Swift-基础语法

Swift-基础语法

1 简单if

if someCondition {
    print("Do something")
}

if speed >= 88 {
    print("Where we're going we don't need roads.")
}

enum Sizes: Comparable {
    case small
    case medium
    case large
}

let first = Sizes.small
let second = Sizes.large
print(first < second)
// “true”，因为枚举类型列表中 small 在 large 之前。

2 多条件if

2.1. if-else

if someCondition {
    print("This will run if the condition is true")
} else {
    print("This will run if the condition is false")
}

2.2. if-else if

let a = false
let b = true

if a {
    print("Code to run if a is true")
} else if b {
    print("Code to run if a is false but b is true")
} else {
    print("Code to run if both a and b are false")
}

2.3. 多条件if

if temp > 20 && temp < 30 {
    print("It's a nice day.")
}
if userAge >= 18 || hasParentalConsent == true {
    print("You can buy the game")
}
if (isOwner == true && isEditingEnabled) || isAdmin == true {
    print("You can delete this post")
}

3 switch 语句检查多个条件

3.1. 基础

Swift 会按顺序检查所有 case，并执行第一个匹配的 case

enum Weather {
    case sun, rain, wind, snow, unknown
}

switch forecast {
case .sun:
    print("It should be a nice day.")
case .rain:
    print("Pack an umbrella.")
case .wind:
    print("Wear something warm")
case .snow:
    print("School is cancelled.")
case .unknown:
    print("Our forecast generator is broken!")
default:
    print("Who are you?")    
}

3.2. fallthrough

继续执行后续的 case

let day = 5
print("My true love gave to me…")

switch day {
case 5:
    print("5 golden rings")
    fallthrough
case 4:
    print("4 calling birds")
    fallthrough
case 3:
    print("3 French hens")
    fallthrough
case 2:
    print("2 turtle doves")
    fallthrough
default:
    print("A partridge in a pear tree")
}

My true love gave to me...
5 golden rings
4 calling birds
3 French hens
2 turtle doves
A partridge in a pear tree

4 什么时候应该使用 switch 语句而不是 if 语句？

以下三个原因可能会让你考虑使用 switch 而不是 if ：

1. Swift 要求 switch 语句必须穷尽所有可能的情况，这意味着你必须为每个可能的值都编写一个 case 代码块（例如枚举类型的所有情况），或者必须有一个 default 情况。if if else if 则没有这样的限制，因此你可能会意外地遗漏某些情况。
2. 当你使用 switch 检查某个值是否存在多个可能的结果时，该值只会读取一次；而如果你使用 if ，则会读取多次。这一点在开始使用函数调用时尤为重要，因为有些函数调用速度较慢。
3. Swift 的 switch 语句允许进行高级模式匹配，而 if 则难以做到这一点

5 三元运算符

let hour = 23
print(hour < 12 ? "It's before noon" : "It's after noon")

6 for 循环

6.1. 基础

let platforms = ["iOS", "macOS", "tvOS", "watchOS"]

for os in platforms {
    print("Swift works great on \(os).")
}

1. 遍历固定范围的数字

   for i in 1...12 {
       print("5 x \(i) is \(5 * i)")
   }
   // 1~12
   

   for i in 1..<5 {
       print("Counting 1 up to 5: \(i)")
   }
   //1~4
   

2. 使用一个范围运行一些代码若干次

   Swift 会识别出你实际上并不需要该变量，因此不会为你创建临时常量

   var lyric = "Haters gonna"
   for _ in 1...5 {
       lyric += " hate"
   }
   print(lyric)
   //Haters gonna hate hate hate hate hate
   

6.2. 范围运算符...、..<

便于便利数组

let names = ["Piper", "Alex", "Suzanne", "Gloria"]
print(names[1...3])
print(names[1...])
//["Alex", "Suzanne", "Gloria"]