All language subtitles for 38. [DART DEEP DIVE] More on if Statements

So in the last lecture, I introduced if statements. You create if statements by writing if and then between

parentheses, your condition.

Now whenever this condition here is met, you make it into this body of the if statement which is found

between these curly braces and just as after all such body statements with curly braces, you don't have

a semicolon there.

Now what goes between these parentheses though?

Now as I said, a condition but to be very precise, what goes between there is actually an expression

that resolves to true or false

and that's a specific type in Dart and this type exists in many programming languages,

it's called a boolean.

So here for example, we have a new variable,

let's say we're tracking the logged in status of our user and that could be set to true or to false

and that would be a boolean. A boolean is a value which is either true or false,

there is no other possible value. So true or false

and of course this is a very handy type of value, a very handy data type because often in programming,

you have these two options and you especially need booleans in conjunction with if statements because

here, you in the end want to find out if some condition is true, then do this or if some condition is

false, then don't do that

but maybe that's also something you have on an if condition, maybe you then do something else.

So that would be your else block here which you can also specify on an if condition to have an alternative

if this is not met. You don't need to have an else block though, it's totally fine if you just have the if

block, then you might execute this code or do nothing

if this condition is not met or if you have an else block, then you have an alternative code to run.

So both is possible and booleans are the key to that.

Now the condition here typically is one of two things, either you're referring to a boolean here,

so to a variable that holds a boolean and then somewhere in your code, you probably have a way of setting

this dynamically because of course, this code snippet here right now doesn't make a lot of sense because

isLoggedIn is hardcoded to be false, hence you can never make it into this block, you would always

go into the else block. But if isLoggedIn is controlled somewhere else in your code, if it is set dynamically

based on some user input or some other action that is occurring, then this here is a typical check.

Now what I'm doing here by the way is a shortcut to equals true,

I'm comparing if isLoggedIn, so the value is stored in this variable, is true.

That's the default check that is performed

if you omit the double equals signs.

So this here, if isLoggedIn, which can be read as if true, is equal to this check,

so it's redundant to check for equality to true because that's the default if you just refer to a variable

or to some expression.

Now as I mentioned, this is one of two common scenarios, that you have some variable or maybe also some

function call where you call some other function which returns true or false, so that you check something

that yields true or false here and that something is a variable or a function call.

The other common scenario is that you write a condition here where you directly compare two or more

values.

Let's say we have the username and that's Max and I have my age, 30 and I want to make sure that we execute

some code here,

logged in, when the username is equal to Max.

Let's also add a password to make this a bit more realistic, tester.

So here when my username and my password are correct, I want to print this, otherwise I want to take my user

or the code execution into the else block and print failed.

Now in such a case, we want to check the values of username and password and we therefore have no single

variable that is true or false but we want to check if username has a certain value and password has

a certain value. In such a scenario, you can check if username is equal to Max

and and that's something new which you haven't seen before, the && sign here means that

you also need some other condition to be true

and here you check if password is equal to tester for example.

So in this scenario, what I'm doing is I'm checking two condition and both have to be true to make it

into this if block, if at least one of them is false, even if the other one is true, you make it into

the else block here.

So now if we run this code, we therefore see logged in because username is Max, I'm setting it here and password

is tester, I'm setting it here.

Please be aware that double equals sign has a totally different meaning than a single equals sign. You

use a single equals sign here to store a value in the variable, you use a double equals sign here to

compare a variable's value with another value and that is something totally different. Such a comparison,

where you use such a boolean comparison operator as it's called, always yields a boolean.

So this here yields true or false and that's where we're back to this boolean value up there

and of course, you could also use some helper variable, like hasCorrectCredentials or whatever you want

to use here

where are you putting this check in, like this,

this is a valid code and then down there, you only use that variable in the if condition to have a leaner

if condition.

This is totally valid,

now I'm basically using this boolean operation here with my comparison and my concatenation of conditions

to derive a boolean value which is true if both conditions are met or false if at least one fails and

therefore I store true or false in hasCorrectCredentials and then I'm using hasCorrectCredentials

down there, which is true or false.

So that's all possible but I'd say the more common case is that you don't use such helper variables

but that you put your condition right into your if statement here.

Now sometimes you also have alternative conditions, here both have to be met but let's say for whatever

reason, we also want to log the user in

if he's older than 20. So we can also add an or statement with two pipe symbols here.

So here, we can add or and check

or age > 20 and that's another boolean comparison operator here. Besides a double equal sign,

you also have greater than, greater than or equal, smaller than or smaller than or equal and also not

equal

with the exclamation mark in front of the equals sign.

Please note that this is only one equal sign in here.

So these are alternative operators

and here I am using the greater than operator so that this part here returns true if age is greater

than 20, which it here of course is.

Now how can we read this overall expression though?

Well let's see what happens if I now change my password, so that it's wrong,

so that this first part here shouldn't be met. If I now run this, we're still logged in,

so how can this be read?

Well this reads as please log me in if this is true and this is true or if this is true

and well, this here is not true and therefore, this combination here is not true

but this here, this is true and since it's an or, this overrules the prior false which this returns because

we have an alternative condition that could be met and only if this condition and this condition would

be false, we would not make it in here because then neither of our two or conditions would be true. And

of course, it's always a great idea that you simply play around with DartPad here and with if statements

to find out how you can configure that and what you can do there and how the different operators behave.

Sometimes you also want to combine conditions differently than they're parsed by default.

So instead of maybe saying this and this should be true, you might also want to say well this should

always be true and then this here should also yield true,

so you want to combine this check, so you want to say yes you're logged in

if the username is Max and then if either the password is tester or the age is greater than 20. Right

now this reads as you're logged in if this is true and this is true or if this is true.

So if you would want to combine password check and age check together, you could also use parentheses in

there,

just like in mathematical equations and now you're saying these two conditions here in the end belong

together,

so now the username always has to be Max and then you're either having the right password or the right

age.

So at the moment, that means we're logged in because the username is Max

but now if I change Max up here to Maximilian, we would actually see that when I run this again,

we're failing because this always needs to be true,

I'm then combining it with the check of the latter condition here.

Now if I would for example remove these parentheses which I just added, you'll see that now, I would

actually be logged in and that's making a difference,

I would be logged in now because I'm checking is this true and this true or is this true

and neither of these first two checks is true

but this here is and therefore that's why we're now logged in.

So using these parentheses correctly is very important

and of course, it's also important that you use these comparison operators correctly,

also be aware of the not equal operator which is also super important and therefore you have a lot of

power to control when your code executes. Now sometimes, you also have a more complex if check where you

don't just have an if and an else statement but you know if this here fails, I might have some alternative

code to execute but that code also depends on some condition and then you can add an else if statement,

you can then also still have a normal else statement if you want to.

Now you would say well if this here is not met, if this first condition is not met, then we make it to

the next block in line and that is an else if statement and there, I can check yet another condition

and only if this condition is then also not met, we make it into this else block.

If this condition here is met, we don't make it into this else block

and here we could check if isLoggedIn is true

and now if I also add this extra parentheses pair up there again, so that the first check will fail,

now we should make it into the else if block and then we could print overruled here to see that this executed

but when I hit run here, you indeed see failed because of course isLoggedIn is false.

As soon as we set this to true however or where we derive this dynamically and it yields true, then

we see overruled because now we're running the else if block because the first condition was not met.

Therefore we got into here,

we checked this and this is true

and therefore now, we're running this,

we're then not making it into the else block too.

These are if statements and they're an important construct in basically any programming language you

can learn and therefore also in Dart. We'll use them a lot throughout the course and I hope that this lecture

here was helpful in understanding how they work and how you may play around with them.