All language subtitles for 7. [Dart] Functions - Part 3

So in the last lesson, we came across this error here where it tells us that this expression here, our built
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Key function, has a type of void and therefore cannot be used here.
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So in order to solve this conundrum, we have to understand a little bit more about how functions work.
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So here I've deleted everything in my dartpad
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other than the main function. And I'm going to create a new function. I'm going to call it myFunction,
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and it's not going to have any input, so I'm going to leave the parentheses here empty, but it will do
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something. In between the curly braces, I'm going to create a new variable that's going to hold values
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of the type double, and it's gonna be called pi. So pi is gonna be its name. And I'm gonna set it to equal
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3.14159, the value of the mathematical pi. And in my function, all that's gonna happen is
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this value pi, it's gonna be multiplied by two.
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Now I've got my function created, we know that the main function is the entry point of all of our Dart
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files.
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So whenever we hit run, it's going to look inside here to see what should happen. Inside here,
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I'm going to call that function I just created, which is called myFunction, and it's gonna have some
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empty parentheses because it doesn't take inputs.
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And now if I click run, you can see that pretty much nothing happens.
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Now firstly because I'm not really printing anything to the console, and all of this just happens behind
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the scenes without us being able to see what happens.
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But rest assured, this does get called and this in fact, does get carried out and we can make sure of this
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by simply adding a print statement here.
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Let's print and say that, 'my function got called'. We know that if this gets printed into the console, then
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this must have been triggered successfully from our main function.
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And indeed it does.
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So we can be sure that these instructions do in fact get carried out.
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But how can we get hold of this value, this pi * 2? Kind of like the result of this function,
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right?
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How can we get hold of it and bring it into our main function?
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What if we wanted to get something out of this function so that once we run this function, it outputs
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the value that it's calculated and then we can store it inside a variable? Let's call it maybe result.
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And then we could maybe just print the result from our main function.
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So in this case, we're basically trying to do something inside my function.
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And then once the results been calculated, we want this to be transported back into our main function
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so that we can use it here to do other things such as printing it.
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Now at the moment, this doesn't work.
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And when I click run, you can see that the error that I get, both from the Dart error checker as well
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as in the console,
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it tells me that the expression here has a type of void and therefore can't be used.
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And it's pointing to this part,
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myFunction.
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Now if this sounds familiar, it's because it's the same error that we got over here.
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So what does this mean?
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Well it means that this function currently doesn't actually output anything. It outputs void, which in
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programming lingo, basically means that there is no output.
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Now what if we wanted it to give us a output?
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Well we would change that from void to the data type of the output.
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So let's say in this case, we times pi by 2.
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It's still going to be a number with decimal places right?
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So the data type of the output is still going to be a double.
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And the final thing we need to do, is we need to mark the thing that we want to come out of the function
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with the keyword return.
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So now that we've updated our code, our error goes away
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and when I click run, we do in fact get 6.28318 printed over here.
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And remember the print statement is inside our main function.
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So in order for it to get a hold of this result, it had to call my function.
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So we had to run these two lines of code and then the result of pi * 2 gets outputed from my function,
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transported back in here and assigned to that variable result.
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And this is how we're able to print it into the console. So this is how we would solve the problem that
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we have with our Xylophone app.
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But how exactly do functions that have outputs work?
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Let's explore this in a little bit more detail. Previously when we talked about the second type of function,
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so functions that can take inputs,
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we saw that we could put a data type inside the parentheses and give the input a name, so that when we
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call getMilk we could provide a value here and that value will then be used inside the function or
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inside the curly braces.
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So in this case for example, our getMilk function takes a value, which is the amount of money that the
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robot got given to buy a single bottle of milk,
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and that value gets used to calculate the amount of change that the robot should get back. Which is the
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amount of money that it got given minus the price of a single bottle of milk.
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So money minus two.
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And we end up with the change.
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Now how can we get this change out of this function?
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Well in that case, we have to use the third type of functions, which are functions that can have a output.
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And in order for our function to have an output, we have to change the way that we created a little bit.
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Instead of having void, which marks the function as having no outputs, we change it to the data type that
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we expect to come out of the function, which in this case can be an integer.
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And we also specify what should come out of the function by using a keyword, which is return.
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So in this case we're saying the output of this function is change.
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So then if we gave our robot five coins and told it to buy some milk, it's the equivalent of calling
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our getMilk function passing in the number five, robot buys us a bottle of milk.
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And if the bottle milk costs 2 coins, then it gives us 3 coins back as the change.
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And so this expression over here, where it says getMilk passing in an input of 5, because it's able
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to have an output, once this line of code runs it will be equivalent to 3.
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So we could assign the output of getMilk to a variable called result.
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And in this case result would be equal to the output which is 3.
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So the result is equal to 3 because getMilk, starting with 5 coins is equal to 3. So
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in this case when we're creating the function, this is what it looks like.
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And when we're calling the function,
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this is what it would look like.
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This function has both an input, which is specified in between the parentheses, and also it can have an
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output which is specified after the keyword return.
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That keyword return marks whatever comes after it as the output of the function, and the int keyword
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at the beginning of our function tells us what is going to be the data type of the output of this function.
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So when we call the function, we can simply call it by its name passing in any inputs if it takes any.
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And once that gets carried out, the result or in this case money - 2, gets assigned to this variable.
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This is how the values would move through the function. If we were calling getMilk passing in the value
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5, 5 goes into the input and it gets used inside the function.
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So we end up with 5 - 2 as the return which is equal to 3 of course. And 3 is definitely
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a whole number
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so it fits the integer data type that we specified when we created the function.
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So that becomes the result of getMilk passing in an input of 5, and it replaces the right hand side
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of the equation.
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So now we have a variable called change which is equal to 3.
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So at this stage, we've seen all three types of functions.
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We've seen the simplest type,
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the vanilla function, which basically just packages a whole set of instructions together into a function.
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And whenever we call the function, we specify its name and a set of parentheses,
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then it will go into the curly braces of where this was created and execute all the instructions in
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that line by line.
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This is kind of just like a machine that does something, right? It does some calculations or it does some
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processing, but essentially there's no input and no output.
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Now the next type of function gets a little bit more exciting,
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this is our chocolate flavor function. And it now can take inputs and it takes those in between our parentheses.
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We have to specify a data type for the input so that when we call the function, we can give it a piece
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of data that matches that data type, and that number or that input gets used inside the function to do
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something or make this function more customized.
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And if this was a machine, then it's kind of like passing in a value into the machine so that it processes
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the data a little bit differently and you can end up with some custom functionality. So that if the input
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is different, then the functionality will also be a little bit different each time.
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Now this leads us to our third type of functions and the most advanced type of functions that we've
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seen so far. And this is where we can have both an input which goes into the parentheses as well as an
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output which we specify in the beginning, and also we specify inside the function to say what should
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be the output.
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And this is equivalent to a fully fledged machine where not only can you provide an input, but you can
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also get something that comes out of the machine, as an output.