為什麼在 Go 範圍循環中修改結構體不會更新原始切片？

Go 中的結構遍歷

在 Go 中，當遍歷結構體切片時，從 range 變數獲得的值是副本。因此，對這些值進行的任何修改都不會影響原始切片。

考慮以下範例：

package main

import (
    "fmt"
)

type myStruct struct {
    Name  string
    Count int
}

func main() {
    chartRecords := []myStruct{}

    for i := 0; i < 5; i++ {
        n := myStruct{Count: i, Name: fmt.Sprintf("Joe%2d", i)}
        chartRecords = append(chartRecords, n)
    }

    fmt.Printf("======ORIGINAL VALUES========\n")
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\n", elem.Count, elem.Name)
    }

    fmt.Printf("======MODIFIED VALUES EXPECTED========\n")
    for _, elem := range chartRecords {
        elem.Count++
        fmt.Printf("No: %2d | Count: %2d | Name = %s\n", elem.Count, elem.Name)
    }

    fmt.Printf("======CHECK AGAIN AND VALUES ARE BACK TO ORIGINAL========\n")
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\n", elem.Count, elem.Name)
    }
}

執行此程式碼將產生以下輸出：

======ORIGINAL VALUES========
No:  0 | Count:  0 | Name = Joe 0
No:  1 | Count:  1 | Name = Joe 1
No:  2 | Count:  2 | Name = Joe 2
No:  3 | Count:  3 | Name = Joe 3
No:  4 | Count:  4 | Name = Joe 4
======MODIFIED VALUES EXPECTED========
No:  1 | Count:  1 | Name = Joe 0
No:  2 | Count:  2 | Name = Joe 1
No:  3 | Count:  3 | Name = Joe 2
No:  4 | Count:  4 | Name = Joe 3
No:  5 | Count:  5 | Name = Joe 4
======CHECK AGAIN AND VALUES ARE BACK TO ORIGINAL========
No:  0 | Count:  0 | Name = Joe 0
No:  1 | Count:  1 | Name = Joe 1
No:  2 | Count:  2 | Name = Joe 2
No:  3 | Count:  3 | Name = Joe 3
No:  4 | Count:  4 | Name = Joe 4

如您所見，即使我們增加了範圍內每個元素的Count 字段，chartRecords 切片中的值也不會被修改。這是因為範圍變數 elem 是切片中原始元素的副本。

要更新原始切片，需要將更新後的元素分配回相應索引處的切片：

for i, elem := range chartRecords {
    elem.Count++
    chartRecords[i] = elem
}

進行此更正後，再次運行程式碼將產生以下輸出：

======ORIGINAL VALUES========
No:  0 | Count:  0 | Name = Joe 0
No:  1 | Count:  1 | Name = Joe 1
No:  2 | Count:  2 | Name = Joe 2
No:  3 | Count:  3 | Name = Joe 3
No:  4 | Count:  4 | Name = Joe 4
======MODIFIED VALUES EXPECTED========
No:  1 | Count:  1 | Name = Joe 0
No:  2 | Count:  2 | Name = Joe 1
No:  3 | Count:  3 | Name = Joe 2
No:  4 | Count:  4 | Name = Joe 3
No:  5 | Count:  5 | Name = Joe 4
======CHECK AGAIN AND VALUES ARE STILL MODIFIED========
No:  1 | Count:  1 | Name = Joe 0
No:  2 | Count:  2 | Name = Joe 1
No:  3 | Count:  3 | Name = Joe 2
No:  4 | Count:  4 | Name = Joe 3
No:  5 | Count:  5 | Name = Joe 4

現在，chartRecords 切片中的值已更新正如預期的那樣。

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