`Select`, `chan`

Suppose we have one or more goroutines running, and they are sending the results of their computations over channels. We can use the select block to receive these results.

Though it is possible to use goroutines without select, this construct makes receiving values in a complicated program easier. We can use select within a for-loop to receive all values.

Example

This program uses 2 channels that send ints back (the chan int type is used). In the select statement, we receive the values sent from within those goroutines.

Part 1 We create two channels that are used in the goroutines (go funcs) that are specified next in the program.

Part 2 This goroutine uses the "go func" keywords and receives an int, and has 2 send statements where it sends values onto the first channel.

Part 3 This goroutine is similar to the previous one, but it uses the second channel instead of the first.

Part 4 We sleep our main thread. This is important so that the goroutines can start up and send some values before the program exits.

Part 5 We use a for-loop to loop infinitely until a break keyword (found in the select default case) is reached.

Part 6 The select statement tries to receive a value from the channels specified in the cases. If nothing is received, it runs the default case.

package main

import (
    "fmt"
    "time"
)

func main() {
    // Part 1: make a couple channels that send ints.
    c := make(chan int)
    c2 := make(chan int)

    // Part 2: use this goroutine to accept 1 argument and send 2 ints.
    go func(x int) {
        c <- 222 + x
        c <- 500 + x
    }(1)

    // Part 3: use (and call) another goroutine that acts on the second channel.
    go func(x int) {
        c2 <- 555 + x
        c2 <- 1000 + x
    }(1)

    // Part 4: sleep, as we need to be sure the goroutines have run.
    time.Sleep(10000)

    // Part 5: continue looping until no more channel activity.
    for {
        r := 0
        // Part 6: use select to receive results from channels, and handle default case where nothing is happening.
        select {
        case r = <-c:
            fmt.Println("RESULT FROM CHAN:  ", r)
        case r = <-c2:
            fmt.Println("RESULT FROM CHAN2: ", r);
        default:
            fmt.Println("END")
            return
        }
    }
}RESULT FROM CHAN2:  556
RESULT FROM CHAN:   223
RESULT FROM CHAN:   501
RESULT FROM CHAN2:  1001
END

The select statement is a critical component to using goroutines in the Go language. It makes receiving values from channels easier and we can merge all results with it.

Select, chan

Example

`Select`, `chan`