Method Declarations

Sometimes we want to repeat a process, but each time we do it we change some part of what we're doing. For example, imagine creating chess boards where each chess board has a different combination of colors. It would be extremely tedious to declare each chess board as we have above for each choice of colors. What we'd like is to be able to define some process for making chess boards that allows the user to specify the color choice for the particular chess board we're making. This is what methods allow us to do.

We have already seen method calls. In this section we are going to see how we can declare our own methods. Like value declarations, method declarations define a name. Instead of giving a name to a value, a method declaration gives a name to a process for creating values:

def twoByTwo(color1: Color, color2: Color): Image = {
  val square1 = Rectangle(30, 30) fillColor color1
  val square2 = Rectangle(30, 30) fillColor color2

  (square1 beside square2) above
  (square2 beside square1)
}

This declares a method called twoByTwo. The method has two parameters, called color1 and color2, which we have declared to have type Color. We have also declared the type of the value returned---Image. The body of the method, which is enclosed with optional braces (the { and } pair) defines how we create the Image. This mirrors the process for creating a two by two chess board that we saw above, but in this case we are using the colors we are passed as parameters.

Exercise: Chess Board

Declare a method called fourByFour that constructs a four-by-four chess board using twoByTwo declared above. The method should have two parameters, both Colors, that it passes on to twoByTwo.

You should be able to call fourByFour like so

fourByFour(Color.cornflowerBlue, Color.seaGreen) beside
fourByFour(Color.chocolate, Color.darkSalmon)

to create this picture:

<div class="solution"> The structure of fourByFour is identical to twoByTwo except that we use twoByTwo to construct the squares we build the board.

def fourByFour(color1: Color, color2: Color): Image = {
  val square = twoByTwo(color1, color2)

  (square beside square) above
  (square beside square)
}

</div>

<div class="callout callout-info"> Syntax Summary

We've seen quite a lot of Scala syntax so far. If you can't remember everything we've covered, don't panic! There's a syntax quick reference in the appendices at the end of the book. </div>