swift 4 函数

• parameter和argument的区别

Note: Take care not to confuse the terms “parameter” and “argument”. A function declares its parameters in its parameter list. When you call a function, you provide values as arguments for the functions parameters.

parameter是函数的形参，argument是函数调用时的实参。

• 函数格式
  函数的定义要像读一条句子，例如下面：

Print multiple of multiplier 4 and value 2

func printMultipleOf(multiplier: Int, andValue: Int) {
  print("\(multiplier) * \(andValue) = \(multiplier * andValue)")
}
printMultipleOf(multiplier: 4, andValue: 2)

• 进一步优化

Print multiple of multiplier 4 and 2

func printMultipleOf(multiplier: Int, and value: Int) {
  print("\(multiplier) * \(value) = \(multiplier * value)")
}
printMultipleOf(multiplier: 4, and: 2)

• 如果你不想要函数在调用时显示参数的名字（下划线）

func printMultipleOf(_ multiplier: Int, and value: Int) {
  print("\(multiplier) * \(value) = \(multiplier * value)")
}
printMultipleOf(4, and: 2)

func printMultipleOf(_ multiplier: Int, _ value: Int) {
  print("\(multiplier) * \(value) = \(multiplier * value)")
}
printMultipleOf(4, 2)

• 给形参添加默认值

func printMultipleOf(_ multiplier: Int, _ value: Int = 1) {
  print("\(multiplier) * \(value) = \(multiplier * value)")
}
printMultipleOf(4)

• 函数的返回值（箭头）

func multiply(_ number: Int, by multiplier: Int) -> Int {
  return number * multiplier
}
let result = multiply(4, by: 2)

func multiplyAndDivide(_ number: Int, by factor: Int)
                   -> (product: Int, quotient: Int) {
  return (number * factor, number / factor)
}
let results = multiplyAndDivide(4, by: 2)
let product = results.product
let quotient = results.quotient

• 传值和传址
  默认swift的形参是let类型的常量，所以我们不能作如下操作。

func incrementAndPrint(_ value: Int) {
  value += 1
  print(value)
}

报错

Left side of mutating operator isn't mutable: 'value' is a 'let' constant

• inout（copy-in copy-out）

func incrementAndPrint(_ value: inout Int) {
  value += 1
  print(value)
}

inout表明要传址了，我们要在调用的时候做点小修改（取地址符是时候出场了）

var value = 5
incrementAndPrint(&value)
print(value)

• 函数作为参数

func add(_ a: Int, _ b: Int) -> Int {
  return a + b
}
func printResult(_ function: (Int, Int) -> Int, _ a: Int, _ b: Int) {
  let result = function(a, b)
  print(result)
}
printResult(add, 4, 2)

• 函数从没返回值（Never关键字）

func noReturn() -> Never {
    while true {
        
    }
}

• stride函数介绍

for index in stride(from: 10, to: 22, by: 4) {
  print(index)
}
// prints 10, 14, 18

for index in stride(from: 10, through: 22, by: 4) {
  print(index)
}
// prints 10, 14, 18, and 22