Breaking out of nested for loops in Java

Question

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Breaking out of nested loops in Java

How can I use break and/or continue statements to return to the first line of the while loop at points 1, 2 and 3, for example, as indicated in the pseudocode?

Suppose I have a scenario reminiscent of the following:

while(condition) {
    // want to return to this point
    for (Integer x : xs) {
        // point 1
        for (Integer y : ys) {
            // point 2
            ...
        }
        ...
    }
    for (Integer a : as) {
        for (Integer b : bs) {
            // point 3
            ...
        }
        ...
    }
}

"Is there some kind of break statement" i believe, there is only one kind of break exist — Ravi, Dec 12 '12 at 16:42
@OldCurmudgeon: I very much disagree that this post is a duplicate. The fact that two different types of loops are involved and that there are multiple sequential inner loops makes this question *very* different from the others on the site. — Froskoy, Dec 12 '12 at 16:48

score 5 · Accepted Answer · answered Dec 12 '12 at 16:39

5

Use a label such as :

outer:
while(condition) {
// want to return to this point
for (Integer x : xs) {
    // point 1
    for (Integer y : ys) {
        // point 2
        ...
    }
    ...
}
for (Integer a : as) {
    for (Integer b : bs) {
        // point 3
        ...
    }
    ...
}

}

and you can then use break outer; to escape the while loop. This works with nested for loops as well, but I try not to overuse labels

As pointed out by @Peter, use continue outer; if you wish to finish the current outer iteration early and continue on to the next, as opposed to escaping the while loop.

answered Dec 12 '12 at 16:39

Darius

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Thanks! I wasn't aware you could use labels with different types of loops. That's really helpful. – Froskoy Dec 12 '12 at 16:41
+1 labels are usually in UPPER_CASE. I assume you want `continue outer;` Break will exit that loop as well. – Peter Lawrey Dec 12 '12 at 16:41
@Froskoy You can break out of any block of code, it doesn't have to be a loop. – Peter Lawrey Dec 12 '12 at 16:42
@PeterLawrey makes a good point, follow the kind of naming convention used for CONSTANTS – Darius Dec 12 '12 at 16:43

PermGenError · Answer 2 · 2012-12-12T16:55:17.440

2

label your Loops with syntax

LABEL: LoopType and later you can use Break Statement to get out of a certain loop with break LABEL;.

OUT: while(somecond){
IN:for(...) {
   INNER: for(...){
break IN;// will get you outta IN and INNER for loop
}
 }
}

edited Dec 12 '12 at 16:55

answered Dec 12 '12 at 16:41

PermGenError

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score 2 · Answer 3 · answered Dec 12 '12 at 16:41

2

loop:
while(condition) {
// want to return to this point
for (Integer x : xs) {
    continue loop:
    for (Integer y : ys) {
        continue loop:
        ...
    }
    ...
}
for (Integer a : as) {
    for (Integer b : bs) {
        continue loop:
        ...
    }
    ...
}

}

answered Dec 12 '12 at 16:41

Nemanja

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What's the difference between using break and continue in this situation? – Froskoy Dec 12 '12 at 16:42
2

Continue returns to beginig of loop, break exits loop – Nemanja Dec 12 '12 at 16:44

score 1 · Answer 4 · answered Dec 12 '12 at 16:40

You can do that :

here:
while(condition) {
   for (Integer x : xs) {
        // point 1
        for (Integer y : ys) {
            break here;
        }
    }

The break statement, with or without label, is described in the Java specification :

A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement of the immediately enclosing method or initializer; this statement, which is called the break target, then immediately completes normally.

A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a switch, while, do, or for statement.

score 0 · Answer 5 · answered Dec 12 '12 at 16:39

0

yes, you can use the break statement with a label:

SOME_LABEL:
   // ... some code here
   break SOME_LABEL;

used sparingly, this is a clean way to break out of multiple nested loops.

answered Dec 12 '12 at 16:39

jtahlborn

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score 0 · Answer 6 · answered Dec 12 '12 at 16:42

0

You could do like this posted here stackoverflow.com/questions/886955/breaking-out-of-nested-loops-in-java handles the same question.

answered Dec 12 '12 at 16:42

8bitcat

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That actually doesn't answer my question, or at least I thought it didn't when I first viewed it before reading the other answers here, because of the mixed for and while loops, and breaking out at the subtly different points. – Froskoy Dec 12 '12 at 16:43

Breaking out of nested for loops in Java

6 Answers6