为什么我不能更改一系列类型结构中的值？

Question

这是我的第一个 post 所以请 "Go" 放轻松。 :) ... 我对许多传统编程语言都非常熟悉，但我是 Go 的新手，无法理解切片和范围的使用。下面的程序代码和评论说明了我的惊愕。谢谢！

package main

import (
    "fmt"
    "time"
)

type myStruct struct {
    Name  string
    Count int
}

写了我自己的 Mod 函数，因为我在 Go 库中找不到 on。

func modMe(mod int, value int) int {
    var m int
    var ret int

    m = value / mod
    ret = value - m*mod
    return ret
}

func main() {
    mod := 4
    cnt := 16
    fmt.Printf("Record mod is %d\r\n", mod)

声明一个结构数组并添加一些记录

    chartRecords := []myStruct{}

    for i := 0; i<=cnt ;i++ {
        n := myStruct{Count: i, Name: fmt.Sprintf("Joe%2d", i)}                         //Load some data
        chartRecords = append(chartRecords,n)
    }

加载数据产生我期望的输出

    fmt.Printf("======ORIGINAL LOADED VALUES========\r\n")                                  
    i := 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out original values in the range
        i = i + 1
    }

现在我修改值并打印它们以查看它们是否已修改。这看起来符合预期。

    fmt.Printf("======MODIFIED VALUES EXPECTED========\r\n")                                    
    i = 0
    for _, elem := range chartRecords {                                                 //looping thru the range of the data records
        mm := modMe(mod, i)                                                             //modMe is my function to return the Mod of a number based on moduls 'mod'
        elem.Count = mm                                                                 //assigning the new mod value to Count
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1                                                                       
    }

现在我只是再次遍历相同的范围并打印出相同的内容。但输出显示原始值。我不明白为什么会这样。我猜它与切片和添加值有关，而不是替换值。

    fmt.Printf("======CHECK AGAIN AND VALUES ARE BACK TO ORIGINAL========\r\n")                                         //Now lets loop through the same range
    i = 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1
    }                                                                                   //But this output shows the original values  WHY??
    time.Sleep(60 * time.Second)
}

输出看起来像这样... Screenshot Output

提前感谢您的建议。

Answer 1

The Go Programming Language Specification

For statements with range clause

A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block.

Go 编程语言规范很容易阅读。

将更新后的 elem 迭代变量放回 chartRecords 切片中：

for i, elem := range chartRecords {
    elem.Count = modMe(mod, i)
    chartRecords[i] = elem
    fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)
}

Answer 2

在我的朋友 Russ 的帮助下添加这些行。

Quote：嗨，Victor，如果我没记错的话，当你在一个集合中进行...范围搜索时，返回的对象是集合中保存的原始对象的副本（“值语义”）。因此，elem 变量是您要为其分配计数的副本。这是有效的代码，但您没有像预期的那样更新集合。考虑将您的 for...range 循环调整为：

    fmt.Printf("======NOW MODIFY VALUES THIS WAY========\r\n")                                  
    i = 0
    for idx := range chartRecords { 
        mm := modMe(mod, i)  
        chartRecords[idx].Count = mm  
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, chartRecords[idx].Count, chartRecords[idx].Name)  //Print out this elem.Count element in the range
        i = i + 1    
    } 

    fmt.Printf("======CHECK AGAIN AND VALUES ARE AS DESIRED========\r\n")                                       //Now lets loop through the same range
    i = 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1
    }

现在我明白了，我希望这能帮助像我这样的新手...

祝你有愉快的一天！

维克多

为什么我不能更改一系列类型结构中的值？

Why can't I change the values in a range of type structure?

range

member

go

slice