Cocoa Programming for OS X: The Big Nerd Ranch Guide

Computed Properties

It is frequently useful to find a vector’s length or magnitude. You could do this by adding a function returning a Double:

struct Vector {
    ...

    func length() -> Double {
        return sqrt(x*x + y*y)
    }
}

However, it is much more natural to think of this as a read-only property. In Swift the general term for this is computed property, which is in contrast to the stored properties you have been using so far. A read-only computed property version of length would look like this:

struct Vector {
    ...

    var length: Double {
        get {
            return sqrt(x*x + y*y)
        }
    }
}

This read-only computed property pattern (called a “getter”) is so common, in fact, that Swift provides a shorthand means of expressing it. Add this to Vector: