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。