Go range creates a copy

Links: 103 Golang Index

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range creates a copy

• range is used to iterate over over slices, maps, arrays and channels.
• A lesser known fact of range is that it returns a copy of the value while iterating.

Example 1

package main

import "fmt"

type MyType struct {
    field int
}

func main() {
    var array [3]MyType

    for _, e := range array {
        e.field = 2
    }

    for _, e := range array {
        fmt.Printf("%d ", e.field)
    }
}
// 0 0 0

• On the other hand if tweak the first for loop a little we can record the changes

for i := range array {
    array[i].field = 2
}
// or 
for i := range array {
    e := &array[i]
    e.field = 2
}

Example 2 - Observing the difference with storage addresses

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

Example 3

package main

import "fmt"

type Record struct {
    Id int
    Name string
}

var records = []Record{
    Record{1, "First"},
    Record{2, "Second"},
    Record{3, "Third"},
}

func findRecod(id int) (foundRecord *Record) {
    for _, record := range records {
        if record.Id == id {
            foundRecord = &record
        }       
        // ...something more here
    }
    return foundRecord
}

func main() {
    foundRecord := findRecod(2)
    if foundRecord == nil {
        fmt.Println("Nothing found")
    } else {
        fmt.Println("Found: ", foundRecord.Name)
    }
}

// Output
// Found Third

// Solution
// foundRecord = &records[i]

You are returning pointer to a record variable which is reused by each iteration of the loop. Last iteration sets the pointer to the third structure.

The reuse of variable has an enormous advantage. It does not allocate memory in every iteration of the loop. This saves a lot of garbage collection time.

This can be verified by adding the following under the if block in the for loop fmt.Printf("%p, %p\n", &record, &records[i])

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Last updated: 2022-07-01