Why Does My Go Range Loop Not Modify Array Elements?

Returning Addresses instead of Values for Range References

Consider the situation where a range statement returns a copy of a value instead of the original address. This can lead to unexpected behavior, as seen in the following Go code:

import "fmt"

type MyType struct {
    field string
}

func main() {
    var array [10]MyType

    for _, e := range array {
        e.field = "foo"
    }

    for _, e := range array {
        fmt.Println(e.field)
        fmt.Println("--")
    }
}

In this example, the intent is to modify the "field" property of each element in the array. However, since the range statement returns a copy of the value, the changes are made to a local copy and don't affect the original array. As a result, the output shows all "field" properties as having the default value.

To address this issue, you cannot return the address of the item in a range loop. Instead, you should iterate through the array using the index, as shown below:

func main() {
    var array [10]MyType

    for idx, _ := range array {
        array[idx].field = "foo"
    }

    for _, e := range array {
        fmt.Println(e.field)
        fmt.Println("--")
    }
}

By using the index instead of the value in the for loop, you ensure that the changes made to the "field" property are reflected in the original array.

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