Swift开发基础02-流程控制
if-slse
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--,是因为从Swift3开始,去除了自增(++)、自减(--)运算符
for
1、基本使用
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
2、for-区间运算符用在数组上
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
3、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
4、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 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
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
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 n 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
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
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
值绑定(必要时let也可以改为var)
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
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)") } }
将来的你会感谢今天如此努力的你!
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