为什么在进行并行处理时可以在Go中重用通道？

Question

Here is a code snippet from the official tutorial

package main

import "fmt"

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, y := <-c, <-c // receive from c

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

Since we are doing the calculation in parallel, and each thread saves its result into the same channel, doesn't this screw up the data?

Answer 1

It's true that when you send two values over a channel from two different goroutines that the ordering is not necessarily guaranteed (unless you've done something else to coordinate their sends).

However, in this example, the ordering doesn't matter at all. Two values are being sent on the channel: the sum of the first half and the sum of the second.

go sum(s[:len(s)/2], c)
go sum(s[len(s)/2:], c)

Since the only thing those two values are used for is to calculate the total sum, the order doesn't matter at all. In fact, if you ran the example enough times you should see that x and y are often swapped, but the sum x+y is always the same.