函数

• 函数名

  描述函数功能，调用函数时使用。

• 定义和调用函数

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

  func 关键字，greetAgain 函数名，person 参数标签，String 参数类型，-> String 返回值极其类型， {} 函数功能代码，"Anna" 实际参数

  函数形式参数和返回值

• 无形式参数的函数

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

• 多形式参数的函数

  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!"

• 无返回值的函数

  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 12
  printWithoutCounting(string: "hello, world")
  // prints "hello, world" but does not return a value

  严格来说，无返回值函数返回了特殊值Void，如有返回值，需要处理返回值，不然函数出错。

• 多返回值的函数

  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"

• 可选元组的返回类型

  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)
  }

  函数实际参数标签和形式参数名

• 指定实际参数标签

  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."    

• 省略实际参数标枪

  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)

  利用下划线（ _ ）来为这个形式参数代替显式的实际参数标签

• 默认形式参数值

  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 6
  someFunction() // parameterWithDefault is 12

• 可变的形式参数

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

  形式参数的类型名称后边插入三个点符号（ ...）来书写可变形式参数

• 输入输出形式参数

  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"

  函数类型

• 使用函数类型

  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 = addTwoInts
  
  print("Result: \(mathFunction(2, 3))")
  // prints "Result: 5"
  
  mathFunction = multiplyTwoInts
  print("Result: \(mathFunction(2, 3))")
  // prints "Result: 6"

• 函数类型作为形式参数类型

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

• 函数类型作为返回类型

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

• 内嵌函数

  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!