Pointer to the loop variable

[Tong Sun]

I'm experiencing a weird problem with my program and finally nail it down to what exactly went wrong, so that I can write a minimum program to duplicate it. See the following program:

package main


import (
 "fmt"
)


type Vertex struct {
 X int
 Y int
}


func main() {
 vs := []Vertex{
 Vertex{1, 2}, // has type Vertex
 Vertex{X: 3}, // Y:0 is implicit
 Vertex{},     // X:0 and Y:0
 }


 fmt.Printf("Vertex Slice %+v\n", vs)
 var vp *Vertex = nil
 for _, v := range vs {
 if v.X == 3 {
 vp = &v
 }
 }
 fmt.Printf("Found: %+v\n", vp)
}

https://play.golang.org/p/R645C5plojx

I was expecting that the found Vertex is "v.X == 3", however the result is not. 

- what exactly is causing the problem?

- how to fix it so that I got what I was looking for? 

Again, the snip is at

https://play.golang.org/p/R645C5plojx

Thanks

[burak serdar]

On Tue, Sep 10, 2019 at 1:13 PM Tong Sun <sunto...@gmail.com> wrote:
>
> I'm experiencing a weird problem with my program and finally nail it down to what exactly went wrong, so that I can write a minimum program to duplicate it. See the following program:
>
> package main
>
>
> import (
> "fmt"
> )
>
>
> type Vertex struct {
> X int
> Y int
> }
>
>
> func main() {
> vs := []Vertex{
> Vertex{1, 2}, // has type Vertex
> Vertex{X: 3}, // Y:0 is implicit
> Vertex{}, // X:0 and Y:0
> }
>
>
> fmt.Printf("Vertex Slice %+v\n", vs)
> var vp *Vertex = nil
> for _, v := range vs {
> if v.X == 3 {
> vp = &v
> }
> }
> fmt.Printf("Found: %+v\n", vp)
> }
>
>
> https://play.golang.org/p/R645C5plojx
>
> I was expecting that the found Vertex is "v.X == 3", however the result is not.

Because you're keeping a pointer to the loop variable. v continues
changing after you store the address of it, so you get the latest
value of v. Instead, save a copy of v, or break the loop as soon as
what you're looking for is found:

if v.x==3 {
t:=v
vp=&t

}

>
> - what exactly is causing the problem?
> - how to fix it so that I got what I was looking for?
>
> Again, the snip is at
> https://play.golang.org/p/R645C5plojx
>
> Thanks
>

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[Yamil Bracho]

vp is taking the address of v, so when the loop ends, v is the last element in the slide and therefore *vp is {0,0}

Add a break to the condition when you assign vp, say

if v.X == 3 {
 vp = &v

 break

}

HTH,

Yamil

[Tong Sun]

 I can't break the loop immediately as my not-minimized real program use a double loop to find the closest item. That's the reason I using the pointer to track my finding...

if v.x==3 {
   t:=v
   vp=&t
}

 OK. will do that. thx

>
> - what exactly is causing the problem?
> - how to fix it so that I got what I was looking for?
>
> Again, the snip is at
> https://play.golang.org/p/R645C5plojx
>
> Thanks
>
> --
> You received this message because you are subscribed to the Google Groups "golang-nuts" group.

> To unsubscribe from this group and stop receiving emails from it, send an email to golan...@googlegroups.com.

[Yamil Bracho]

Then turn vo into a Vertex variable, not a pointer

var vp Vertex = Vertex{}

...

if v.X == 3 {
   vp = v
  }

….

This time vp will be a copy of v,

Maybe you have to give some known values and exceptional case for your domain, I mean, vp  Vertex = { -1, -1} or any values your are 

positive does not exists because maybe {0,0} is good data.

HTH

PS: You can break nested loops from an inside loop using break [label]