iOS 10 Programming Fundamentals with Swift

Chapter 2. Functions

Nothing is so characteristic of Swift syntax as the way you declare and call functions. Probably nothing is so important, either! As I said in Chapter 1, all your code is going to be in functions; they are where the action is.

Function Parameters and Return Value

A function is like one of those pseudoscientific machines for processing miscellaneous stuff that you probably drew in your math textbook in elementary school. You know the ones I mean: with a funnel-like “hopper” at the top, and then a bunch of gears and cranks, and then a tube at the bottom where something is produced. A function is a machine like that: you feed some stuff in, the stuff is processed in accordance with what this particular machine does, and something is produced.

The stuff that goes in is the input; what comes out is the output. More technically, a function that expects input has parameters; a function that produces output has a result. For example, here’s a very silly but perfectly valid function that expects two Int values, adds them together, and produces that sum:

func sum (_ x:Int, _ y:Int) -> Int {
    let result = x + y
    return result
}

The syntax here is very strict and well-defined, and you can’t use Swift unless you understand it perfectly. Let’s pause to appreciate it in full detail; I’ll break the first line into pieces so that I can call them out individually:

func sum                       (_ ...