Swift Control Flow控制流

一.For-in

1.  ...

在下面的例子是打印从1到6的数,  1...6  1是开头 6是结尾,开头和结尾都包括在其中。

第一个例子：

for index in 1...6{
    print("index = \(index)")
}

打印:

index = 1
index = 2
index = 3
index = 4
index = 5
index = 6

第二个例子：

这个例子相当于普通的for循环。 for(int i = 1;i < 10;i++)

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

打印：

3 to the power of 10 is 59049

2.数组的遍历

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

打印：

Hello, Anna!
Hello, Alex!
Hello, Brian!
Hello, Jack!

3.字典的遍历

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

打印：

ants have 6 legs
cats have 4 legs
spiders have 8 legs

4.字符串的遍历

for character in "Dog!".characters {
    print(character)
}

打印：

D
o
g
!

二、Repeat-While

先执行循环，知道条件为假就结束，和do-while一样。

repeat {
    statements
} while condition

三、if

这个没什么好讲的。

var 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.")
}

打印：

It's really warm. Don't forget to wear sunscreen.

四、Switch

可以多个条件，可以不要break。

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
}

例子1：

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"

例子2：

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a":
    print("The letter a")
case "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// this will report a compile-time error

例子3：

元组的switch - case

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")
}
// prints "(1, 1) is inside the box"

例子4：

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"

例子5：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")
}
// prints "(1, -1) is on the line x == -y"

例子6：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)
// prints "grtmndsthnklk"

打印的时case之外的字符串

例子7：break

let numberSymbol: Character = "三"  // Simplified Chinese 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."

例子8：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)
// prints "The number 5 is a prime number, and also an integer."

例子9：#available

检查API的有效性

if #available(platform name version, ..., *) {
    statements to execute if the APIs are available
} else {
    fallback statements to execute if the APIs are unavailable
}

if #available(iOS 9, OSX 10.10, *) {
    // Use iOS 9 APIs on iOS, and use OS X v10.10 APIs on OS X
} else {
    // Fall back to earlier iOS and OS X APIs
}