Swift - 流程控制

if-else

• if后面的条件可以省略小括号
• 条件后面的大括号不可以省略

let age = 4
if age >= 22 {print("Get married")
} else if age >= 18 {print("Being a adult")
} else if age >= 7 {print("Go to school")
} else {print("Just a child")
}

• if后面的条件只能是Bool类型

2. while

• repeat-while相当于C语言中的do-while
• 这里不用num–，是因为
• • 从Swift3开始，去除了自增（++）、自减（--）运算符

var num1 = 5
while num1 > 0 {print("num is \(num1)")num1 -= 1
} //打印了5次var num2 = -1
repeat {print("num is \(num2)")
} while (num2 > 0) //打印了1次

3. for

3.1 闭区间运算符

a…b，a <= 取值 <= b

遍历一个闭区间：

let names = ["Anna", "Alex", "Brian", 
"Jack"]

for i in 0...3 {print(names[i])
} // Anna Alex Brian Jack

闭区间赋值给一个常量

let range = 1...3
for i in range {print(names[i])
} // Alex Brian Jack

使用变量表示区间的开头、结果

let a = 1
let b = 2
for i in a...b {print(names[i])
} // Brian Jackfor i in a...3 {print(names[i])
} // Alex Brian Jack

i默认是let，有需要是可以声明为var

for var i in 1...3 {i += 5print(i)
} //6 7 8

没有用到i，建议写个下划线_

for _ in 1...3 {print("123")
}

3.2 半开区间运算符

a…<b，a <= 取值 < b

// 半开区间运算符
for i in 1..<5 {print(i)
} // 1 2 3 4

3.3 for - 区间运算符用在数组上

// for - 区间运算符用在数组上
for name in names[0...3] {print(name)
} // Anna Alex Brian Jack

3.3.1 单侧区间

单侧区间：让区间朝一个方向尽可能的远

// 单侧区间：让区间朝一个方向尽可能的远
for name in names[2...] {print(name)
} // Brian Jackfor name in names[...2] {print(name)
} // Anna Alex Brianfor name in names[..<2] {print(name)
} // Anna Alex

查看区间是否包含

// 查看区间是否包含
let range2 = ...5
range2.contains(7) // false
range2.contains(4) // true
range2.contains(-3) // true

3.4 区间类型

前面写的区间并没有写类型，编译器自动推断类型了。

区间类型

// 区间类型
let range1: ClosedRange<Int> = 1...3
let range2: Range<Int> = 1..<3
let range3: PartialRangeThrough<Int> = ...5

字符、字符串也能使用区间运算符，但默认不能用在for-in中

// 字符、字符串也能使用区间运算符，但默认不能用在for-in中
let stringRange1 = "cc"..."ff" // ClosedRange<String>
stringRange1.contains("cb") // false
stringRange1.contains("dz") // true
stringRange1.contains("fg") //falselet stringRange2 = "a"..."f"
stringRange2.contains("d") // true
stringRange2.contains("h") //false

写明类型ClosedRange<Character>

//\0到～囊括了所有可能要用到的ASCII字符
let characterRange: ClosedRange<Character> = "\0"..."~"
characterRange.contains("G") //true

3.5 带间隔的区间值

// 带间隔的区间值
let hours = 11
let hourInterval = 2
// tickMark的取值：从4开始，累加2，不超过11
for tickMark in stride(from: 4, through: hours, by: hourInterval) {print(tickMark)
} // 4 6 8 10

4. switch

case、default后面不能写大括号{}

var number = 1
switch number {
case 1:print("number is 1")break
case 2:print("number is 2")break
default:print("number is other")break
} // number is 1

默认可以不写break，并不会贯穿到后面的条件

var number = 1
switch number {
case 1:print("number is 1")
case 2:print("number is 2")
default:print("number is other")
} // number is 1

4.1 fallthrough

使用fallthrough可以实现贯穿效果

// fallthrough 实现贯穿
var number = 1
switch number {
case 1:print("number is 1")fallthrough
case 2:print("number is 2")
default:print("number is other")
} // number is 1   // number is 2

4.2 switch注意点

switch必须要保证能处理所有情况

case、default后面至少要有一条语

如果不想做任何事，加个break即可

如果能保证已处理所有情况，也可以不必使用default

// 如果能保证已处理`所有`情况，也可以不必使用
enum Answer { case right, wrong}
let answer = Answer.right
switch answer {
case Answer.right:print("right")
case Answer.wrong:print("wrong")
}

由于已确定answer是Ansewer类型，因此可以省略Answer

// 由于已确定answer是Ansewer类型，因此可以省略Answer
switch answer {
case .right:print("right")
case .wrong:print("wrong")
}

5. 复合条件

• switch也支持Character、String类型

实现复合条件

这样写汇报case至少需要一条语句，需要写个fallthrough

// switch也支持Character、String类型
let string = "Jack"
switch string {
case "Jack":fallthrough
case "Rose":print("Right person")
default:break
}

也可以使用,实现复合条件

switch string {
case "Jack", "Rose":print("Right person")
default:break
}

Character

let character: Character = "a"
switch character {
case "a", "A":print("The letter A")
default:break
}

6. 区间匹配、元组匹配

区间匹配

let count = 62
switch count {
case 0:print("none")
case 1..<5:print("a few")
case 5..<12:print("several")
case 12..<100:print("dozens of")
case 100..<1000:print("hundreds of")
default:print("many")
} // dozens of

元组匹配（坐标系）

// 元组匹配（坐标系）
let point = (1, 1)
switch point {
case (0, 0):print("the origin")
case (_, 0):print("on the y-axis")
case (0, _):print("on the y-axis")
case (-2...2, -2...2):print("inside the box")
default:print("outside of the box")
} // inside the box

可以使用下划线 _ 忽略某个值

关于case匹配问题，属于模式匹配（Pattern Matching）的范畴，以后会再次详细展开讲解

7. 值绑定

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

必要时let也可以改为var

8. where

坐标

// 坐标
let point3 = (1, -1)
switch point3 {
case let (x, y) where x == y:print("on the line x == y")
case let (x, y) where x == -y:print("on the line x == -y")
case let (x, y):print("(\(x), \(y)) is just some arbitrary point")
} // on the line x == -y

将所有正数加起来

// 将所有正数加起来
var numbers = [10, 20, -10, -20, 30, -30]
var sum = 0
for num in numbers where num > 0 {sum += num
}
print(sum) // 60

9. 标签语句

如下代码，正常情况我们在for k in里面使用continue、break，针对的是for k in这个循环，这里我们使用标签语句，实现在for k in里面控制for i in这一层循环

outer: for i in 1...4 {for k in 1...4 {if k == 3 {continue outer}if i == 3 {break outer}print("i == \(i), k == \(k)")}
}
/* 打印内容i == 1, k == 1i == 1, k == 2i == 2, k == 1i == 2, k == 2*/

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