迭代时更改值 [英] Change values while iterating

问题描述

假设我有这些类型:

type 属性 struct {键，Val 字符串}类型节点结构{属性 []属性}

并且我想迭代节点的属性以更改它们.

我希望能够做到:

for _, attr := range n.Attr {如果 attr.Key == "href" {attr.Val = "东西"}}

但由于 attr 不是指针，这不起作用，我必须这样做:

for i, attr := range n.Attr {如果 attr.Key == "href" {n.Attr[i].Val = "某事"}}

有更简单或更快捷的方法吗?是否可以直接从range获取指针?

显然我不想仅仅为了迭代而改变结构，更详细的解决方案是没有解决方案的.

解决方案

不，您想要的缩写是不可能的.

这样做的原因是 range 从您正在迭代的切片中复制值.关于范围的规范 说:

<块引用>

范围表达式第一个值第二个值(如果存在第二个变量)数组或切片 a [n]E、*[n]E 或 []E 索引 i int a[i] E

因此，range 使用 a[i] 作为数组/切片的第二个值，这实际上意味着值被复制，使原始值不可触碰.

此行为由以下代码演示:

x := make([]int, 3)x[0], x[1], x[2] = 1, 2, 3对于 i, val := range x {println(&x[i], "vs.", &val)}

代码为您打印了完全不同的内存位置，用于范围内的值和实际值切片中的值:

0xf84000f010 vs. 0x7f095ed0bf680xf84000f014 与 0x7f095ed0bf680xf84000f018 与 0x7f095ed0bf68

所以你唯一能做的就是使用指针或索引，正如 jnml 和 peterSO 已经提出的那样.

Let's suppose I have these types:

type Attribute struct {
    Key, Val string
}
type Node struct {
    Attr []Attribute
}

and that I want to iterate on my node's attributes to change them.

I would have loved to be able to do:

for _, attr := range n.Attr {
    if attr.Key == "href" {
        attr.Val = "something"
    }
}

but as attr isn't a pointer, this wouldn't work and I have to do:

for i, attr := range n.Attr {
    if attr.Key == "href" {
        n.Attr[i].Val = "something"
    }
}

Is there a simpler or faster way? Is it possible to directly get pointers from range?

Obviously I don't want to change the structures just for the iteration and more verbose solutions are no solutions.

解决方案

No, the abbreviation you want is not possible.

The reason for this is that range copies the values from the slice you're iterating over. The specification about range says:

Range expression                          1st value             2nd value (if 2nd variable is present)
array or slice  a   [n]E, *[n]E, or []E   index    i  int       a[i]       E

So, range uses a[i] as its second value for arrays/slices, which effectively means that the value is copied, making the original value untouchable.

This behavior is demonstrated by the following code:

x := make([]int, 3)

x[0], x[1], x[2] = 1, 2, 3

for i, val := range x {
    println(&x[i], "vs.", &val)
}

The code prints you completely different memory locations for the value from range and the actual value in the slice:

0xf84000f010 vs. 0x7f095ed0bf68
0xf84000f014 vs. 0x7f095ed0bf68
0xf84000f018 vs. 0x7f095ed0bf68

So the only thing you can do is to either use pointers or the index, as already proposed by jnml and peterSO.

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