Lesson 8: How Input, Parameters, and Arguments Work

Input is information added to a program and used by code to make changes.

The function examples given in the previous lesson didn’t need any input:

GetNumberOfMousetrapsYouCanAfford() : int

You know this because the parentheses () in the function signature above are empty.

Parameters

You can define the input a function needs by adding a parameter to the function signature.

A parameter is a constant that’s declared in the function signature between the parentheses. When a parameter is set, you can use it in the body of the function.

The syntax for a function that includes a parameter looks like this:

name(parameter : type) : type =
	codeblock

In the following example, CoinsPerMousetrap is now a parameter for the function BuyMousetrap():

var Coins : int = 500

BuyMousetrap(CoinsPerMousetrap : int) : void =
    set Coins = Coins - CoinsPerMousetrap
    Print("Mousetrap bought! You have {Coins} coins left.")

What this code is saying is:

• You have a variable called Coins with a starting integer value of 500: var Coins : int = 500.

• The function called BuyMousetrap() takes a parameter of CoinsPerMousetrap. However, you don't want this function to return a value, so the type is void.

Arguments

When you call a function that expects parameters, you've got to assign values to the parameters, the same way you assign values to constants.

These assigned values for functions are called arguments to the function.

To call the BuyMousetrap() function, you do so by adding an argument inside the parentheses. There are a few ways to do this.

For example:

• You could use BuyMousetrap(CoinsPerMousetrap := 10), which mimics how you define the parameters for the function. The 10 is an argument to the function, and changes the value of the result.

• Remember literals from Lesson 2 and Lesson 3? You could also use a literal value of the same type as the parameter.

• A third way is to use a previously declared variable or constant as your argument. You can call the function using different values for the argument and get different results.

  Verse

        var Coins : int = 500
        # After this call, Coins is 490
        BuyMousetrap(CoinsPerMousetrap := 10)
  		
        # After this call, Coins is 485
        BuyMousetrap(5)
  		
        CoinsPerMouseTrap : int = 20
        # After this call, Coins is 465
        BuyMousetrap(CoinsPerMousetrap)
  

  Copy full snippet(10 lines long)

Multiple Parameters

You can define as many parameters to the function as you want as long as you separate the parameters with commas.

The syntax for a function with two parameters looks like this:

name(parameter : type, parameter : type) : type =
    codeblock

When you call the function, you'll need to separate the arguments with commas again, which mimics how you define the parameters to the function.

Summary

• A parameter is a constant that's declared in the function signature. It’s inside parentheses.

• An argument is the value assigned to the constant used in the parameter for that function.

• You have to separate multiple parameters in a single function with commas.

Practice Time!

• 

  Lesson 8: Practice Time!

  Practice setting parameters and arguments for functions.