Modifying a struct in a goroutine?

Question

I was experimenting with goroutines, and it seems I can't modify the value of a struct within a goroutine (example below). Is there any work around for this?

EDIT: It appears that the code runs if I place a sleep statement, indicating that the goroutines would run if given more time, but they finish running after everything in main() has already executed. How do I "wait" for my goroutines to finish before proceeding?

package main

import (
    "fmt"
)

type num struct {
    val int
}

func (d *num) cube_val() {
    fmt.Println("changing value...")
    d.val = d.val*d.val*d.val 
}

func main() {
    a := []num{num{1},num{3},num{2},num{5},num{4}}
    for i := range a {
        go a[i].cube_val()
    }
    // code that waits for go routines to finish should get inserted here ...
    fmt.Println(a) // change does NOT happen

    for i := range a {
        a[i].cube_val()
    }
    fmt.Println(a) // change happens, and fmt.Println statements worked?
}

Answer 1

The changes do happen. But they happened for you after the fmt.Println(a) . In fact there are no guarantees on the execution order of the goroutines and the print statement, without synchronization .

If you want the fmt.Println(a) happen after the goroutines have completed, you have to wait for them, for example: ( see also on Playground )

func main() {
    var wg sync.WaitGroup
    a := []num{num{1}, num{3}, num{2}, num{5}, num{4}}
    for i := range a {
        wg.Add(1)
        go func(d *num) {
            defer wg.Done()
            d.cube_val()
        }(&a[i])
    }

    wg.Wait()

    fmt.Println(a)
}