Swift Cheat Sheet [2] - Control Flow

If条件语句

if else

let number = 100
if number < 10 {
    print("The number is small")
} else if number > 100 {
    print("The number is pretty big")
} else {
    print("The number is between 10 and 100")
}

if else + for in

let individualscores = [75,43,103,87,12]
var teamscore = 0
for score in individualscores {
    if score > 50 {
        teamscore += 3
    } else {
        teamscore += 1
    }
}
print(teamscore)

使用可选绑定

var optionalName: String?
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello,\(name)"
}else
{
    print("optionalName is nil")
}

在if条件判断语句中使用where关键字

var optionalHello: String? = "Hello"
if let hello = optionalHello where hello.hasPrefix("H"),let name = optionalName {
    greeting = "\(hello),\(name)"
    print("greeting:\(greeting)")
}else
{
    //跳到这里，因为hello还没有被复制，它没有“H”前缀，仅当where模式匹配成功，if条件语句才执行。
}

Switch

一条case分支可匹配多个模式

let vegetable = "red pepper"
switch vegetable {
    case "celery":
        let vegetableComment = "Add some raisins and make ants on a log."
    case "cucumber","watercress":
        let vegetableComment = "That would make a good tea sandwich."
    case let x where x.hasSuffix("pepper"):
        let vegetableComment = "Is it a spicy \(x)?"
    default://必须有default分支
        let vegetableComment = "Everything tastes good in soup."
}

不存在隐式的贯穿

下面的代码会有编译错误！

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

区间匹配

使用闭区间操作符..或开区间操作符..<

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."

使用元组

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"

如果存在多个匹配，那么只会执行第一个被匹配到的 case 分支。剩下的能够匹配 case 分支都会被忽视掉.

case分支中使用值绑定

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"

使用where 模式匹配

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"

For循环

使用For-In

var firstForLoop = 0
for i in 0..<4 {
    firstForLoop += i
}
print(firstForLoop)


var secondForLoop = 0
for _ in 0...4 {
    secondForLoop += 1
}
print(secondForLoop)

使用下划线_替代循环变量名

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

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"

code2:

var secondForLoop = 0
for _ in 0...4 {
    secondForLoop += 1
}
print(secondForLoop)
//输出5

遍历数组元素

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

遍历字典的键值对

字典元素的遍历顺序和插入顺序可能不同

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

使用条件判断和递增方法的标准 C 样式for循环

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

While循环

while

var count = 1
while count < 3 {
    println("count is \(count)")
    ++count
}
// count is 1
// count is 2

count = 1
while count < 1 {
    println("count is \(count)")
    ++count
}

repeat-while

var count = 1
repeat {
    print("count is \(count)")
    ++count
} while count < 3
// count is 1
// count is 2

count = 1
repeat {
    print("count is \(count)")
    ++count
} while count < 1
// count is 1

原文地址：https://www.jb51.cc/swift/325573.html