Break out of Nested Loops in Go

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🗓 November 21, 2018 | 👱 By: Hugh

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How do you break out of nested loops in Go? A simple break sometimes just won't do!

One example where I run into this issue is in in a ticker function like below, unfortunately it doesn't behave as you might think:

package main
import (
	"fmt"
	"time"
)
func main() {
	stop := make(chan interface{})
	go func() {
		for {
			select {
			case <-time.After(time.Second):
				fmt.Println("tick")
			case <-stop:
				fmt.Println("stopping")
				break
			}
		}
		fmt.Println("stopped")
	}()
	time.Sleep(3 * time.Second)
	stop <- true
	time.Sleep(3 * time.Second)
}

You might expect the output to be along the lines of:

tick
tick
tick
stopping
stopped

WRONG!

Instead you'll see the following:

tick
tick
tick
stopping
tick
tick
tick

Check it out here.

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The break only breaks out of the select, but the for loop continues indefinitely! To break out of the for, we need to use a label, like so:

package main
import (
	"fmt"
	"time"
)
func main() {
	stop := make(chan interface{})
	go func() {
	ForLoop:
		for {
			select {
			case <-time.After(time.Second):
				fmt.Println("tick")
			case <-stop:
				fmt.Println("stopping")
				break ForLoop
			}
		}
		fmt.Println("stopped")
	}()
	time.Sleep(3 * time.Second)
	stop <- true
	time.Sleep(3 * time.Second)
}

The output is now as expected:

tick
tick
tick
stopping
stopped

Try it out here!

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