Swift笔记-控制流

For循环

For-In

你可以使用for-in循环来遍历一个集合里面的所有元素

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

如果你不需要知道区间序列内每一项的值，你可以使用下划线（_）替代变量名来忽略对值的访问：

let base = 3
let power = 10
var answer = 1
for _ in 1...power {
    answer *= base
}
print("\(base) to the power of \(power) is \(answer)")
// 输出 "3 to the power of 10 is 59049"

For

for var index = 0; index < 3; ++index {
    print("index is \(index)")
}
// index is 0
// index is 1
// index is 2

While循环

• while循环，每次在循环开始时计算条件是否符合；
• repeat-while循环，每次在循环结束时计算条件是否符合。while循环，每次在循环开始时计算条件是否符合；
  repeat-while循环，每次在循环结束时计算条件是否符合。

While

while condition {  
    statements
}

Repeat-While

类似于java的do while

repeat {
    statements
} while condition

条件语句

if

var temperatureInFahrenheit = 30
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
}
// 输出 "It's very cold. Consider wearing a scarf."

if else

temperatureInFahrenheit = 40
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else {
    print("It's not that cold. Wear a t-shirt.")
}
// 输出 "It's not that cold. Wear a t-shirt."

if else if else

temperatureInFahrenheit = 90
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
    print("It's really warm. Don't forget to wear sunscreen.")
} else {
    print("It's not that cold. Wear a t-shirt.")
}
// 输出 "It's really warm. Don't forget to wear sunscreen."

if else if

temperatureInFahrenheit = 72
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
    print("It's really warm. Don't forget to wear sunscreen.")
}

Switch

switch some value to consider {
case value 1:
    respond to value 1
case value 2, value 3:
    respond to value 2 or 3
default:
    otherwise, do something else
}

let someCharacter: Character = "e"
switch someCharacter {
case "a", "e", "i", "o", "u":
    print("\(someCharacter) is a vowel")
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
    print("\(someCharacter) is a consonant")
default:
    print("\(someCharacter) is not a vowel or a consonant")
}
// 输出 "e is a vowel"

区间匹配

let approximateCount = 62
let countedThings = "moons orbiting Saturn"
var naturalCount: String
switch approximateCount {
case 0:
    naturalCount = "no"
case 1..<5:
    naturalCount = "a few"
case 5..<12:
    naturalCount = "several"
case 12..<100:
    naturalCount = "dozens of"
case 100..<1000:
    naturalCount = "hundreds of"
default:
    naturalCount = "many"
}
print("There are \(naturalCount) \(countedThings).")
// 输出 "There are dozens of moons orbiting Saturn."

元组

let somePoint = (1, 1)
switch somePoint {
case (0, 0):
    print("(0, 0) is at the origin")
case (_, 0):
    print("(\(somePoint.0), 0) is on the x-axis")
case (0, _):
    print("(0, \(somePoint.1)) is on the y-axis")
case (-2...2, -2...2):
    print("(\(somePoint.0), \(somePoint.1)) is inside the box")
default:
    print("(\(somePoint.0), \(somePoint.1)) is outside of the box")
}
// 输出 "(1, 1) is inside the box"

值绑定

let anotherPoint = (2, 0)
switch anotherPoint {
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))")
}
// 输出 "on the x-axis with an x value of 2"

Where

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

控制转移语句

Swift 有五种控制转移语句：

• continue
• break
• fallthrough
• return
• throw

Continue

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
for character in puzzleInput.characters {
    switch character {
    case "a", "e", "i", "o", "u", " ":
        continue
    default:
        puzzleOutput.append(character)
    }
}
print(puzzleOutput)
// 输出 "grtmndsthnklk"

Break

break语句会立刻结束整个控制流的执行。

循环语句中的 break

当在一个循环体中使用break时，会立刻中断该循环体的执行，然后跳转到表示循环体结束的大括号(})后的第一行代码。

Switch 语句中的 break

当在一个switch代码块中使用break时，会立即中断该switch代码块的执行，并且跳转到表示switch代码块结束的大括号(})后的第一行代码。

fallthrough

let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2, 3, 5, 7, 11, 13, 17, 19:
    description += " a prime number, and also"
    fallthrough
default:
    description += " an integer."
}
print(description)
// 输出 "The number 5 is a prime number, and also an integer."

return

像if语句一样，guard的执行取决于一个表达式的布尔值。我们可以使用guard语句来要求条件必须为真时，以执行guard语句后的代码。不同于if语句，一个guard语句总是有一个else分句，如果条件不为真则执行else分句中的代码。

func greet(person: [String: String]) {
    guard let name = person["name"] else {
        return
    }
  
    print("Hello \(name)")
  
    guard let location = person["location"] else {
        print("I hope the weather is nice near you.")
        return
    }
  
    print("I hope the weather is nice in \(location).")
}
  
greet(["name": "John"])
// prints "Hello John!"
// prints "I hope the weather is nice near you."
greet(["name": "Jane", "location": "Cupertino"])
// prints "Hello Jane!"
// prints "I hope the weather is nice in Cupertino."

检测 API 可用性

if #available(iOS 9, OSX 10.10, *) {
    // 在 iOS 使用 iOS 9 的 API, 在 OS X 使用 OS X v10.10 的 API
} else {
    // 使用先前版本的 iOS 和 OS X 的 API
}