Go functions writing to the same map

Question

I am trying to familiarize with go routines. I have written the following simple program to store the squares of numbers from 1-10 in a map.

func main() {
    squares := make(map[int]int)
    var wg sync.WaitGroup
    for i := 1; i <= 10; i++ {
        go func(n int, s map[int]int) {
            s[n] = n * n
        }(i, squares)
    }
    wg.Wait()
    fmt.Println("Squares::: ", squares)
}

At the end, it prints an empty map. But in go, maps are passed by references. Why is it printing an empty map?

Answer 1

As pointed out in the comments, you need to synchronize access to the map and your usage of sync.WaitGroup is incorrect.

Try this instead:

func main() {
    squares := make(map[int]int)
    var lock sync.Mutex
    var wg sync.WaitGroup
    for i := 1; i <= 10; i++ {
        wg.Add(1) // Increment the wait group count
        go func(n int, s map[int]int) {
            lock.Lock() // Lock the map
            s[n] = n * n
            lock.Unlock()
            wg.Done() // Decrement the wait group count
        }(i, squares)
    }
    wg.Wait()
    fmt.Println("Squares::: ", squares)
}