[Swift] Functions

Function Parameters and Return Values

Optional Tuple Return Types

An optional tuple type such as (Int, Int)? is different from a tuple that contains optional types such as (Int?, Int?). With an optional tuple type, the entire tuple is optional, not just each individual value within the tuple.

func minMax(array: [Int]) -> (min: Int, max: Int)? {
    if array.isEmpty { return nil }
    var currentMin = array[0]
    var currentMax = array[0]
    for value in array[1..<array.count] {
        if value < currentMin {
            currentMin = value
        } else if value > currentMax {
            currentMax = value
        }
    }
    return (currentMin, currentMax)
}

Function Argument Labels and Parameter

Default Parameter Values

이렇게 default Value를 사용할 때 매개 변수의 순서를 지켜주는 것이 좋다. 앞쪽에 default Value를 정의하지 않은 매개변수들을 두고 뒤쪽에 default value를 사용한 매개변수들을 두는 것이 좋다.

func someFunction(parameterWithoutDefault: Int, parameterWithDefault: Int = 12) {
    // If you omit the second argument when calling this function, then
    // the value of parameterWithDefault is 12 inside the function body.
}
someFunction(parameterWithoutDefault: 3, parameterWithDefault: 6) // parameterWithDefault is 6
someFunction(parameterWithoutDefault: 4) // parameterWithDefault is 12

Variadic Parameters

A variadic parameter accepts zero or more values of a specified type. Write variadic parameters by inserting three period characters (...) after the parameter’s type name.

unc arithmeticMean(_ numbers: Double...) -> Double {
    var total: Double = 0
    for number in numbers {
        total += number
    }
    return total / Double(numbers.count)
}
arithmeticMean(1, 2, 3, 4, 5)
// returns 3.0, which is the arithmetic mean of these five numbers
arithmeticMean(3, 8.25, 18.75)
// returns 10.0, which is the arithmetic mean of these three numbers

In-Out Parameters

If you want a function to modify a parameter’s value, and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead.

You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an in-out parameter, to indicate that it can be modified by the function

In-out parameters can’t have default values, and variadic parameters can’t be marked as inout.

func swapTwoInts(_ a: inout Int, _ b: inout Int) {
    let temporaryA = a
    a = b
    b = temporaryA
}

var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
// Prints "someInt is now 107, and anotherInt is now 3"

Function Types

Function Types as Parameter Types

func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {
    print("Result: \(mathFunction(a, b))")
}
printMathResult(addTwoInts, 3, 5)
// Prints "Result: 8"

Function Types as Return Type!!!!

Nested Functions

functions inside the bodies of other functions, known as nested functions.

func chooseStepFunction(backward: Bool) -> (Int) -> Int {
    func stepForward(input: Int) -> Int { return input + 1 }
    func stepBackward(input: Int) -> Int { return input - 1 }
    return backward ? stepBackward : stepForward
}
var currentValue = -4
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
// moveNearerToZero now refers to the nested stepForward() function
while currentValue != 0 {
    print("\(currentValue)... ")
    currentValue = moveNearerToZero(currentValue)
}
print("zero!")
// -4...
// -3...
// -2...
// -1...
// zero!