Re: [go-nuts] Beginner Question : Reassignment of counter variable inside for loop in https://github.com/golang/mobile/tree/master/example/flappy

[Victor Kovacs]

Ah Thanks. Sorry i missed it in the FAQ. 

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Re: [go-nuts] Beginner Question : Reassignment of counter variable inside for loop in https://github.com/golang/mobile/tree/master/example/flappy

[Jesse McNelis]

On Thu, Feb 23, 2017 at 8:59 AM, Victor Kovacs  wrote:
>
> // The ground.
>> for i := range g.groundY {
>> i := i
>> // The top of the ground.
>> newNode(func(eng sprite.Engine, n *sprite.Node, t clock.Time) {
>>
>
>  While this solves the issue I would like to understand what is happening.
> Any insight is appreciated.
>
>
This is a variant of https://golang.org/doc/faq#closures_and_goroutines

Your function literal captures a reference to the variables 'i' and 'j',
These variables are modified by the loop on the next iteration.

The reason for using i := i is to copy the value from the loop variable 'i'
in to a new variable also called 'i', which won't be modified by the next
iteration of the loop.

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[go-nuts] Beginner Question : Reassignment of counter variable inside for loop in https://github.com/golang/mobile/tree/master/example/flappy

[Victor Kovacs]

 Hi, 

I've recently started learning go as a way to learn mobile apps and have 
started experimenting with flappy from the mobile examples 
(https://github.com/golang/mobile/tree/master/example/flappy)
While doing some changes I've run into a problem that I don't understand. 
The code : 


   for i :=0; i <2; i++{
> for j :=0; j<2 ; j++{
> newNode(func(eng sprite.Engine, n *sprite.Node, t clock.Time) {
> fmt.Println(i, j)


produces this error with i and j being 2 :  

panic: runtime error: index out of range
>

The example code 
https://github.com/golang/mobile/tree/master/example/flappy solves this by 
assigning i again inside the loop-block  : 
 starting ligne 96  : 

>
> newNode := func(fn arrangerFunc) { 
> n := {Arranger: arrangerFunc(fn)} 
> eng.Register(n) 
> scene.AppendChild(n) 
> } 
>
> // The ground. 
> for i := range g.groundY { 
> i := i 
> // The top of the ground. 
> newNode(func(eng sprite.Engine, n *sprite.Node, t clock.Time) {
>

 While this solves the issue I would like to understand what is happening. 
Any insight is appreciated. 

Thanks for your help, 
Victor


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