02-流程控制

• if-else

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 后面的条件可以省略小括号
• 条件后面的大括号不可以省略

• if 后面的条件只能是 Bool 类型

• while

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

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

• repeat-while相当于C语言中的do-while

• 这里不用num–，是因为 p从Swift3开始，去除了自增(++)、自减(–)运算符

• for

• 闭区间运算符:a…b， a <= 取值 <= b

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

// i默认是let，有需要时可以声明为var 
for var i in 1...3 {
	i += 5
	print(i)
} // 6 7 8

for _ in 1...3 {
	print("for")
} // 打印了3次

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

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

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

• 半开区间运算符:a…<b， a <= 取值 < b

for i in 1..<5 { 
	print(i)
} // 1 2 3 4

• for - 区间运算符运用在数组上

let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names[0...3] {
    print(name)
} // Anna Alex Brian Jack

• 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 Jack
for name in names[...2] {
	print(name)
} // Anna Alex Brian
for name in names[..<2] {
	print(name)
} // Anna Alex

let range = ...5 
range.contains(7) // false 
range.contains(4) // true 
range.contains(-3) // true

• 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

let stringRange1 = "cc"..."ff" // ClosedRange<String> 
stringRange1.contains("cb") // false 
stringRange1.contains("dz") // true 
stringRange1.contains("fg") // false
let stringRange2 = "a"..."f" 
stringRange2.contains("d") // true 
stringRange2.contains("h") // false

// \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

• 带隔离的区间值

let hours = 11
let hourinterval = 2
// tickMark 的 取值，从 4 开始，累计加 2，不超过 11
for tickMark in stride(from: 4, through: hours, by: hourinterval) {
    print(tickMark)
}

• 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

• fallthrough

• 使用fallthrough可以实现贯穿效果

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

• 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句
• 如果不想做任何事，加个break即可

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

• 如果能保证已处理所有情况，也可以不必使用default

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 
switch answer {
	case .right:
		print("right")
	case .wrong:
    	print("wrong")
}

• 复合条件

• switch也支持Character、String类型

let string = "Jack"
switch string {
	case "Jack":
    	fallthrough
	case "Rose":
    	print("Right person")
	default:
		break
} // Right person

let character: Character = "a"
switch character {
	case "a", "A":
    	print("The letter A")
	default:
    	print("Not the letter A")
} // The letter A

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

• 区间匹配、元组匹配

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

let point = (1, 1)
switch point {
	case (0, 0):
    	print("the origin")
	case (_, 0):
		print("on the x-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

• 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-axis 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

• where

 let point = (1, -1)
switch point {
	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
var numbers = [10, 20, -10, -20, 30, -30]
var sum = 0
for num in numbers where num > 0 { // 使用where来过滤num
	sum += num 
}
print(sum) // 60

• 标签语句

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)")
    }
}

本文章只是本人的学习笔记！