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 1 (x:Int, _ y:Int) ...

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