How to use the range keyword in go language-Golang-php.cn

In the Go language, the range keyword is used in a for loop to iterate the elements of an array, slice, channel, or set; the "for range" structure is similar to the foreach statement in other languages. In arrays and slices it Return the index of the element and the value corresponding to the index, and return the "key-value" pair in the collection; the syntax is "for key, value := range data{//}".

How to use the range keyword in go language

The operating environment of this tutorial: Windows 7 system, GO version 1.18, Dell G3 computer.

Go language range (Range)

The range keyword in Go language is used to iterate arrays and slices in for loops ), channel, or map element. In arrays and slices it returns the index of the element and the value corresponding to the index, and in collections it returns a key-value pair.

For mapping, it returns the next key-value pair. Range returns one value or two values. If only one value is used on the left side of the Range expression, that value is the first value in the table below.

The key-value for loop in Go language uses the syntax form of for range, which is similar to the foreach statement in other languages

Go language Detailed explanation of for range loop

Syntax

for key, value := range data{
    //
}

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Parameters	Description
for	for range Keyword used for looping.
key	The key returned by the for range loop.
value	A copy of the value returned by the for range loop.
range	for range Keyword used for looping.
data	for range loops the data that needs to be traversed.

Instructions:

If you only want to read the key, the format is as follows:

for key := range oldMap

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Or this:

for key, _ := range oldMap

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If you only want to read the value, the format is as follows:

for _, value := range oldMap

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Go language for range loop instructions

#value of the index map##value returns the data accepted by the channel#

通过 for range 遍历的返回值有一定的规律：

数组、切片、字符串返回索引和值。
map 返回键和值。
通道（channel）只返回通道内的值。

遍历数组、切片——获得索引和值

在遍历代码中，key 和 value 分别代表切片的下标及下标对应的值，下面的代码展示如何遍历切片，数组也是类似的遍历方法：

for key, value := range []int{1, 2, 3, 4} {
    fmt.Printf("key:%d  value:%d\n", key, value)
}

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代码输出如下：

key:0  value:1
key:1  value:2
key:2  value:3
key:3  value:4

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遍历字符串——获得字符

Go语言和其他语言类似，可以通过 for range 的组合，对字符串进行遍历，遍历时，key 和 value 分别代表字符串的索引和字符串中的每一个字符。

下面这段代码展示了如何遍历字符串：

var str = "hello 你好"
for key, value := range str {
    fmt.Printf("key:%d value:0x%x\n", key, value)
}

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代码输出如下：

key:0 value:0x68
key:1 value:0x65
key:2 value:0x6c
key:3 value:0x6c
key:4 value:0x6f
key:5 value:0x20
key:6 value:0x4f60
key:9 value:0x597d

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代码中的变量 value，实际类型是 rune 类型，以十六进制打印出来就是字符的编码。

遍历 map——获得 map 的键和值

对于 map 类型来说，for range 遍历时，key 和 value 分别代表 map 的索引键 key 和索引对应的值，一般被称为 map 的键值对，因为它们是一对一对出现的，下面的代码演示了如何遍历 map。

m := map[string]int{
    "hello": 100,
    "world": 200,
}
for key, value := range m {
    fmt.Println(key, value)
}

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代码输出如下：

hello 100
world 200

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注意

对 map 遍历时，遍历输出的键值是无序的，如果需要有序的键值对输出，需要对结果进行排序。

遍历通道（channel）——接收通道数据

for range 可以遍历通道（channel），但是通道在遍历时，只输出一个值，即管道内的类型对应的数据。

下面代码为我们展示了通道的遍历：

c := make(chan int)
go func() {
    c <- 1
    c <- 2
    c <- 3
    close(c)
}()
for v := range c {
    fmt.Println(v)
}

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代码说明如下：

第 1 行创建了一个整型类型的通道。
第 3 行启动了一个 goroutine，其逻辑的实现体现在第 5～8 行，实现功能是往通道中推送数据 1、2、3，然后结束并关闭通道。
这段 goroutine 在声明结束后，在第 9 行马上被执行。
从第 11 行开始，使用 for range 对通道 c 进行遍历，其实就是不断地从通道中取数据，直到通道被关闭。

在遍历中选择希望获得的变量

在使用 for range 循环遍历某个对象时，一般不会同时需要 key 或者 value，这个时候可以采用一些技巧，让代码变得更简单，下面将前面的例子修改一下，参考下面的代码示例：

m := map[string]int{
    "hello": 100,
    "world": 200,
}
for _, value := range m {
    fmt.Println(value)
}

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代码输出如下：

100
200

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在上面的例子中将 key 变成了下划线_，这里的下划线就是匿名变量。

可以理解为一种占位符。
匿名变量本身不会进行空间分配，也不会占用一个变量的名字。
在 for range 可以对 key 使用匿名变量，也可以对 value 使用匿名变量。

再看一个匿名变量的例子：

for key, _ := range []int{1, 2, 3, 4} {
    fmt.Printf("key:%d \n", key)
}

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代码输出如下：

key:0
key:1
key:2
key:3

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在该例子中，value 被设置为匿名变量，只使用 key，而 key 本身就是切片的索引，所以例子输出索引。

我们总结一下 for 的功能：

Go语言的 for 包含初始化语句、条件表达式、结束语句，这 3 个部分均可缺省。
for range 支持对数组、切片、字符串、map、通道进行遍历操作。
在需要时，可以使用匿名变量对 for range 的变量进行选取。

【相关推荐：Go视频教程、编程教学】

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Data type	key	value	Description
String	The index of the string	A copy of the character value of the index corresponding to the string	If the key is not written, the index is returned
Array	Index of the array	Copy of the value corresponding to the index	If the key is not written, then return is the index
slice	the index of the slice	a copy of the value corresponding to the index	if If the key is not written, then the key
		map is returned, and the value corresponding to the key is returned. Copy of value	If the key is not written, the value of the map is returned
channel