了解去无缓冲通道的例子

official code example

func sum(s []int, c chan int) {
    sum := 0
    for _, v := range s {
        sum += v
    }
    c <- sum // send sum to c
}

func main() {
    s := []int{7, 2, 8, -9, 4, 0}
    c := make(chan int)
    go sum(s[:len(s)/2], c)
    go sum(s[len(s)/2:], c)
    x := <-c
    y := <-c
    fmt.Println(x, y, x+y)
}

printed : -5, 17, 12

why not printed 17, -5, 12 ?

I think x should be 17

There's no defined order of which value gets written to the channel first, it's purely at the mercy of the scheduler. As all the example is doing is adding the results, it doesn't actually matter which way round the results turn up.

There is no guarantee in what order your go sum(...) goroutines will start and which will be the first to finish (and send to channel).