仅选择一个通道的打印输出

Question

I am learning Go and I am on channels now. I have written a simple program using channels. I have created two channels and the channels are passed to a function which is called concurrently.

My expectation is to print output form both the channels but in reality only one channel output is getting printed:

package main

import "fmt"

func square(dat int, ch chan<- int) {

    ch <- dat * dat

}

func main() {

    resp1 := make(chan int)
    resp2 := make(chan int)

    go square(20, resp1)
    go square(10, resp2)

    select {
    case msg1 := <-resp1:
        fmt.Println(msg1)
    case msg2 := <-resp2:
        fmt.Println(msg2)
    }
}

Either the message from resp1 is printed or that from resp2 during each execution. Channels should block until something is pushed into it, right?

Answer 1

The Go Programming Language Specification

Select statements

A "select" statement chooses which of a set of possible send or receive operations will proceed.

Select chooses one of a set. For example,

package main

import "fmt"

func square(dat int, ch chan<- int) {

    ch <- dat * dat

}

func main() {

    resp1 := make(chan int)
    resp2 := make(chan int)

    go square(20, resp1)
    go square(10, resp2)

    // Choose one
    select {
    case msg1 := <-resp1:
        fmt.Println(msg1)
    case msg2 := <-resp2:
        fmt.Println(msg2)
    }
    // Choose the other
    select {
    case msg1 := <-resp1:
        fmt.Println(msg1)
    case msg2 := <-resp2:
        fmt.Println(msg2)
    }
}

Playground: https://play.golang.org/p/TiThqcXDa6o

Output:

100
400