Swift教程之控制流

控制流

Swift包含很多控制流语句：while循环、if、guard、switch和for-in循环。

## For-In循环 **for-in**循环用于迭代序列，如数组遍历、数字范围、字符串中的字符。

下面为for-in循环遍历数组：

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

遍历字典：

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    print("(animalName)s have (legCount) legs")
}
// ants have 6 legs
// spiders have 8 legs
// cats have 4 legs

遍历数字范围：

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

若不需要序列中的值时，使用下划线代替变量名来忽略这些值：

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

遍历半开数字范围：

let minutes = 60
for tickMark in 0..<minutes {
    // render the tick mark each minute (60 times)
}

使用stride(from:to:by:)方法跳过不需要的遍历：

let minuteInterval = 5
for tickMark in stride(from: 0, to: minutes, by: minuteInterval) {
    // render the tick mark every 5 minutes (0, 5, 10, 15 ... 45, 50, 55)
}

stride(from:through:by:)方法遍历包括结尾值：

let hours = 12
let hourInterval = 3
for tickMark in stride(from: 3, through: hours, by: hourInterval) {
    // render the tick mark every 3 hours (3, 6, 9, 12)
}

## While循环 Swift有两种**while**循环：

• 每次循环开始时判断循环条件的while循环
• 每次循环结束时判断循环条件的repeat-while循环

While

while循环语法：

while condition {

    statements

}

Repeat-While

Swift的repeat-while循环与其他语言的do-while循环类似。

repeat-while循环语法：

repeat {

    statements

} while condition

## 条件语句 Swift提供两种条件语句：**if**和**switch**语句。**if**判断简单条件，可能结果较少；**switch**适合复杂条件且可能情况较多的情况，常用于模式匹配。

If

Swift中的if语句可忽略判断条件的括号，但执行语句的大括号不可或缺：

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

if-else语句：

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.")
}

多重if语句：

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.")
}

Switch

switch语句比较多个匹配模式，且能匹配任意类型值，语法如下：

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

}

switch语句必须列出所有可能情况。

下面例子匹配单个小写字符：

let someCharacter: Character = "z"
switch someCharacter {
case "a":
    print("The first letter of the alphabet")
case "z":
    print("The last letter of the alphabet")
default:
    print("Some other character")
}

Swift的switch语句不需要在每个语句块最后显式使用break语句(也可使用)。当匹配某个条件时，之间结束执行整个switch语句。

switch语句的每个条件必须只要包含一个可执行语句，否则会抛出编译错误：

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a": // Invalid, the case has an empty body
case "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// This will report a compile-time error.

要将两个判断条件合并，使用逗号隔开：

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

注意

要想显示跳过某次判断条件，进入下个条件，使用fallthrough关键字。

间隔匹配

使用间隔匹配可在一条判断语句中匹配多个值：

let approximateCount = 62
let countedThings = "moons orbiting Saturn"
let 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).")
// Prints "There are dozens of moons orbiting Saturn."

匹配元组

使用元组可在一条switch语句中匹配复合值，并使用下划线匹配元组中某个元素的所有可能值。

当已经匹配某个条件时，之后的所有条件都将被忽略。
let somePoint = (1, 1)
switch somePoint {
case (0, 0):
print("(somePoint) is at the origin")
case (_, 0):
print("(somePoint) is on the x-axis")
case (0, _):
print("(somePoint) is on the y-axis")
case (-2...2, -2...2):
print("(somePoint) is inside the box")
default:
print("(somePoint) is outside of the box")
}
// Prints "(1, 1) is inside the box"

值绑定

值绑定表示将匹配到的值命名为临时常量或变量。

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))")
}
// Prints "on the x-axis with an x value of 2"

当某个条件正好匹配所有可能的剩余情况时，则default语句可忽略。

Where

使用where语句对某个case添加额外条件：

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")
}
// Prints "(1, -1) is on the line x == -y"

复合case

使用逗号在一个case匹配多个条件，条件过长时可多行显示：

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")
}
// Prints "e is a vowel"

复合匹配中也可包含值绑定：

let stillAnotherPoint = (9, 0)
switch stillAnotherPoint {
case (let distance, 0), (0, let distance):
    print("On an axis, (distance) from the origin")
default:
    print("Not on an axis")
}
// Prints "On an axis, 9 from the origin"

## 控制转移语句 Swift有五种控制转移语句：

• continue
• break
• fallthrough
• return
• throw

Continue

continue语句表示在循环中跳过本次循环，直接开始下一次循环，而不结束循环：

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
let charactersToRemove: [Character] = ["a", "e", "i", "o", "u", " "]
for character in puzzleInput {
    if charactersToRemove.contains(character) {
        continue
    } else {
        puzzleOutput.append(character)
    }
}
print(puzzleOutput)
// Prints "grtmndsthnklk"

Break

break语句立即结束执行整个控制流语句。

用于循环语句的break

当在循环语句中使用break时，break会立即结束执行整个循环语句。

用于switch语句的break

当在switch语句中使用break时，break会立即结束执行整个switch。

Swift的switch语句中不用显示使用break跳出匹配，但可以在某个case中可以插入break结束匹配以使意图明确。

let numberSymbol: Character = "三"  // Chinese symbol for the number 3
var possibleIntegerValue: Int?
switch numberSymbol {
case "1", "١", "一", "๑":
    possibleIntegerValue = 1
case "2", "٢", "二", "๒":
    possibleIntegerValue = 2
case "3", "٣", "三", "๓":
    possibleIntegerValue = 3
case "4", "٤", "四", "๔":
    possibleIntegerValue = 4
default:
    break
}
if let integerValue = possibleIntegerValue {
    print("The integer value of (numberSymbol) is (integerValue).")
} else {
    print("An integer value could not be found for (numberSymbol).")
}
// Prints "The integer value of 三 is 3."

Fallthrough

Swift中，switch语句在匹配到某个case条件后会忽略之后所有case，不会自动下一个case。若需要像C语言那样跳入下个case中，可使用fallthrough关键字。

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)
// Prints "The number 5 is a prime number, and also an integer."

注意

fallthrough关键字不会检查下个进入的case判断条件，只是简单地跳入下一个case并执行内部代码，与C语言中的switch语句相似。

标签语句

当使用循环或条件嵌套语句时，需要明确说明要使用break或continue语句来结束哪层控制语句，这时使用标签语句对需要提前结束的控制语句做标签，然后在break或continue语句后接上已声明的标签名来结束带相同标签的语句。语法如下：

label name: while condition {

    statements

}

## guard语句 **guard**语句是**if**语句的变体，通过将判断条件的主体执行代码放在**guard**语句末尾以使代码意图更加清晰。

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(person: ["name": "John"])
// Prints "Hello John!"
// Prints "I hope the weather is nice near you."
greet(person: ["name": "Jane", "location": "Cupertino"])
// Prints "Hello Jane!"
// Prints "I hope the weather is nice in Cupertino."

## 检查API可用性 Swift支持检查API可用性，避免代码中使用不可用API导致意外错误。

当尝试使用不可用API时，Swift会报告编译错误。

if #available(iOS 10, macOS 10.12, *) {
    // Use iOS 10 APIs on iOS, and use macOS 10.12 APIs on macOS
} else {
    // Fall back to earlier iOS and macOS APIs
}

语法如下：

if #available(platform name version, ..., *) {

    statements to execute if the APIs are available

} else {

    fallback statements to execute if the APIs are unavailable

}