swift4--function, self-study notes

Function

• function names describe function functions, which are used when calling functions.
• Defining and calling Functions

  func greetAgain(person: String) -> String {    return "Hello again, " + person + "!"}print(greetAgain(person: "Anna"))// Prints "Hello again, Anna!"

  Func keyword, greetagain function name, person parameter label, string parameter type,-> string return value and its type, {} function function code, "Anna" actual argument

  function form parameters and return values
• Functions with no formal parameters

  func sayHelloWorld() -> String {    return "hello, world"}print(sayHelloWorld())// prints "hello, world"

• Functions of multi-form parameters

  func greet(person: String, alreadyGreeted: Bool) -> String {    if alreadyGreeted {        return greetAgain(person: person)    } else {        return greet(person: person)    }}print(greet(person: "Tim", alreadyGreeted: true))// Prints "Hello again, Tim!"

• function with no return value

  func printAndCount(string: String) -> Int {    print(string)    return string.characters.count}func printWithoutCounting(string: String) {    let _ = printAndCount(string: string)}printAndCount(string: "hello, world")// prints "hello, world" and returns a value of 12printWithoutCounting(string: "hello, world")// prints "hello, world" but does not return a value

  Strictly speaking, no return value function returns a special value void, and if there is a return value, the return value needs to be processed or the function is faulted.
• Functions with multiple return values

  func minMax(array: [Int]) -> (min: Int, max: Int) {    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)}let bounds = minMax(array: [8, -6, 2, 109, 3, 71])print("min is \(bounds.min) and max is \(bounds.max)")// Prints "min is -6 and max is 109"

• return type of an optional 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 actual parameter label and formal parameter name
• Specify the actual parameter label

  func greet(person: String, from hometown: String) -> String {    return "Hello \(person)!  Glad you could visit from \(hometown)."}print(greet(person: "Bill", from: "Cupertino"))// Prints "Hello Bill!  Glad you could visit from Cupertino."    

• Omit actual parameter Javelin

  func someFunction(_ firstParameterName: Int, secondParameterName: Int) {    // In the function body, firstParameterName and secondParameterName    // refer to the argument values for the first and second parameters.}someFunction(1, secondParameterName: 2)

  Use an underscore (_) to replace the explicit actual parameter label for this form parameter
• Default form parameter values

  func someFunction(parameterWithDefault: Int = 12) {    // In the function body, if no arguments are passed to the function    // call, the value of parameterWithDefault is 12.}someFunction(parameterWithDefault: 6) // parameterWithDefault is 6someFunction() // parameterWithDefault is 12

• Variable formal parameters

  func 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 numbersarithmeticMean(3, 8.25, 18.75)// returns 10.0, which is the arithmetic mean of these three numbers

  The type name of the formal parameter is inserted behind the three dot symbol (...). ) to write variable form parameters
• Input and output form parameters

  func swapTwoInts(_ a: inout Int, _ b: inout Int) {    let temporaryA = a    a = b    b = temporaryA}var someInt = 3var anotherInt = 107swapTwoInts(&someInt, &anotherInt)print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")// prints "someInt is now 107, and anotherInt is now 3"

  function type
• Using function types

  func addTwoInts(_ a: Int, _ b: Int) -> Int {    return a + b}func multiplyTwoInts(_ a: Int, _ b: Int) -> Int {    return a * b}var mathFunction: (Int, Int) -> Int = addTwoIntsprint("Result: \(mathFunction(2, 3))")// prints "Result: 5"mathFunction = multiplyTwoIntsprint("Result: \(mathFunction(2, 3))")// prints "Result: 6"

• function type as formal parameter type

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

• function type as return type

    func stepforward (_ Input:int), Int {return input + 1}func Stepbackward (_ Input:int)-& Gt int {return input-1}func choosestepfunction (backwards:bool)-(int), int {return backwards? stepbackw Ard:stepforward}var CurrentValue = 3let Movenearertozero = choosestepfunction (Backward:currentvalue > 0)//moveNeare    Rtozero now refers to the Stepbackward () Functionprint ("Counting to Zero:")//counting to zero:while CurrentValue! = 0 { Print ("\ (CurrentValue) ... ") CurrentValue = Movenearertozero (currentvalue)}print (" zero! ") 3...//2...//1...//zero!  

• inline 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 = -4let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)// moveNearerToZero now refers to the nested stepForward() functionwhile currentValue != 0 {    print("\(currentValue)... ")    currentValue = moveNearerToZero(currentValue)}print("zero!")// -4...// -3...// -2...// -1...// zero!

swift4--function, self-study notes