Go loop pointer changes

Issue

I am using a for range loop in Go to iterate through a slice of structs.

In each loop, I a pointer to the current item to a variable.

I am confused why the pointer changes value in the next loop.

For example this code:

package main

import "fmt"

type t struct {
    val int
}

func main() {
    l := []t{{1}, {2}}
    var p *t
    for _, i := range l {
        fmt.Println("begin", p)
        p = &i
        fmt.Println("end", p)
    }
}

I would expect to produce:

begin <nil>
end &{1}
begin &{1}
end &{2}

But actually does:

begin <nil>
end &{1}
begin &{2}
end &{2}

For reference, in my actual code, I am checking for a condition during the loop, and returning the current item and previous one. So I am trying to save a pointer to it, so that in the next iteration it will have access to the previous as well.

Solution

The problem is that you’re taking the address of the loop/range variable and not the address of the item in slice. However, you’re just making a lot of unnecessary work for yourself. For one, why don’t you use the i, v := range or better yet i, _ := and then you can do i-1 to get the previous item? Secondly, even if you want it saved in a pointer, still use this syntax and then assign p = &l[i] so you have the address of the item in the slice rather than the address of the loop/range variable.

People are way too eager to use for/each style constructs when it’s obviously better to work with the index… If you want index-1 on every iteration, using the index should be your go to way of doing that.

Answered By – evanmcdonnal

Answer Checked By – Timothy Miller (GoLangFix Admin)