Go loop pointer changes

Question

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.

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.

At the start of each loop iteration, `for` is **copying** the current element of `l` into `i`. — , Sep 04 '15 at 15:48
So it reuses one `i` basically for the all the iterations? I think that makes sense. How would I save a pointer to the actual object then, if I am in my loop? — saul.shanabrook, Sep 04 '15 at 15:52

score 1 · Answer 1 · edited May 23 '17 at 11:52

1

Building off Tim's comment, it seems like you can copy the value on each loop, instead of the pointer, and dereference it after.

package main

import "fmt"

type t struct {
    val int
}

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

edited May 23 '17 at 11:52

Community

1
1

answered Sep 04 '15 at 15:58

saul.shanabrook

3,068
3
31
49

2

see here for the FAQ entry: http://golang.org/doc/faq#closures_and_goroutines. The `i := i` idiom is probably the most common solution when you're not using a closure. – JimB Sep 04 '15 at 15:59
@JimB Thanks, that is very relevant. – saul.shanabrook Sep 04 '15 at 16:03

score 1 · Answer 2 · answered Sep 04 '15 at 16:02

Another option is to get the pointer to the current item by using the index:

package main

import "fmt"

type t struct {
    val int
}

func main() {
    l := []t{{1}, {2}}
    var p *t

    for index, _ := range l {
        fmt.Println("begin", p)
        p = &l[index]
        fmt.Println("end", p)
    }
}

score 1 · Accepted Answer · answered Sep 04 '15 at 16:22

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.

Go loop pointer changes

3 Answers3

Linked