Why Doesn't Modifying Values in Go's Range Loop Change the Original Array?

Range References Instead Values

In Go, the range keyword iterates over a sequence, such as an array or a slice, and provides access to both the key (index) and the value of each element. However, for performance reasons, the range keyword operates on a copy of the original value. This means that any modifications made to the value returned by range will not be reflected in the original array.

Example

Consider the following code:

package main

import "fmt"

type MyType struct {
    field string
}

func main() {
    var array [10]MyType

    // Attempting to modify a value returned by range
    for _, e := range array {
        e.field = "foo"
    }

    // Printing the values after the range loop
    for _, e := range array {
        fmt.Println(e.field)
        fmt.Println("--")
    }
}

Running this code will print "000000" for all elements because the modifications made to e.field inside the first range loop are applied to a copy and do not affect the original value in the array.

Solution

To modify the values of an array using a range loop, you need to use the array index instead of the value returned by range. This can be achieved by capturing the index using the _ placeholder in the range loop.

package main

import "fmt"

type MyType struct {
    field string
}

func main() {
    var array [10]MyType

    // Using the array index to modify values
    for idx, _ := range array {
        array[idx].field = "foo"
    }

    // Printing the values after the range loop
    for _, e := range array {
        fmt.Println(e.field)
        fmt.Println("--")
    }
}

This modified code will successfully change the values of the field for all elements in the array and print "foo" for each element.

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