range遍历指针类型时注意事项

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package main

import "fmt"

func main()  {
	arr := []int{1,2,3,4,5,6,7,8}
	odd := make([]int, 0, 5)

	for _, v := range arr{
		if v % 2 == 1{
			odd = append(odd, v)
		}
	}

	for _, v := range odd{
		fmt.Println(v)
	}
}

/*
output:
1
3
5
7
*/
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package main

import "fmt"

func main()  {
	arr := []int{1,2,3,4,5,6,7,8}
	**odd := make([]*int, 0, 5)**

	for _, v := range arr{
		if v % 2 == 1{
			**odd = append(odd, &v)**
		}
	}

	for _, v := range odd{
		**fmt.Println(*v)**
	}
}

/*
output:
8
8
8
8
*/

上面的区别主要是slice里面装的是标准类型还是指针类型，标准类型可以得到正常结果，而指针类型则得到重复的结果

The Go Programming Language Specification

根据Go range的文档中得到

The iteration variables may be declared by the “range” clause using a form of short variable declaration (:=). In this case their types are set to the types of the respective iteration values and their scope is the block of the “for” statement; they are re-used in each iteration. If the iteration variables are declared outside the “for” statement, after execution their values will be those of the last iteration.

变量在迭代中会重复使用，如果变量声明在for外，那么迭代结束后变量值为最后一次赋值。

因为变量v被重复使用，而它的地址不会变更。

那么上面指针类型的得到错误的结果原因也是这个

解决方法主要有两个：

  1. 使用下标

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    for i, _ := range arr{
    		if arr[i] % 2 == 1{
    			odd = append(odd, &arr[i])
    		}
    	}

  2. 使用临时变量

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    for _, v := range arr{
    		if v % 2 == 1{
    			tmp := v
    			odd = append(odd, &tmp)
    		}
    	}