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Welcome back everyone in this lecture.
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We're going to discuss if.
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Elif.
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Else.
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Statements.
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So as I mentioned in this lecture we're going to discuss control flow in general and control flow basically
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allows us to use logic to execute code only when we want to.
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So often you have a larger piece of code and you only want certain code to be executed when a particular
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condition has been met.
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For example let's imagine that I'm trying to program a robot to feed my dogs that I could say if my
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dog is hungry.
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So that's the condition my dog being hungry then I'll have the robot feed the dog.
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They'll have the actual code execute or perform some action.
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So in order to control this flow of logic we use some keywords and the keywords we're going to be introducing
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in this lecture are if.
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Elif.
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Else.
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So let's see the syntax for these three keywords in order to understand the syntax.
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We have to understand that control flow syntax in Python makes use of colons and indentation otherwise
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known as whitespace.
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And this indentation system is absolutely crucial to Python.
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And it's really what sets it apart from other programming language.
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This use of whitespace and in the notation allows Python code to be easily readable and very quick to
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prototype.
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So here's the syntax of a basic IF statement we're going to say.
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If so that's a keyword some condition so some condition is usually some sort of comparison operation
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that we just saw in the previous section of the course.
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So that could be something like If hungry is equal to true colon and then notice that that blue line
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is indented further than the if statement.
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So that says anything along that indentation is going to be executed if that condition happens to be
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true.
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Now on top of an IF statement we can add an else to this.
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So let's say that condition doesn't happen to be true.
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We can have another block of code execute.
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So in this logic we say if some condition happens to be true we execute some code else meaning that
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condition that it has to be true.
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We do something else.
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And notice how the else doesn't have a condition attached to it.
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It only actually executes if the condition is above that it happens to be true.
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You should also notice that the if else in the notation lies are lined up with each other.
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If you want to check for multiple conditions before that else statement executes you can have an elf
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statement or E L I F statement and basically you say if some condition that executes some code Elif
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some other condition do something different and you can have as many of these LFA as you want.
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And then finally all the way at the end you can have an else statement do something else OK.
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Let's explore all these concepts by actually coding out some examples in a Jupiter notebook to begin
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all of this.
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We're going to start with the simplest example we can do just a single line of an IF statement with
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just a boolean condition we're saying if true colon and note what happens when I hit enter.
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I have this indentation automatically done for me.
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And if using any text editor and you have defined the file as a PI script already you should see this
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in the notation occur automatically for you as well.
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Then we're going to say Prince it's true.
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So then we're going to run this and we see if true print it's true.
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So notice we're saying if some condition is true print it's true.
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Now typically you won't have just a boolean like this.
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Otherwise you'll always be printing that instead what you may have is something like a comparison operation.
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So we'll say if three is greater than to print it's true.
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We run that and we get back it's true.
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And then to make this even more realistic we'll say hungry set a variable there will say Hungry is equal
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to true and then I'll say if hungry Prince feed me.
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And if you run that we see that we get feed me.
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We can also then get hungry to false.
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Now if I run this code again notice I don't get back anything else.
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So I have some condition and it happened to be false meaning this block of code that execute what I
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could do is add in an else statement to execute.
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If this condition doesn't happen to be true.
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So he hit enter again and then we hit backspace in order to lineup up or else block with the if statement.
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So if we want this else to be lined up with this if they need to be at the same indentation in our code
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in a lot of times when you're working if any text editor will kind of automatically line things up for
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you.
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So keep that in mind.
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So else does it have any other conditions attached to it.
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Because we're only going to execute.
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Else if none of the conditions above happened to be true.
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So right now we're saying if you're hungry print's feed me otherwise.
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Prince I'm not hungry.
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I'll run this right now because Hungary was equal to false.
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We're getting back.
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I'm not hungry.
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If we change hungry to be true we get back.
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Feed me.
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What's also important to notice is that I'm just passing right here hungry by itself as a bullion.
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I don't actually need to do something like this.
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Check that Hungary is equal to true because hungry by itself is already a boolean.
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And we'll explore that example later on in more detail.
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So if hungry print's feed me else Prince I'm not hungry.
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OK.
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Now let's discuss multiple branches using if Ellefson else.
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So let's look at another example.
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I'm going to say EHLO see which stands for location.
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And I'm going to set that equal to bank.
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So have a location.
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It's equal to bank and I'm going to say if my location is equal to an auto shop I will for it.
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Cars are cool.
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Else I'll Prince.
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I do not know much.
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So when I run this it says I do not know much because the location was bank and we have location equal
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to our shop print cards.
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Cool.
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So that's an excuse.
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So then we have else printing.
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I don't know much what we can do is check for other conditions using elif.
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So let's pass in another condition here we'll say if the location is equal to the bank.
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Then Prince money is cool.
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As I'm sure everyone at the bank says and then we run this when we get back.
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Money is cool.
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So here we can stack on as many conditions using an if statement so we can add and more lives for more
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conditions.
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We can say Alosi is equal to store Colin Prince Welcome to the store we run that and we still get back.
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Money is cool.
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But as soon as we start changes condition let's change it to auto shop.
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We run that.
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We get back.
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Cars are cool.
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If we change it to store we get back.
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Welcome to the store.
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And if we change it to something else that's not in any of these conditions then we'll have the else
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block could x.
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So let's see what that looks like.
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Let's say we're going to some game and it says I do not know much.
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Perfect.
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All right.
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Just to drive this point home of indentation and white.
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We're going to do one last very simple example.
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Pretty much exactly the same as the last one.
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Let's define a name we'll say Sammy and I will say if this person's name is equal to let's say Frankie
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will print out Hello Frankie then we'll say well if we have some other name if elif name is equal to
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Sammy print out Hello Sammy and untypically for your L's condition it's going to be something where
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none of the other conditions were met.
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So good thing to do here would be ask the person what their really.
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What is your name.
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And later on in another lecture we'll actually learn how to get input from the user.
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But for now let's focus on a couple of things here.
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Note the indentation and know how.
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If Elif and else are all lined up for each other and their respective blocks are all indented and then
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we also have this colon at the end of these conditions.
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So now when I read this we should expect to see Hello Sammy and if we change this to Franki and run
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it again I get back Villefranche if it changes Tony and it's not there because they like it back.
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What is your name.
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All right that's the basics.
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If elephant.
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Else they seem pretty simple and hopefully they're pretty straight forward to you.
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Later on we're going to use them to create really nice large pieces of code that can execute more complex
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tasks.
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We'll see you at the next lecture.
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