2、Swift 基础运算

赋值运算

let b = 10
var a = 5
a = b
//a现在等于10

let (x, y) = (1, 2)
//x等于1，y等于2

算数运算

1 + 2       // 3
5 - 3       // 2
2 * 3       // 6
10.0 / 2.5  // 4.0

"hello, " + "world"  // "hello, world"

余数运算

9 % 4    // 1
-9 % 4   // -1

一元加减法运算

let three = 3
let minusThree = -three       //  -3
let plusThree = -minusThree   //  3

加号没有起任何作用
let minusSix = -6
let alsoMinusSix = +minusSix  //  -6

混合赋值运算

var a = 1
a += 2 // 3，a=a+2

比较运算

Equal to (a == b)
Not equal to (a != b)
Greater than (a > b)
Less than (a < b)
Greater than or equal to (a >= b)
Less than or equal to (a <= b)

(1, "zebra") < (2, "apple")   // true because 1 is less than 2; "zebra" and "apple" are not compared
(3, "apple") < (3, "bird")    // true because 3 is equal to 3, and "apple" is less than "bird"
(4, "dog") == (4, "dog")      // true because 4 is equal to 4, and "dog" is equal to "dog"

三元运算

let contentHeight = 40
let hasHeader = true
let rowHeight = contentHeight + (hasHeader ? 50 : 20)
// rowHeight = 90

空合运算符

表达式：a??b
a为非空则返回a,否则返回默认值b，a 是optional类型，此表达式等价于：
a  !=  nil ? a! : b

例如：
let defaultColorName = "red"
var userDefinedColorName: String?   // 默认为 nil
 
var colorNameToUse = userDefinedColorName ?? defaultColorName
//userDefinedColorName为空，返回"red"

userDefinedColorName = "green"
colorNameToUse = userDefinedColorName ?? defaultColorName
//userDefinedColorName不为空，返回"green"

范围运算

没有下标
for index in 10...15 {
    print("\(index) times 5 is \(index * 5)")
}
// 10 times 5 is 50
// 11 times 5 is 55
// 12 times 5 is 60
// 13 times 5 is 65
// 14 times 5 is 70

需要下标
let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
    print("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

部分范围：
for name in names[2...] {
    print(name)
}
// Brian
// Jack

for name in names[...2] {
    print(name)
}
// Anna
// Alex
// Brian

for name in names[..<2] {
    print(name)
}
// Anna
// Alex

let range = ...5
range.contains(7) // false
range.contains(4) // true
range.contains(-1) // true

逻辑运算

非： !a
且：a && b
或：a || b