Control Flow

2017-10-13 本文已影响0人夜雨聲煩_

固定次数遍历

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}

可以使用forin加数组遍历整个数组，也可以在数组后的[]中加入...指定数组内部的遍历范围
当循环内不需要循环变量时使用_代替index
使用stride设定步长，to不包含右端，through包含
```
for tickMark in stride(from: 0, to: 60, by: 5) {  
}
```
while适用于循环长度未知的情况

一个经典蛇和梯子的游戏逻辑实现

let finalSquare = 25
var board = [Int](repeating: 0, count: finalSquare + 1)

board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08

var square = 0
var diceRoll = 0
while square < finalSquare {
    // roll the dice
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    // move by the rolled amount
    square += diceRoll
    if square < board.count {
        // if we're still on the board, move up or down for a snake or a ladder
        square += board[square]
    }
}
print("Game over!")

//如果使用repeat-while来实现的话，代码如下
repeat {
    // move up or down for a snake or ladder
    square += board[square]
    // roll the dice
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    // move by the rolled amount
    square += diceRoll
} while square < finalSquare
print("Game over!")

switch不同于OC中顺序执行，在执行某一case之后自动退出switch，不需要手动加入break
case中至少要有一行代码，不能为空

case中可以包含多条件，中间用逗号分隔

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a", "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// Prints "The letter A"

case中可以使用...来做区间判断

case中可以使用元组，元组中可以使用...做区间判断

let somePoint = (1, 1)
switch somePoint {
case (0, 0):
    print("\(somePoint) is at the origin")
case (_, 0):
    print("\(somePoint) is on the x-axis")
case (0, _):
    print("\(somePoint) is on the y-axis")
case (-2...2, -2...2):
    print("\(somePoint) is inside the box")
default:
    print("\(somePoint) is outside of the box")
}
// Prints "(1, 1) is inside the box"

case中元组其中某些项可以定义为变量，其余项符合时这些变量项可以用于body中
case中可以where做条件判断
```
case let (x, y) where x == y:
```

case中可以使用compound

case (let distance, 0), (0, let distance)

控制转移语句：continue, break, fallthrough, return, throw
continue表示立即停止本次循环继续开始下一次循环
break表示终止该整个control flow，在switch中case的body内部不允许为空，所以使用break
使用fallthrough完成C中switch继续执行下一次case的情况
使用标签定义循环，使用break或continue加标签表示退出或继续的循环体
```
gameLoop: while square != finalSquare 
break gameLoop
```

guard类似于if, 不同之处在于guard用于判断某一条件是否为真，如果为真继续，不为真执行else中代码（else多用return直接退出），guard必须带有else。区别if作为专用，用于判断不满足需求条件的情况，利于阅读

  //某一元素缺失时
   guard let location = person["location"] else {
      //打印相关缺失内容
      print("I hope the weather is nice near you.")
      //直接退出该方法
      return
  }

  //当location存在时才会执行，不存在时该代码不会执行
  print("I hope the weather is nice in \(location).")

使用if #available 判断版本，*表示任意其他平台

if #available(iOS 10, *) {
    //Use ios 10 APIs on ios
} else {
    //Fall back to earlier ios
}

代码：

gameLoop: while square != finalSquare {
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    switch square + diceRoll {
    case finalSquare:
        // diceRoll will move us to the final square, so the game is over
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // diceRoll will move us beyond the final square, so roll again
        continue gameLoop
    default:
        // this is a valid move, so find out its effect
        square += diceRoll
        square += board[square]
    }
}
print("Game over!")

Control Flow

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