How to break out of nested loops in Go?
LoopsGoLoops Problem Overview
I have an outer and inner loop, each iterating over a range. I want to exit the outer loop when a condition is satisfied inside the inner loop.
I have a solution which works using two 'break's, one inside the inner loop and one inside the outerloop, just outside the inner loop (a very simplified case for demonstration):
package main
import (
"fmt"
)
func main() {
word := ""
for _, i := range("ABCDE") {
for _,j := range("ABCDE") {
word = string(i) + string(j)
fmt.Println(word)
if word == "DC" {
break
}
}
if word == "DC" {
break
}
}
// More logic here that needs to be executed
}
There is no problem with this solution, but it just looks patched and ugly to me. Is there a better way to do this?
I can try and have another for conditional loop outside the outer loop in the previous solution and have a label and use continue with the label. But as you can see, this approach isn't any more elegant than the solution with break.
package main
import (
"fmt"
)
func main() {
word := ""
Exit:
for word != "DC" {
for _, i := range "ABCDE" {
for _, j := range "ABCDE" {
word = string(i) + string(j)
fmt.Println(word)
if word == "DC" {
continue Exit
}
}
}
}
// More logic here that needs to be executed
}
I have seen similar questions here pertaining to other languages (C, C#, Python etc). But what I am really interested to see is whether there is any trick with Go constructs such as 'for select'.
Loops Solutions
Solution 1 - Loops
Use break {label} to break out of any loop as nested as you want. Just put the label before the for loop you want to break out of. This is fairly similar to the code that does a goto {label} but I think a tad more elegant, but matter of opinion I guess.
package main
func main() {
out:
for i := 0; i < 10; i++ {
for j := 0; j < 10; j++ {
if i + j == 20 {
break out
}
}
}
}
More details: https://www.ardanlabs.com/blog/2013/11/label-breaks-in-go.html
Solution 2 - Loops
use function
package main
import (
"fmt"
)
func getWord() string {
word := ""
for word != "DC" {
for _, i := range "ABCDE" {
for _, j := range "ABCDE" {
word = string(i) + string(j)
fmt.Println(word)
if word == "DC" {
return word
}
}
}
}
return word
}
func main(){
word := getWord()
}
Edit: thanks to @peterSO who points on some mistakes in the details and provides this playground https://play.golang.org/p/udcJptBW9pQ
Solution 3 - Loops
How about goto?
package main
import (
"fmt"
)
func main() {
word := ""
for _, i := range "ABCDE" {
for _, j := range "ABCDE" {
word = string(i) + string(j)
fmt.Println(word)
if word == "DC" {
goto Exit
}
}
}
Exit: // More logic here that needs to be executed
}
Solution 4 - Loops
The most straightforward seems to be something like:
func main() {
word := ""
isDone := false
for _, i := range("ABCDE") {
for _,j := range("ABCDE") {
word = string(i) + string(j)
fmt.Println(word)
isDone = word == "DC"
if isDone {
break
}
}
if isDone {
break
}
}
// other stuff
}
An Alternative using a Generator
However you could also do a generator to create the sequence of words as in:
func makegen () chan string {
c:= make(chan string)
go func () {
for _, i := range ("ABCDE") {
for _, j := range ("ABCDE") {
c <- string(i) + string(j)
}
}
close (c)
}()
return c
}
func main() {
word := ""
for word = range makegen() {
fmt.Println (word)
if word == "DC" {
break
}
}
// other code
}
Solution 5 - Loops
Just defer anything you need to do and return as normal.
package main
import (
"fmt"
)
func main() {
defer func() {
// More logic here that needs to be executed
}()
word := ""
for _, i := range "ABCDE" {
for _, j := range "ABCDE" {
word = string(i) + string(j)
fmt.Println(word)
if word == "DC" {
return
}
}
}
}
Solution 6 - Loops
Wrap your for loops in an anonymous self-invoked function, then just return whenever you want to break out
package main
func main() {
func() {
for i:= 0; i < 100; i++ {
for j:= 0; j < 100; j++ {
if (i == 5 && j == 5) {
return
}
}
}
}()
}