Mapping an array to array of function

Question

package main

import (
    "fmt"
)

func main() {
    a := []int{1, 2, 3}
    var fs []func()
    for idx, x := range a {
        f := func() {
            fmt.Println(idx, " ", x)
        }
        fs = append(fs, f)

    }
    for _, f := range fs {
        f()
    }
}

I have this piece of code. The output of this code is

2 3
2 3
2 3

Which isn't matched my expectation

0 1
1 2
2 3

As I guest is that after the for loop, the ref of idx and x will point to the last element of array a and the function is executed after this for loop so it will print out 2 3 for an fs[i].

score 2 · Accepted Answer · 2020-01-12 04:05:52Z

2

The functions refer to the single idx and x variables. The variables are modified on each step through the loop and have the values 2 and 3 when the loop exits.

Fix by declaring variables for each function:

for idx, x := range a {
    idx := idx // <-- add this line and next
    x := x
    f := func() {
        fmt.Println(idx, " ", x)
    }
    fs = append(fs, f)
}

The inner idx and x variables are set once with the values you expected.

This issue is covered in the FAQ.

edited Jan 12, 2020 at 4:05

answered Jan 12, 2020 at 4:00

user12258482

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2 Comments

Quang Lê Over a year ago

Great, is it good practice when using the same variable name like idx := idx ?

user12258482 Over a year ago

I think think it is a good practice use the same name, but some people do not like shadowed variables (the inner variables shadow the variables declared in the for range. For what it's worth, the last example in the FAQ entry on closures uses the same name.

Madhan Ganesh · Accepted Answer · 2020-01-12 04:03:40Z

This is due do a concept called closure. Basically the idx and x variable that you are using inside the function f is using the outside variables. So when the function is actually called it still "refers" to the outside variables which has be iterated to the end. There are many solution to fix this, for your code one fix is to assign new variables inside the for loop. In the code below a and b takes copy for each iteration and use the same when function is invoked.

package main

import (
    "fmt"
)

func main() {
    a := []int{1, 2, 3}
    var fs []func()
    for idx, x := range a {
        a := idx
        b := x

        f := func() {
            fmt.Println(a, " ", b)
        }
        fs = append(fs, f)

    }
    for _, f := range fs {
        f()
    }
}

This code will produce the output

0   1
1   2
2   3

You can see some details in here.

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