 to the eth

power and in fact that gives us roughly

6 quadrillion 95 trillion 689 Bill 385

mil 410,000 and

86 possible passcodes now what does that

mean well the adversary's algorithm the

step-by-step code that they write to try

to hack into your phone is no different

and honestly if your passcode is eight

characters long but there're zeros

00000000000000 you're no more secure

fundamentally you really want to be

somewhere in The Sweet Spot of that

massive range of values so that if the

adversary tries this Brute Force attack

just running through all possibilities

they will eventually reach your passcode

just mathematically it will be there

hopefully though well maybe not

hopefully you and I and they will be

gone from this world because that much

time will have passed and if we fact if

we do out the math here uh this number

of seconds for instance is long past uh

when uh uh we will uh no longer be here

so that's the sort of measure is we

don't sort of fundamentally change the

equation for the adversary it's still

the same risk it's still the same attack

but you significantly drive down the

probability of success on their part or

conceptually you drive up the cost to

the adversary and indeed even in the

physical world this is true you just

want your passcode in the digital world

really to be better than someone else's

because you want someone else's passcode

to be the one that the adversary does

something with with just like in the

physical world even though it's a bit uh

uncomfortable to consider your house

doesn't need to be 100% secure and

indeed it's difficult to make it such

there's always going to be a a point of

weakness maybe it's that window the door

or something like that but if your home

is more secure than the next door home

just probabilistically you are more

secure you're not secure and indeed any

website you see down the road that says

we are secure because we do X Y or Z

like that's nonsense security is really

about comparisons and Val ating things

if quantitatively relative to some other

system relative to some other code so

what's the takeaway here well hopefully

a non-trivial number of you we'll go

home this weekend on Monday and change

at least one passcode um but there's

going to be a trade-off here and we talk

about this all the time in cs50 anytime

we improve something we pay some price

in time and performance and cost

somewhere else so what's the downside

then of this advice that you should use

minimally eight character

passcodes why might you want to say nay

and not do

this say again you have to remember you

have to remember it right and so here

there's sort of some sociology there's

some human behavior you know some of you

might have colleagues if you're working

in the real world at least back in

healthier times when you had colleagues

with desks and cubicles and there's

probably one person in the office with

like a Post-It note on their monitor

with their passcode you know it's a bit

of a cyber security offense but it's

also a sort of real world side effect

maybe of corporate policies that aren't

really calibrated for human behavior so

we'll see if there's some other defenses

and indeed let me propose that we talk

briefly about one that actually tends to

kick in automatically even if your

passcode is not as strong as we've just

seen one of these six quadrillion

possibilities well what could we do

instead well as anyone and I'll zoom in

on this here accidentally locked themsel

out of their own phone before like when

does that happen yeah when you try the

password too many times yeah so too many

times maybe your finger's slightly off

maybe you're slightly off and you just

don't input the same passcode correctly

after like five times 10 times there's

some reasonable threshold and why does

that happen well Apple and Google

equivalently figure just

probabilistically if after 10 guesses

you still haven't typed in the right

passcode probably you're not you you're

someone else who's picked up your phone

so we're just going to go ahead and lock

you out now what's the effect of this

well this means now that each of those

possible passcodes no longer takes

roughly 1 second now it takes roughly 1

minute so the attack is still the same

but if it's now one passcode or 10

guesses per minute we have significantly

by a factor of 60 in this story slowed

things down and unfortunately does

anyone know what happens if you screw up

again after a

minute yeah it goes longer it's like

five minutes and then 10 minutes and uh

Google is kind of obnoxious about it

they don't even give you a time frame

they just say try again later and so

that keeps not only the adversary out

but also potentially you so there in

lies that trade-off if you've forgotten

your code if nowadays your finger is

slightly wet so the screen isn't

responding correctly these could be

usability downsides too so security is

really just about finding The Sweet Spot

among these various tradeoffs here but

there's other mechanisms too and some of

you might recognize this screen from

Gmail via which of course you log in but

after you log into Gmail or similar

websites or apps or systems at work

nowadays especially you might be

presented with uh what's called

two-factor authentication and what is

this in a nutshell in Lay person

terms most many of you if you do

anything digitally at work might have to

do this now

yeah exactly you get texted at your

phone an additional code that's not your

same password it's typically a numeric

code maybe six digits long it expires

after a minute or 10 minutes but why is

this a good thing well one it's no

longer just a piece of information that

you know or that you might have written

down it's information that changes every

time you try to log in but more

importantly it's a fundamentally second

factor which means it's not just

something you know now it's something

you have so you for instance are the

only one theoretically that should be

receiving that code and so now the

adversary if they want to get into your

account not only have to guess or brute

force or maybe read off of a Post-It

note your password they also have to

physically have access now to that phone

so there's still a threat absolutely but

it's not everyone on the internet with

an internet connection now it's only the

people in Starbucks now it's only the

people at work now it's only the people

in your home who might have access to

that second Factor so there too it just

raises the bar to the adversary making

it harder more timec consuming more

geographically impossible for them to

attack you but what's the downside of

two-factor authentication whether it's a

device or even nowadays it's in software

whether it's on your keychain or on your

phone where you're prompted for this

code what's a downside as some of us

have probably experienced

too forg you forget your cell phone

absolutely right the the factor that you

have you don't have with you or maybe

you're in a basement somewhere don't

have reception you're on a plane you

can't get the code and so there too are

these tradeoffs and even it departments

need to keep that in mind because what

does that mean for them well if you

don't have your phone with you and you

are in the habit of calling it to help

you fix this now there's a a cost a

human cost maybe even a financial cost

and so it policy nowadays is really just

about finding the right balance and

where we want to spend our resources but

at least raise the bar to the adversary

but of course there's other ways too and

this is going to be one of our homework

assignments if you will after today

there's this software called password

managers and no need to buzz in on your

phone but maybe with a physical hand how

many folks here use a password manager

okay let me ballpark this at 10 20%

perhaps Okay so we've got 80% upside

here and a lesson learned potentially so

a password manager is just a piece of

software on your Mac your PC or your

phone nowadays that manages your

passwords well what is that mean when

you go to a website for the first time

or you download an app for the first

time and you have to create an account

you can can still use your email address

or David as your username or whatever

your name might be so you don't have to

change that methodology but instead of

typing in one 2 3 4 5 six as your same

password for that website or app as well

as for every other now you use the

password manager software to generate

something difficult to guess for you

that is you tell the password manager

give me an8 character random passcode

not 0000 but something with punctuation

with numbers with letters and better yet

the password manager is the name

suggests remembers that password for you

and the next time you go to another

website you do it again with a

completely different password maybe same

username maybe two-factor authentication

but different password different

password different password and it

doesn't have to be eight I mean I'm in

the habit of using a dozen two dozen

characters in total and at that point I

can't even pronounce the number of

possibilities because it goes well be

Beyond uh the quadrillions so the

probability that someone's going to get

into one of those accounts for me now is

very very very low and they're going to

take less interest in me and maybe more

interest in someone else that's not

using as good of a password now what

does this mean in real terms well when

you go to log into that managed site you

don't manually type your password

anymore in fact you don't generally even

need to know it nowadays I probably

don't know 90 plus 99% of my passwords I

entrust them to this password manager

now of course you'd like to think that

the password manager itself is secure so

what might that mean well those of you

who do use a password manager how do you

access that software itself what's

protecting your data in your

understanding so maybe Biometrics like

your your face ID or maybe your

fingerprint or maybe more simply what

else password maybe just a password and

hopefully that password that primary

password that gatekeeper is not itself 1

2 3 4 5 6 otherwise it doesn't matter

how secure all of the others are but if

you're willing to put in the effort and

pick one pretty long somewhat random

very unguessable password that you just

promise to commit to memory and maybe

for backup you literally print it out

and put it in a safe deposit box or a

safe or just kind of hide it somewhere

physically that there's very low

probability someone's going to find the

backup copy that might be Al loan but of

course the flip side is now if you

forget that Ma that primary password

you've now lost all of the eggs in the

basket if someone gets that primary

password now they have access to

everything so that's rather the

trade-off but I dare say you're probably

less threatened depending on your family

uh uh by the people immediately around

you than the billions of other people on

the internet that have access

potentially to those same systems so

there to it's a trade-off and it's up to

you to decide whether or not to manage

your passwords in this way but if you

were on that top 10 list or even if

you're not but you can think of several

accounts that all have the same password

you're probably going to benefit from

something like this and why is it bad to

be clear to use the same password on

multiple sites in case that's never sort

of dawned

in thought why is that a bad thing to

reuse a password on different websites

different apps any intuition yeah and

back exactly once it's attacked you can

the adversary presumably by transitivity

can see oh well if this user's username

is mailin harvard.edu on this website

and their password is foolishly 1 2 3 4

56 or even something way more

complicated they can probably just

assume with high probability that if I'm

being a little reckless let's try access

mailin at harvard.edu other accounts

other apps using that exact same

password and so by transitivity

essentially you're putting your other uh

Accounts at risk so what's maybe a

takeaway minimally here I would start to

reconsider your passcodes on your most

important data Maybe it's medical maybe

it's Financial maybe it's email anything

remotely personal that you really

wouldn't want to have access uh do you

necessarily need the same form level of

security on eCommerce sites or sites

that you don't really care about or that

you signed up for once and after that H

that's it probably not so you can decide

for yourself but again software like a

password manager and these are just some

of the possibilities out there um are

probably to be your friend a couple of

these are free they come with Windows or

Mac OS a couple are commercial Harvard

has a site license for students for uh

one of these as well so there are

options out there but what else do

people use what else can people use to

keep their system secure so most of us

nowadays have probably heard of

encryption this technique for just kind

of scrambling information so when you

want to send a message an email or

upload a a photograph or use your credit

card hopefully it's not just being sent

out for all to see but there's some kind

of scrambling going on and some fancy

mathematics ensure that encryption

ensures that only you the sender and

someone else the receiver can

theoretically see what that credit card

number is what that message is what that

photograph is instead so encryption is

sort of Common Place nowadays both in

websites and apps and ATMs and other

such devices but how does it work well

back in week two of cs50 uh your child

learned learned a little something about

encryption otherwise known as

cryptography and one of the algorithms

we talked about was quite simply

something like this this is what we

might call uh not only cs50 but plain

text so very plain text that in this

case is English and obviously everyone

in the room can read it but what if I

wanted to send this message out to

someone in this room or out on the

Internet or maybe equivalently back in

the day maybe write a message down on a

scrap of paper in grade school and pass

a secret note a secret love note to

someone in class with hopes that the

teach or any other students in the class

can't intercept it and read it well you

probably don't want to say this is cs50

or I love you or anything remotely

sensitive but rather maybe you want to

encrypt it and let's change the the T to

a u maybe change the H to an i the I to

a j the S to a t the I to a j the S to a

t again the C to a d the S to a T and

we'll just leave the numbers alone even

though I worry someone could probably

guess what this now does say nonetheless

but what was the algorithm as I rattled

those ch changes off whether student

from week two or parent from week now

yeah a on shift just a onlet shift and

this is more sophisticatedly called a

rotational Cipher or a Caesar Cipher

after Caesar back in the day it's

relatively simplistic but back in the

day it's not so simplistic if you're the

first person in the world to ever use it

or think of it but nowadays this is not

actually what we use but it's similarly

mathematical in nature it's not quite as

simple as just adding one or subtracting

one to go from now what we'd call text

to plain test but it's similarly math

that's involved and let me just

stipulate that the way the math works is

that the sender and the receiver just

have to have in mind some kind of secret

and the Secret in this case would very

trivially be one but it could be a much

bigger much more unguessable number or

maybe some other secret we share the

presumption being that my classmates my

teacher in that grade school classroom

if they don't know what that secret is

that number is yeah they could try to

brute force it and try all possible

mathematics plus one plus 2 plus three

but that's going to take them some time

and they probably don't care enough and

so my data might be therefore relatively

secure but we use encryption all the

time nowadays and so for instance this

is at the start of most URLs nowadays

even if you don't type it yourself with

that said Safari and even Chrome now or

kind of simplifying if not dumbing down

user interfaces to just hide details

that you and I as sort of normal users

don't need to see 24/7 but it is there

and if in fact on your phone or laptop

you click on the URL even if it's super

short initially you'll probably see the

whole thing starting with this and the s

means secure the s means that encryption

is being used but there's other forms of

this not just when you visit websites

there's this endtoend encryption which

is being talked about more nowadays

especially during coid times with so

many more of us on video and talking

about uh more sensitive things tele

medicine talking to doctors things that

you also wouldn't want to verbally or

visually get out into the wild just like

text what's different about endtoend

encryption versus HTT GPS and the type

of encryption that most of us use every

day on websites

alone end to end encryption is sort of a

better feature that you want to

increasingly seek when using services

like Zoom or Microsoft teams or Whatsapp

or the

like any instincts here yeah over on the

right good so the encryption the

scrambling of information happens in The

Source the sender and the destination

the receiver without a so-called

middleman in between and this is

actually very different from most

contexts nowadays that use just https

because when you're using https to buy

something on Amazon securely with your

credit card well of course Amazon needs

to be able to decrep the message at the

end of the day and so that's fine but

even when you're using services like

video conferencing or maybe text

messaging nowadays well if you're using

Whatsapp that's owned by meta and if

you're using Instagram that's owned by

meta there's a lot of middlemen in these

apps that we're using and if they were

only using encryption period or only

using something like https yes your en

your connection from you to WhatsApp and

in turn to the recipient might very well

be secure on each end of that channel

but meta in the between the company and

any other company in between could

theoretically For Better or For Worse be

looking at that data whether it's to

mine it for advertising purposes whether

it's to Snoop on data that you're

sending that is not end to-end

encryption if the middleman a company

typically has technically access to that

data now zoom and Microsoft teams and

WhatsApp and iMessage and other services

with which you're familiar increasingly

are offering stronger guarantees of

encryption whereby it's indeed between

parties A and B and not the one in the

middle now there's downsides here and

you can actually see this kind of

functionality manifest in certain

settings for instance besides iMessage

uh which just does this for you on

iPhones or Macs besides Zoom um you can

actually fine-tune these settings indeed

within Zoom itself s so here's a

screenshot that I took last night of

just what the user interface looks like

today to create a new Zoom meeting with

the latest version of Zoom software and

maybe unbeknownst to you there's a

choice of buttons down here and most

likely yours is by default on enhanced

encryption which is brilliant marketing

speak because it's just encryption it's

not enhanced it actually ironically

means worse than this um but they want

you using it most likely why well it's a

little easier to implement it's a little

less expensive for them computationally

and to be fair enhanced encryption does

scramble the data but not in a way that

Zoom can't see it Zoom can indeed see it

but that's actually a plus in some

context because if you want to do like

Cloud recordings and you want a meeting

recorded not on your Mac or PC but like

let Zoom deal with that if you want

automatic transcription nowadays so the

words to appear whether it's English or

something else on the screen well you

can't really lock Zoom or any other

middleman out of that because someone

needs to save it to the cloud someone

needs to translate the voice to those

English or some other language words so

enhanced encryption enables those

features but they also allow a bad actor

malicious employee someone who's just

nosy at Zoom or the equivalent middleman

to just kind of poke around your video

conference and hear what you've said or

see what you've typed as well unless you

instead check this box as well so

increasingly look for mentions of end

to-end encryption or give that some

thought when you choose a technology via

which to communicate with someone

whether it's within your family or

without as well now last but not least

there's other applications of encryption

too and this too might be a lesson

learned as well full dis encryption so a

dis is like where your data stored in

your Mac or PC or even your phone and

full dis encryption just means ideally

that all of your data is encrypted that

is somehow scrambled now hopefully your

password for your computer or phone is

good enough that even though the device

is encrypted with that password at least

you'll remember it and your phone or

your Mac or PC will automatically

decrypt it for you of course you can't

scramble the information and hide it

from from ourselves one of us at least

for these devices needs to have access

but full dis encryption typically means

that at least when you close the laptop

lid or power down for the night that

even if someone else steals that device

opens the lid unless they don't unless

they have your passcode they can't even

plug in fancy cables to the device and

just rip the zeros and ones off of the

device and see what's actually there

full dis encryption means they could do

that but they would just see seemingly

random zeros and ones now there's a

downside here too this might slow things

down potentially but it is a feature

increasingly that's offered and is

absolutely something you should consider

enabling um in general especially if

your laptop or phone travels with you

and certainly your phone does or if you

plan to donate or sell or give away a

device you don't want to leave all of

the zeros and ones the remnants of your

own sensitive data passed on there so

Windows has a feature called bit Locker

Mac OS has a feature called file Vault

there's commercial options as well but

generally we're at the point now in 2022

we're clicking a button is suff

efficient to enable these features with

that said don't rush into all of these

decisions I would make backups of your

data and don't maybe email cs50 if

something goes wrong with that process

but I would do your own due diligence

but this too would be a menu of

possibilities and now the bad side the

downside of what seems to be great this

notion of full dis encryption

unfortunately just as we can encrypt our

data to protect it from the adversaries

so can the adversaries if they get into

our devices encrypt our data and do what

not tell us that secret key and so this

is generally applied in the context of

ransomware which tragically you

increasingly hear about in Hospital

Systems school systems municipalities

where systems are getting attacked and

the data is not just getting stolen

because what is the adversary typically

need with like local municipal or even

Hospital data the value to the adversary

is encrypting all of the hospital all of

the mpali data preventing them from

accessing it if they have no backups or

the like and so ransomware is liter

Lally about trying to convince someone

to pay you money or pay you Bitcoin or

something like that to give you that

secret key and this key in this case is

surely more sophisticated than the

number one but it's really the same idea

so here too yet again a trade-off just

as we sort of invent something for good

it can also be used for evil and so to

speak as well but it's really the same

underlying principles even though we

keep seeing it and hearing about it in

these different forms and lastly if only

because folks are generally familiar but

don't necessarily know what it is that

it's doing for them browsers nowadays

have what's often called incognito mode

or private mode which has nothing to do

with encryption but does have to do with

cyber security or really cyber privacy

keeping your data from prying eyes uh

incognito mode if you open it in Chrome

for instance looks a little something

like this and we use it in cs50 when

introducing students as we did last week

to web programming because it in effect

lets you start with a clean slate like a

brand new browser that has never visited

any websites before which is good for

just diagnosing problems but it's often

commonly used if you want to log into

maybe your Gmail account on someone

else's computer and you don't want your

password being saved or you want to

visit some website where you don't want

the URL or the Search terms ending up in

your autocomplete history so there's

multiple uses for incognito mode but

what does it really do well it doesn't

stop your company it doesn't stop your

University your internet service

provider be it Comcast Verizon or the

like from knowing what websites you go

to because ask your student a couple

weeks ago we talked about actually a

week ago we talked about how the

internet works and unfortunately

computer has an IP address which is

unique identifier which goes out anytime

you go anywhere incognito mode or not so

this isn't really covering your tracks

outside of your office or outside of

your home or outside of your company but

it is at least throwing away local

information and so we'll talk in fact in

cs50 is week nine this coming Monday

about cookies which you might generally

know about and what are called sessions

and so long story short what incognito

mode does is it throws away when you

close the window any locally stored

information to the these things called

cookies which are sort of like virtual

handstamps that just remember what

you've logged in as or what's in your

shopping cart or the like but it doesn't

hide any information from anyone outside

of your own Mac or PC it only prevents

those local prying eyes so there too

even though we have tools that many of

you are probably in the habit of using

or thinking you should use to um be more

private be more secure on the internet

what we do really in cs50 boast week's

uh past and future is talk about how

these Technologies work so that

ultimately we have all the more of an

educated citizenry um here among

undergrads and here as well as on line

so that you can apply these same Lessons

Learned to problems you'll encounter in

the future so as promised the homework

one should probably use a password

manager doesn't have to be one of those

ones on the list but at least starting

that conversation maybe with someone who

does maybe the you know it's often the

you know the the the student in your

family perhaps who can advise you on

some of these Technologies consider

using a password manager to using

two-factor authentication whether it's

your phone or some key fob or or the

like but at least seeking out that

feature at least for accounts that you

really care about your email social

media Financial medical anything where

you'd be embarrassed at best or really

violated at worst if that kind of

information got out and then

increasingly using not just encryption

which you kind of get automatically for

most Technologies today but increasingly

choosing technologies that offer

stronger guarantees that keep those

middlemen those companies out of the way

if only so that you can trust with

higher probability that only party B

knows what party a has said or sent now

this of course was a whirlwind tour

there's so much more that you can do

online indeed this course cs50 can be

taken for free online via platforms like

edex at edx.org cs50 I thought it might

be appropriate to end on this note if

anyone would like to conjecture before

we start playing music and adjourn for

lunch what our final message here

is if we reverse the plus one and maybe

start minus one here minus one here and

indeed thank you so much for coming this

was

[Applause]

[Music]

[Music]

cs