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Method Return Values

6:30 with Jay McGavren

Let's get back to our cat food store program. We're still working on asking the user how many cans they want to order, and storing the number they enter. Now that we know how to write methods and pass arguments to them, we know most of what we need to add this feature.

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Let's get back to our cat food store program. We're still working on asking the user how many cans they want to order, and storing the number they enter.

Cat Food Store Features

~~Display welcome message~~
Ask for quantity
Calculate total
Discount for large orders

Now that we know how to write methods and pass arguments to them, we know most of what we need to add this feature. Let's give it a try now.

We'll create a new Ask method that takes a question parameter in the form of a string.
- For now, we'll just print out the question parameter.
In our Main method, we'll call Ask with the question: Ask("How many cans are you ordering?");

    static void Ask(string question)
    {
        Console.WriteLine(question);
    }

    static void Main()
    {
        Console.WriteLine("Welcome to the cat food store!");

        Ask("How many cans are you ordering?");
    }

We need to get text user enters at keyboard
- The Console.ReadLine method can do that for us
- When we call ReadLine, it waits for the user to type something and press Enter, and then returns what the user typed.
- In this version of Ask, we assign the value the user typed to an answer variable.
- Then we print the value in answer.

    static void Ask(string question) 
    {
        Console.WriteLine(question);
        string answer = Console.ReadLine();
        Console.WriteLine(answer);
    }

But really, we need to be able to access the user's answer in the Main method.
- We learned about variable scope earlier, and so we know that the answer variable is only in scope within the Ask method.
- I could try to access it within Main: Console.WriteLine(answer);
- ...But I'll get a compile error when I try to run this: dotnet run

We need a way to get the user's answer from the Ask method back to the Main method.

Some methods have a return value, a value they send back to the code that called them.
- At the top of a method definition, in place of void, write the type of value the method will return.
- In the case of our Add and Subtract methods, they'll return a double value.
- Return type doesn't have to match the parameter types; you can accept an int and a string parameter and return a bool, for example.

using System;

class Program
{
    static double Add(double first, double second)
    {
        return first + second;
    }

    static double Subtract(double first, double second)
    {
        return first - second;
    }

    static void Main(string[] args)
    {
        double total = Add(3, 5);
        Console.WriteLine(total);
        double remaining = Subtract(21.3, 7.1);
        Console.WriteLine(remaining);
    }
}

Pass return values to other functions

    static void Main(string[] args)
    {
        // We don't have to assign the return value to a variable first.
        // We can just pass it straight to WriteLine.
        Console.WriteLine(Add(3, 5));
        Console.WriteLine(Subtract(21.3, 7.1));
    }

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Let's get back to our cat food store program. 0:00

We're still working on asking the user, how many cans they want to order and 0:02

store the number they enter. 0:06

Now that we know how to write methods and pass arguments to them, 0:08

we know most of what we need to add this feature, let's give a try now. 0:11

We'll create a new ask method that takes a question parameter in the form 0:15

of a string. 0:19

For now will just print out the question parameter. 0:20

In our main method, we'll co ask with the question, how many cans are you ordering? 0:26

Next, we need to get the text, the user enters at the keyboard. 0:39

The Console.ReadLine method can do that for us. 0:42

When we call ReadLine, it waits for the user to type something and 0:46

press Enter, and then returns what the user typed. 0:49

In this version of Ask, 0:52

we assign the value the user typed to an answer variable. 0:54

Then we print the value and answer, let's try saving this and 0:57

running it with dotnet run. 1:02

And you can see that our new ask method gets called, it prints the question, 1:06

how many cans are you ordering? 1:10

And then it's waiting for us to type a response, so 1:11

let's enter the number 10 and hit Enter. 1:14

And the call to Console.ReadLine then returns the number 10, 1:17

which gets assigned to this answer variable. 1:22

And that's what gets printed on the following line, 1:25

you can see it prints out what we typed in again 10. 1:28

But really, we need to be able to access the user's answer in the main method. 1:31

We learned about Variable Scope earlier, and so 1:36

we know that the answer variable is only in scope within the ask method. 1:38

I could try to access it within Maine, but 1:43

I'll only get a compile error if I try running it now. 1:45

Oops I forgot to save, let me save my work, and cancel this and rerun it. 1:50

There is our compile error. 1:58

The name answer does not exist in the current context. 1:59

And you will notice this refers to line 18, which is down here in main. 2:03

There is no answer variable down here, 2:07

because its only declared up in the ask method. 2:09

So we need the way to get the users answer from the ask method back to 2:13

the main method. 2:17

Some methods have a return value, 2:19

a value they send back to the code that called them. 2:21

At the top of a method definition, in place of void, 2:26

you should write the type of value the method will return. 2:29

So for this Add method, it's going to accept a first parameter and 2:32

a second parameter, both of which are doubles. 2:37

And I'm gonna have it return a double parameter as well. 2:40

The type of the return value doesn't have to match the type of the parameters, 2:43

but it can't match, then within your method body. 2:48

You need to type the return keyword followed by a value you want to return. 2:52

This code here will add the first and second parameters together and 2:56

then return the resulting value. 3:00

Down here in Main, we'll make a call to add. 3:10

Add, parenthesis, and we'll give it parameters of 3 and 5, so 3:12

it'll add those two numbers together. 3:17

But then add is going to return a value, so 3:19

we need to do something with that value. 3:21

What we're gonna do is, we're going to take that return value and 3:23

we're gonna assign it to a variable that will have a type of double. 3:26

Because that's the type of value that the Add method is returning, and 3:30

I'll name that total. 3:34

So, Add 3, 5, is going to return eight and we'd take that value and 3:38

assign it to the total variable. 3:42

That won't show us what the total variable contains though, so 3:49

I'm gonna make a separate call to cancel that WriteLine, 3:52

and pass the total variable to it so it prints that out. 3:55

Let me save this. 3:58

And I'll go down to my console and try running it. 4:02

Dotnet run, so we call add with the arguments 3 and 5. 4:08

Three gets assigned to the first parameter, 4:13

five gets assigned to the second parameter. 4:16

We add those two values together to get eight, and 4:19

that's what gets returned from the add method. 4:21

That value then gets assigned to the total variable, which we print down here, and 4:25

there's our result, eight. 4:29

Let's try declaring a subtract method as well, 4:33

that's going to take two parameters as well. 4:37

Both of which will have a type of double, and it'll return a double value. 4:42

It'll subtract the first parameter from the second parameter, and 4:46

return the result. 4:49

Then down here in Main, we'll make a call to subtract. 4:55

I'll pass it, floating point numbers 21.3 and 7.1. 4:58

We can do that since we specify the type of double, and 5:02

we'll take the return value and assign it to the remaining variable. 5:06

Obviously, that won't do anything by itself. 5:12

So we make a call here to print the value of remaining. 5:14

Let me save this and let's try running it. 5:22

And here's our same result from before for calling add the number eight, but 5:26

here's our new result for calling subtract. 5:30

We subtract 7.1 from 21.3 and we get the result 14.2. 5:32

You don't have to assign a return value to a variable though. 5:40

We can also pass it directly to another function. 5:46

So, we can actually get rid of our total variable here, and 5:49

just pass the return value from add directly to Console.WriteLine. 5:52

So let's try saving that and running it, dotnet run. 5:58

And you can see we get the same result, eight. 6:02

The return value of Add was passed directly to Console.WriteLine. 6:05

Let's go and do the same with our subtract call, I'll just cut this code out of here. 6:09

And get rid of our remaining variable, and instead, 6:14

pass the return value of subtract directly to Console.WriteLine, save that. 6:17

Ctrl+L to clear my console, and let's try re-running it. 6:23

Again, same result as before. 6:28

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