Select

In this tutorial, we will learn about the select statement in Go.

The select statement blocks the code and waits for multiple channel operations simultaneously.

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

package main

import (
	"fmt"
	"time"
)

func main() {
	one := make(chan string)
	two := make(chan string)

	go func() {
		time.Sleep(time.Second * 2)
		one <- "One"
	}()

	go func() {
		time.Sleep(time.Second * 1)
		two <- "Two"
	}()

	select {
	case result := <-one:
		fmt.Println("Received:", result)
	case result := <-two:
		fmt.Println("Received:", result)
	}

	close(one)
	close(two)
}

Similar to switch, select also has a default case that runs if no other case is ready. This will help us send or receive without blocking.

func main() {
	one := make(chan string)
	two := make(chan string)

	for x := 0; x < 10; x++ {
		go func() {
			time.Sleep(time.Second * 2)
			one <- "One"
		}()

		go func() {
			time.Sleep(time.Second * 1)
			two <- "Two"
		}()
	}

	for x := 0; x < 10; x++ {
		select {
		case result := <-one:
			fmt.Println("Received:", result)
		case result := <-two:
			fmt.Println("Received:", result)
		default:
			fmt.Println("Default...")
			time.Sleep(200 * time.Millisecond)
		}
	}

	close(one)
	close(two)
}

It’s also important to know that an empty select {} blocks forever.

func main() {
	...
	select {}

	close(one)
	close(two)
}