swift2 控制流

For循环

for循环有两种形式，一种是for in 可以方便的变量数组等集合类型，另一张是与c语言类型的基本for循环。

for in

let numberOfLegs = ["spider": 8,"ant": 6,"cat": 4]
for (animalName,legCount) in numberOfLegs {
    print("\(animalName)s have \(legCount) legs")
}
// spiders have 8 legs
// ants have 6 legs
// cats have 4 legs

条件递增

for var index = 0; index < 3; ++index {
    print("index is \(index)")
}
// index is 0
// index is 1
// index is 2

在初始化表达式中声明的常量和变量（比如var index = 0）只在for循环的生命周期里有效。

如果想在循环结束后访问index的值，你必须要在循环生命周期开始前声明index。

while循环

while也包括两种，while循环和repeat while循环（repeat while 就是其他语言的 do while)

while

var square = 0
var diceRoll = 0
while square < finalSquare {
    // 掷骰子
    if ++diceRoll == 7 { diceRoll = 1 }
    // 根据点数移动
    square += diceRoll
    if square < board.count {
        // 如果玩家还在棋盘上，顺着梯子爬上去或者顺着蛇滑下去
        square += board[square]
    }
}
print("Game over!")

repeat while

repeat {
    // 顺着梯子爬上去或者顺着蛇滑下去
    square += board[square]
    // 掷骰子
    if ++diceRoll == 7 { diceRoll = 1 }
    // 根据点数移动
    square += diceRoll
} while square < finalSquare
print("Game over!")

代码摘自 the swift programing language 2

if语句

let temperatureInFahrenheit = 90
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
    print("It's really warm. Don't forget to wear sunscreen.")
} else {
    print("It's not that cold. Wear a t-shirt.")
}
// 输出 "It's really warm. Don't forget to wear sunscreen."

switch语句

let someCharacter: Character = "e"
switch someCharacter {
case "a","e","i","o","u":
    print("\(someCharacter) is a vowel")
case "b","c","d","f","g","h","j","k","l","m","n","p","q","r","s","t","v","w","x","y","z":
    print("\(someCharacter) is a consonant")
default:
    print("\(someCharacter) is not a vowel or a consonant")
}
// 输出 "e is a vowel"

与 C 语言和 Objective-C 中的switch语句不同，在 Swift 中，当匹配的 case 分支中的代码执行完毕后，程序会终止switch语句，而不会继续执行下一个 case 分支。

这也就是说，不需要在 case 分支中显式地使用break语句。这使得switch语句更安全、更易用，也避免了因忘记写break语句而产生的错误。

区间匹配

let count = 3_000_000_000_000
let countedThings = "stars in the Milky Way"
var naturalCount: String
switch count {
case 0:
    naturalCount = "no"
case 1...3:
    naturalCount = "a few"
case 4...9:
    naturalCount = "several"
case 10...99:
    naturalCount = "tens of"
case 100...999:
    naturalCount = "hundreds of"
case 1000...999_999:
    naturalCount = "thousands of"
default:
    naturalCount = "millions and millions of"
}
print("There are \(naturalCount) \(countedThings).")
// 输出 "There are millions and millions of stars in the Milky Way."

元组匹配

你可以使用元组在同一个switch语句中测试多个值。

元组中的元素可以是值，也可以是区间。另外，使用下划线（_）来匹配所有可能的值。

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

值绑定

case 分支的模式允许将匹配的值绑定到一个临时的常量或变量.

这些常量或变量在该 case 分支里就可以被引用了——这种行为被称为值绑定（value binding）。

let anotherPoint = (2,0)
switch anotherPoint {
case (let x,0):
    print("on the x-axis with an x value of \(x)")
case (0,let y):
    print("on the y-axis with a y value of \(y)")
case let (x,y):
    print("somewhere else at (\(x),\(y))")
}
// 输出 "on the x-axis with an x value of 2"

where语句

let yetAnotherPoint = (1,-1)
switch yetAnotherPoint {
case let (x,y) where x == y:
    print("(\(x),\(y)) is on the line x == y")
case let (x,y) where x == -y:
    print("(\(x),\(y)) is on the line x == -y")
case let (x,y):
    print("\(x),\(y)) is just some arbitrary point")
}
// 输出 "(1,-1) is on the line x == -y"

转移控制语句

fallthrough

如果你确实需要 C 风格的贯穿（fallthrough）的特性，你可以在每个需要该特性的 case 分支中使用fallthrough关键字。

下面的例子使用fallthrough来创建一个数字的描述语句。

let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2,3,5,7,11,13,17,19:
    description += " a prime number,and also"
    fallthrough
default:
    description += " an integer."
}
print(description)
// 输出 "The number 5 is a prime number,and also an integer."

带标签的语句

跟c语言的goto很像

gameLoop: while square != finalSquare {
    if ++diceRoll == 7 { diceRoll = 1 }
    switch square + diceRoll {
    case finalSquare:
        // 到达最后一个方块，游戏结束
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // 超出最后一个方块，再掷一次骰子
        continue gameLoop
    default:
        // 本次移动有效
        square += diceRoll
        square += board[square]
    }
}
print("Game over!")

如果上述的break语句没有使用gameLoop标签，那么它将会中断switch代码块而不是while循环体。

使用gameLoop标签清晰的表明了break想要中断的是哪个代码块。

同时请注意，当调用continue gameLoop去跳转到下一次循环迭代时，这里使用gameLoop标签并不是严格必须的。

因为在这个游戏中，只有一个循环体，所以continue语句会影响到哪个循环体是没有歧义的。

然而，continue语句使用gameLoop标签也是没有危害的。

这样做符合标签的使用规则，同时参照旁边的break gameLoop，能够使游戏的逻辑更加清晰和易于理解。

最后还有 guard语句，看起来很像断言。