Swift基础语法 - 流程控制

for i in 0..<5{
    print(i)
}//0 1 2 3 4

for - 区间运算符在数组上使用

let nums = [1,2,3,4,5,6,7,8]

for num in nums[0...3] {
    print(num)
} //1 2 3 4

• 单侧区间：让区间朝一个方向尽可能的远

for num in nums[5...] {
    print(num)
}//6 7 8

for num in nums[...2] {
    print(num)
}//1 2 3

for num in nums[..<2] {
    print(num)
}//1 2

let range = ...3

range.contains(2) //true
range.contains(-2) //true
range.contains(5) //false

区间类型

let range1: ClosedRange<Int> = 1...3
let range2: Range<Int> = 1..<3
let range3: PartialRangeThrough<Int> = ...3

• 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 cd cf dz ....
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ASSCII字符
let characterRange : ClosedRange<Character> = "\0"..."~"
characterRange.contains("G") //true

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

switch

• case、default后面不能写大括号 { }

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

• 默认可以不写break，并不会贯穿到后面的条件

var num = 1
switch num {
case 1:
    print("1")
case 2:
    print("2")
default:
    print("default")
}// 1

fallthrough

• 使用 fallthrough 可以实现贯穿效果

var num = 1
switch num {
case 1:
    print("1")
    fallthrough
case 2:
    print("2")
default:
    print("default")
}
// 1
// 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 num = 1
switch num {
case 1:
    print("1")
    fallthrough
case 2:
    print("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Ansewer
enum Answer {case right, wrong}
let answer = Answer.right
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 string = "Jack"
switch string {
case "Jack", "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

区间匹配、元组匹配

//区间匹配
let count = 55
switch count {
case 0:
    print("0")
case 1...5:
    print("1-5")
case 5..<12:
    print("5-11")
case 12..<100:
    print("12-99")
case 100..<1000:
    print("100-999")
default:
    print("many")
}//12-99

//元组匹配
let point = (1, 1)
switch point {
case (0, 0):
    print("0 - 0")
case (_, 0):
    print("x - 0")
case (0, _):
    print("0 - y")
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）范畴

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 an 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

//where 可以与 for 循环一起使用
//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)")
    }
}