同一频道中的两个goroutine-如何执行？

Question

When I take the tour of Golang from golang.org, there's one code snippet I don't understand:

func sum(a []int, c chan int, order int) {
    sum := 0
    for _, v := range a {
        sum += v
    }
    fmt.Println(order, a)
    c <- sum // 将和送入 c
}

func main() {
    a := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    a1, a2 := a[:len(a)/2], a[len(a)/2:]

    go sum(a1, c, 1)
    x := <-c
    go sum(a2, c, 2)
    y := <-c
    //x := <-c
    //y := <-c
    // x, y := <-c, <-c // 从 c 中获取

    fmt.Println(x, y, x+y)
}

This is the output I expected:

1 [7 2 8]
2 [-9 4 0]
17 -5 12

and when I changed the code:

func main() {
    a := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    a1, a2 := a[:len(a)/2], a[len(a)/2:]

    go sum(a1, c, 1)
    //x := <-c
    go sum(a2, c, 2)
    //y := <-c
    x := <-c
    y := <-c
    // x, y := <-c, <-c // 从 c 中获取

    fmt.Println(x, y, x+y)
}

Why is the output like this:

2 [-9 4 0] 
1 [7 2 8]
-5 17 12

Answer 1

the thing is u've missed the concept of concurrency there is no guarantee in executing functions in exact order of calling them with go

the reason first code snippet works orderly is the x := <-c part this line force your app to wait until c channel is filled with data so second goroutine wont be called