swift 控制流

swift 控制流

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For 循环

• for-in循环
• for-condition-increment循环

for-in循环

for-in用了遍历一个区间(range),序列(sequence),集合(collection),系列(progression)里面所有的元素执行一系列语句。

for条件递增(for-condition-increment)

while 循环

• while 循环
• do-while循环

条件语句

• if 语句
• switch 语句

switch 语句

switch 语句会尝试把某个值与若干个模式(pattern)进行匹配。

• switch 语句必须是完备的。每个可能的值都必须至少有一个 case 分支与之对应.
• 不存在隐式的贯穿

switch 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 = "default case"
}

元组(tuple)

let somePoint = (1, 1)
switch somePoint {
    case (0, 0) : println("origin point.")
    case (_, 0) : println("x-axis")
    case (0, _) : println("y-axis")
    case (-2...2, -2...2) : println("inside a box")
    default : println("some point")
}

_ 匹配所有可能的值。

值绑定(Value Bindings)

case 分支的模式允许将匹配的值绑定到一个临时的常量或者变量，这些常量或者变量在该 case 分支就可以被引用了。

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

Where

case 分支的模式可以使用 where 语句来判断额外的条件。

let yetAnotherPoint = (1, -1)
switch yetAnotherPoint {
    case let (x, y) where x == y : 
        println("(\(x), \(y)) is on the line x == y")
    case let (x, y) where x == -y :
        println("(\(x), \(y)) is on the line x == -y")
    case let (x, y) : 
        println("somewhere point .")
}