Functions

In this chapter, we will look at the basics of function declaration in Swift, and we'll leave the more advanced features until Chapter 5, Advanced Swift.

To define a function in Swift, we use the following syntax:

func functionName(argument: argumentType) -> returnType 
{ 
    // code 
} 

We can define a function that takes no arguments and returns no value as follows:

func printDate() 
{ 
    print(Date()) 
} 

Note that it is not necessary to declare a Void return value, though we may do so if we choose.

We would call that function as follows:

printDate() 

Arguments to the function are supplied in the parentheses following the function name, in the form argName: argType; if there are more than one, they are separated by commas:

func printPoint(x: Int, y:Int) 
{ 
    print(x, y) 
} 

We would call that function as follows:

printPoint(x: 11, y: 59) 

The argument value(s) must be preceded by the argument name(s), as can be seen previously.

This is how we define a return value:

func increment(x: Int) -> Int 
{ 
    return x + 1 
} 

Calling this function looks as follows:

let y = increment(x: 0) 

A function can have one (and only one) variadic argument, an arbitrary number of arguments of the same type, as well as other "normal" arguments:

func printMyInts(args: Int..., label: String) 
{ 
    print(label) 
    for x in args 
    { 
        print(x) 
    } 
} 

The variadic arguments are separated by commas in the call to the function:

printMyInts(args: 1, 1, 2, 3, 5, 0, label: "Fibonnacci Five-0") 

It is quite simple to return any number of values, without having to resort to arrays, or dictionaries, or some other complex data structure, by using a tuple to enclose the return values:

func sumAndDifference(x: Int, y: Int) -> (sum: Int, diff: Int) 
{ 
    let sum = x + y 
    let difference = abs(x - y) 
    return (sum, difference) 
} 

We can access the results together as a tuple:

let results = sumAndDifference(x: 3, y: 5) 

These can then be decomposed by index or argument name, as with any tuple:

print(results.0) 
print(results.1) 
print(results.sum) 
print(results.diff) 

But we can also do that in one step:

let (sum, difference) = sumAndDifference(x: 5, y: 8) 

So far, so good; we have covered the basics of functions in Swift. But most of the good stuff is yet to come.