Swift笔记 - 流程控制

if-else

let age = 4
if age >= 22{
  print("Get married")
}else if age >= 18{
  print("Being a adult")
}else{
  prin("just a child")  
}

• if 后面的条件可以省略
• 条件后面的大括号不可以省略
• if 后面的条件只能是bool值

while

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

var num = -1
repeat {
    print("num is \(num)")
}while num > 0 //打印一次

• repeat-while 相当于C语言里面的 do-while
• swift3 去掉了 ++，-- 的操作 因为++，–的可读性很差,且在不同的平台上的结果可能不同

for 循环

1. 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

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
// 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)
}

2. 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 - 区间运算符在数组的应用

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

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

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)
}

switch

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
}

case、default 后面不能写 { }

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

默认可以不写break，不会贯穿到后面的条件

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
}

如果能保证已经处理所有情况，也可以不使用default

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

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

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,之后会再仔细讲

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 可以改成 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")
}

// 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)")
    }
}

标签可以用来控制外部循环