Swift控制流

概览

　　Swift
的条件语句包含if和switch，循环语句包含for-in、for、while和do-while，循环/判断条件不需要括号，但循环/判断体（body）必需括号：

1 let individualScores = [75, 43, 103, 87, 12]

2 var teamScore = 0 for score in individualScores {

3     if score > 50 {

4         teamScore += 3

5     } else {

6         teamScore += 1

7     }

8 }

可空类型

　　结合if和let，可以方便的处理可空变量（nullable
variable）。对于空值，需要在类型声明后添加?显式标明该类型可空。

1 var optionalString: String? = "Hello"

2 optionalString == nil

3

4 var optionalName: String? = "John Appleseed" var gretting = "Hello!" if let name = optionalName {

5     gretting = "Hello, \(name)"

6 }

灵活的
switch

　　Swift
中的switch支持各种各样的比较操作：

1 let vegetable = "red pepper" switch vegetable {

2 case "celery":

3     let vegetableComment = "Add some raisins and make ants on a log." case "cucumber", "watercress":

4     let vegetableComment = "That would make a good tea sandwich." case let x where x.hasSuffix ("pepper"):

5     let vegetableComment = "Is it a spicy \(x)?" default:

6     let vegetableComment = "Everything tastes good in soup."

7 }

其它循环

　　for-in除了遍历数组也可以用来遍历字典：

 1 let interestingNumbers = [

 2     "Prime": [2, 3, 5, 7, 11, 13],

 3     "Fibonacci": [1, 1, 2, 3, 5, 8],

 4     "Square": [1, 4, 9, 16, 25],

 5 ]

 6 var largest = 0 for (kind, numbers) in interestingNumbers {

 7     for number in numbers {

 8         if number > largest {

 9             largest = number

10         }

11     }

12 }

13 largest

while循环和do-while循环：

1 var n = 2 while n < 100 {

2     n = n * 2

3 }

4 n

5

6 var m = 2 do {

7     m = m * 2

8 } while m < 100

9 m

Swift
支持传统的for循环，此外也可以通过结合..（生成一个区间）和for-in实现同样的逻辑。

1 var firstForLoop = 0 for i in 0..3 {

2     firstForLoop += i

3 }

4 firstForLoop

5

6 var secondForLoop = 0 for var i = 0; i < 3; ++i {

7     secondForLoop += 1

8 }

9 secondForLoop

注意：Swift
除了..还有...：..生成前闭后开的区间，而...生成前闭后闭的区间。

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