Swift4 基础部分:Basic Operators

本文是学习《The Swift Programming Language》整理的相关随笔，基本的语法不作介绍，主要介绍Swift中的一些特性或者与OC差异点。

系列文章：

• Swift4 基础部分:The Basics

Basic Operators

等号操作符(Assignment Operator)

Unlike the assignment operator in C and 
Objective-C, the assignment operator in 
Swift does not itself return a value. 

• Swift 中的等号操作符，是不会有返回值的；为了避免以下例子中的错误用法(安全性的体现点)。

例子：

var color1 = "red"
var color2 = "green";
if color2 = color1 {
    
}

编译报错：

error: MyPlayground.playground:50:11: 
error: use of '=' in a boolean context, did 
you mean '=='?
if color2 = color1 {
   ~~~~~~ ^ ~~~~~~
          ==

比较操作符(Comparison Operators)

Swift also provides two identity operators 
(=== and !==), which you use to test 
whether two object references both refer to 
the same object instance. For more 
information, see Classes and Structures.

• Swift中引入的===,!==只能用来比较引用数据类型

例子1：

var color1 = NSString(string: "red");
var color2 = color1;
if color2 === color1{
    print("Equal");
}

执行结果：

Equal

例子2：

var color1 = "red";
var color2 = color1;
if color2 === color1{
    print("Equal");
}

执行结果:

error: MyPlayground.playground:50:11: error: binary 
operator '===' cannot be applied to two 'String' operands
if color2 === color1{
   ~~~~~~ ^   ~~~~~~

同时得出另一个结论：NSString是引用类型,String是值类型。

等号合并操作符(Nil-Coalescing Operator)

The nil-coalescing operator is shorthand for the code 
below: a != nil ? a! : b

例子：

let num1 = 1;
var num2:Int?;
var num3 = num2 ?? num1;
print("num3:\(num3)")
num2 = 2;
num3 = num2 ?? num1;
print("num3:\(num3)")

执行结果：

num3:1
num3:2

区间操作符(Range Operators)

1.The closed range operator (a...b) defines a range that 
runs from a to b, and includes the values a and b. The 
value of a must not be greater than b. 

2.The half-open range operator (a..<b) defines a range
that runs from a to b, but does not include b.

直接查看下面例子即可了解使用:

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}

for index in 1..<5 {
    print("\(index) times 5 is \(index * 5)")
}

执行结果：

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
1 times 5 is 5
2 times 5 is 10
3 times 5 is 15
4 times 5 is 20

3.The closed range operator has an alternate form for ranges that continue as far as possible in one direction—for 
example, a range that includes all the elements of an 
array, from index 2 to the end of the array. In these 
cases, you can omit the value from one side of the range 
operator. This kind of range is called a one-sided range 
because the operator has a value on only one side.

例子：

let nums = [1, 2, 3, 4]
for num in nums[2...]{
    print("num:\(num)");
}

for num in nums[...2]{
    print("num:\(num)");
}

执行结果：

num:3
num:4
num:1
num:2
num:3