if else

if 后面条件可以省略小括号
条件后面打括号不可以省略

while

var num = 5
while num > 0 {
    print("num is \(num)")
    num -= 1
}
var num = -1
repeat {
    print("num is \(num)")
} while num > 0

repeat-while 相当于C语言中的do-while
这里不是num—，是因为Swift3开始，去除了自增（++）、自减(—)运算符
(阅读性较差)

for

闭区间运算符： a…b，a <= 取值 <= b

let names = ["a", "b", "c", "d"]
for i in 0...3 {
    print(names[i])
}

或者 let range = 0…3， for i in range这样遍历
如果没有用到i，可以用下划线代替

for _ in 0...3 {
}

半开区间运算符：a..区间运算符用在数组上

for name in names[0...3] {
    print(name)
}

单侧区间：让区间朝一个方向尽可能的远

for name in names[...3] {
    print(name)
}
for name in names[..<3] {
    print(name)
}

也可以将区间赋值给range

// 负无穷到5
let range = ...5
range.contains(7) // true
range.contains(3) // false

字符、字符串也能使用区间运算符，但默认不能在for-in中

let range = "a"..."z"
range.contains("c")
range.contains("e")

带间隔的区间值

/// 打印0-100间的偶数
for i in stride(from: 0, through: 100, by: 2) {
    print(i)
}

switch

let num = 1
switch num {
case 1:
    print("num is 1")
case 2:
    print("num is 2")
case 3:
    print("num is 3")
default:
    print("num is other")
}
// num is 1

case、default后面不能写大括号{}
默认可以不写break，并不会贯穿到后面的条件
fallthrough，可以实现贯穿效果

let num = 1
switch num {
case 1:
    print("num is 1")
    fallthrough
case 2:
    print("num is 2")
case 3:
    print("num is 3")
default:
    print("num is other")
}
// num is 1
// num is 2

switch必须保证能处理所有情况，下面这种情况不写default就会报错

case、default后面至少有一条语句，如果不想做任何事情，加个break即可

如果能保证处理所有情况，也不必使用default，例如枚举

enum Answer {
    case right
    case wrong
}
let answer = Answer.right
switch answer {
case .right:
    print("answer is right ")
case .wrong:
    print("answer is right ")
}

复合条件
switch也支持Character、String类型

let string = "Jack"
switch string {
case "Jack":
    print("is jack")
case "Tom":
    print("is Tom")
default:
    break
}

复合条件可以使用逗号分隔条件，也可以用fallthrough关键字

switch name {
case "Jack", "Rose":
    print("is Jack and Rose")
case "Tom":
    print("is Tom")
default:
    break
}
switch name {
case "Jack":
    print("is jack")
    fallthrough
case "Rose":
    print("is Rose")
case "Tom":
    print("is Tom")
default:
    break
}

字符类型也一样：

let c: Character = "a"
switch c {
case "a":
    print("is a")
case "A":
    print("is A")
default
    break
}

区间匹配、元组匹配

let count = 62
switch count {
case 1..<5:
    print("1..<5")
case 6..<100:
    print("6..<100")
default:
    break
}

let point = (1, 1)
switch point {
case (0, 0):
    print("is zero point")
case (_, 0):
    print("is on x-axis")
case (0, _):
    print("is on y-axis")
default:
    break
}

可以使用下划线忽略某个值，case匹配问题，属于模式匹配（Pattern Matching）的范畴
值绑定

switch point {
case (let x, 0):
    print("x is \(x)")
case (0, let y):
    print("y is \(y)")
case let (x, y):
     print("x is \(x), y is \(y)")
default:
    break
}

where

在switch语句中作为条件

switch point {
case let (x, y) where x == y:
    print("x == y")
case let (x, y) where x == -y:
    print("x == -y")
default:
    break
}

在for循环中作为条件

// 打印区间内所有的偶数
for i in -100...100 where i%2 == 0  {
    print("i = \(i)")
}

标签语句

outer: for i in 0...3 {
    for k in 0...3 {
        if k == 3 {
            continue outer
        }
        if i == 3 {
            break outer
        }
        print("i = \(i), k = \(k)")
    }
}