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Breaking out of nested loops in Java

boutta
1#
boutta Published in 2009-05-20 09:07:43Z

I've got a nested loop construct like this:

for (Type type : types) {
    for (Type t : types2) {
         if (some condition) {
             // Do something and break...
             break; // Breaks out of the inner loop
         }
    }
}

Now how can I break out of both loops. I've looked at similar questions, but none concerns Java specifically. I couldn't apply these solutions because most used gotos.

I don't want to put the inner loop in a different method.

Update: I don't want to rerun the loops, when breaking I'm finished with the execution of the loop block.

Jon Skeet
2#
Jon Skeet Reply to 2012-10-25 09:46:03Z

(EDIT: Like other answerers, I'd definitely prefer to put the inner loop in a different method. This answer just shows how the requirements in the question can be met.)

You can use break with a label for the outer loop. For example:

public class Test {
  public static void main(String[] args) {
    outerloop:
    for (int i=0; i < 5; i++) {
      for (int j=0; j < 5; j++) {
        if (i * j > 6) {
          System.out.println("Breaking");
          break outerloop;
        }
        System.out.println(i + " " + j);
      }
    }
    System.out.println("Done");
  }
}

This prints:

0 0
0 1
0 2
0 3
0 4
1 0
1 1
1 2
1 3
1 4
2 0
2 1
2 2
2 3
Breaking
Done
jermenkoo
3#
jermenkoo Reply to 2016-01-21 14:10:08Z

You can use labels:

label1: 
    for (int i = 0;;) {
        for (int g = 0;;) {
          break label1;
        }
    }
Miguel Ping
4#
Miguel Ping Reply to 2009-05-20 09:11:59Z

You can use a temporary variable:

boolean outerBreak = false;
for (Type type : types) {
   if(outerBreak) break;
    for (Type t : types2) {
         if (some condition) {
             // Do something and break...
             outerBreak = true;
             break; // Breaks out of the inner loop
         }
    }
}

Depending on your function, you can also exit/return from the inner loop:

for (Type type : types) {
    for (Type t : types2) {
         if (some condition) {
             // Do something and break...
             return;
         }
    }
}
Joey
5#
Joey Reply to 2009-05-20 09:12:12Z

You can use a named block around the loops:

search: {
    for (Type type : types) {
        for (Type t : types2) {
            if (some condition) {
                // Do something and break...
                break search;
            }
        }
    }
}
Fortega
6#
Fortega Reply to 2009-05-20 12:25:57Z

maybe with a function?

public void doSomething(List<Type> types, List<Type> types2){
  for(Type t1 : types){
    for (Type t : types2) {
      if (some condition) {
         //do something and return...
         return;
      }
    }
  }
}
Blundell
7#
Blundell Reply to 2012-06-03 10:31:41Z

Technically the correct answer is to label the outer loop. In practice if you want to exit at any point inside an inner loop then you would be better off externalizing the code into a method (a static method if needs be) and then call it.

That would pay off for readability.

The code would become something like that:

private static String search(...) 
{
    for (Type type : types) {
        for (Type t : types2) {
            if (some condition) {
                // Do something and break...
                return search;
            }
        }
    }
    return null; 
}

Matching the example for the accepted answer:

 public class Test {
    public static void main(String[] args) {
        loop();
        System.out.println("Done");
    }

    public static void loop() {
        for (int i = 0; i < 5; i++) {
            for (int j = 0; j < 5; j++) {
                if (i * j > 6) {
                    System.out.println("Breaking");
                    return;
                }
                System.out.println(i + " " + j);
            }
        }
    }
}
dabluck
8#
dabluck Reply to 2014-08-04 17:18:10Z

I never use labels. It seems like a bad practice to get into. Here's what I would do:

boolean finished = false;
for (int i = 0; i < 5 && !finished; i++) {
    for (int j = 0; j < 5; j++) {
        if (i * j > 6) {
            finished = true;
            break;
        }
    }
}
boutta
9#
boutta Reply to 2016-11-10 08:54:50Z

I needed to do a similar thing, but I chose not to use the enhanced for loop to do it.

int s = type.size();
for (int i = 0; i < s; i++) {
    for (int j = 0; j < t.size(); j++) {
        if (condition) {
            // do stuff after which you want 
            // to completely break out of both loops
            s = 0; // enables the _main_ loop to terminate
            break;
        }
    }
}
boutta
10#
boutta Reply to 2012-10-25 08:39:43Z

You just use label for breaking inner loops

public class Test {
public static void main(String[] args) {
    outerloop:
for (int i=0; i < 5; i++) {
  for (int j=0; j < 5; j++) {
    if (i * j > 6) {
      System.out.println("Breaking");
      break outerloop;
    }
    System.out.println(i + " " + j);
  }
}
System.out.println("Done");
}
}
Ankush soni
11#
Ankush soni Reply to 2015-10-09 11:34:58Z

Even creating a flag for outer loop and checking that after each execution of inner loop can be the answer.

Like this :

for (Type type : types) {
      boolean flag=false;
         for (Type t : types2) {
                if (some condition) {
                    // Do something and break...
                    flag=true;
                    break; // Breaks out of the inner loop
                }
            }
            if(flag)
                break;
        }
Edd
12#
Edd Reply to 2013-05-07 18:57:20Z

Check if the inner loop is exited with an if statement, by checking the inner loop's variable. You could also create another variable such as a boolean to check if the inner loop is exited.

In this example it uses the inner loop's variable to check if it has been exited:

int i, j;
for(i = 0; i < 7; i++){

for(j = 0; j < 5; j++) {

     if (some condition) {
         // Do something and break...
         break; // Breaks out of the inner loop
     }
}
     if(j < 5){    // Checks if inner loop wasn't finished
     break;    // Breaks out of the outer loop   
     } 
}
Panzercrisis
13#
Panzercrisis Reply to 2013-09-11 22:14:59Z
boolean broken = false; // declared outside of the loop for efficiency
for (Type type : types) {
    for (Type t : types2) {
        if (some condition) {
            broken = true;
            break;
        }
    }

    if (broken) {
        break;
    }
}
zord
14#
zord Reply to 2013-12-21 22:56:34Z

If you don't like breaks and gotos, you can use a "traditional" for loop instead the for-in, with an extra abort condition:

int a, b;
bool abort = false;
for (a = 0; a < 10 && !abort; a++) {
    for (b = 0; b < 10 && !abort; b++) {
        if (condition) {
            doSomeThing();
            abort = true;
        }
    }
}
Tom
15#
Tom Reply to 2015-04-25 22:44:38Z

You can break from all loops without using any label: and flags.

It's just tricky solution.

Here condition1 is the condition which is used to break from loop K and J. And condition2 is the condition which is used to break from loop K , J and I.

For example:

public class BreakTesting {
    public static void main(String[] args) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                for (int k = 0; k < 9; k++) {
                    if (condition1) {
                        System.out.println("Breaking from Loop K and J");
                        k = 9;
                        j = 9;
                    }
                    if (condition2) {
                        System.out.println("Breaking from Loop K, J and I");
                        k = 9;
                        j = 9;
                        i = 9;
                    }
                }
            }
        }
        System.out.println("End of I , J , K");
    }
}
Milindu Sanoj Kumarage
16#
Milindu Sanoj Kumarage Reply to 2014-06-08 16:40:27Z

Use Labels.

INNER:for(int j = 0; j<numbers.length; j++){
            System.out.println("Even number: " + i + ", break  from INNER label");
            break INNER;
        }

Refer this article http://javarevisited.blogspot.com/2012/05/break-continue-and-lablel-in-loop-java.html

astryk
17#
astryk Reply to 2014-07-11 16:46:28Z
boolean condition = false;
for (Type type : types) {
  for (int i = 0; i < otherTypes.size && !condition; i ++) {
    condition = true; // if your condition is satisfied
  }
}

Use condition as a flag for when you are done processing. Then the inner loop only continues on while the condition has not been met. Either way the outer loop will keep on chuggin'.

ddyer
18#
ddyer Reply to 2014-08-04 17:29:32Z

I prefer to add an explicit "exit" to the loop tests. It makes it clear to any casual reader that the loop may terminate early.

boolean earlyExit = false;
for(int i=0;i<10&&!earlyExit; i++) {
 for(int j=0;i<10&&!earlyExit; j++) { earlyExit=true; }
}
mtyson
19#
mtyson Reply to 2014-09-13 19:00:17Z

Like @1800 INFORMATION suggestion, use the condition that breaks the inner loop as a condition on the outer loop:

boolean hasAccess = false;
for (int i = 0; i < x && hasAccess == false; i++){
  for (int j = 0; j < y; j++){
    if (condition == true){
      hasAccess = true;
      break;
    }
  }
}
ursa
20#
ursa Reply to 2014-10-10 14:13:18Z

Another one solution, mentioned without example (it actually works in prod code).

try {
    for (Type type : types) {
        for (Type t : types2) {
            if (some condition #1) {
                // Do something and break the loop.
                throw new BreakLoopException();
            }
        }
    }
}
catch (BreakLoopException e) {
    // Do something on look breaking.
}

Of course break exception should be internal, private and accelerated with no-stack-trace:

private static class BreakLoopException extends Exception {
    @Override
    public StackTraceElement[] getStackTrace() {
        return new StackTraceElement[0];
    }
}
user2875404
21#
user2875404 Reply to 2015-04-25 22:01:06Z

Rather unusual approach but in terms of code length (not performance) this is the easiest thing you could do:

for(int i=0; i++; i<j){
    if(wanna exit){
        i=i+j; //if more nested, also add the 
               //maximum value for the other loops
    }
}
Keshav bansal
22#
Keshav bansal Reply to 2015-08-23 21:04:40Z

Best and Easy Method..

outerloop:
 for(int i=0; i<10; i++){
    // here we can break Outer loop by 
    break outerloop;

    innerloop:
      for(int i=0; i<10; i++){
          // here we can break innerloop by 
          break innerloop;
       }
 }
Oleksii
23#
Oleksii Reply to 2016-01-28 19:14:59Z

Rather for a long I was thinking to share this type of answer for this type of question.

Usually such cases are come in scope of a more meaningful logic, let's say some searching or manipulating over some of the iterated 'for'-objects in question, so I usually use the functional approach:

public Object searching(Object[] types) {//or manipulating
    List<Object> typesReferences = new ArrayList<Object>();
    List<Object> typesReferences2 = new ArrayList<Object>();

    for (Object type : typesReferences) {
        Object o = getByCriterion(typesReferences2, type);
        if(o != null) return o; 
    }
    return null;
}
private Object getByCriterion(List<Object> typesReferences2, Object criterion) {
    for (Object typeReference : typesReferences2) {
        if(typeReference.equals(criterion)) {
             // here comes other complex or specific logic || typeReference.equals(new Object())
             return typeReference;
        }
    }
    return null;
}

Major cons:

  • roughly twice more lines
  • more consumption of computing cycles, meaning it is slower from algorithmic point-of-view
  • more typing work

The pros:

  • the higher ratio to separation of concerns because of functional granularity
  • the higher ratio of re-usability and control of searching/manipulating logic without
  • the methods are not long, thus they are more compact and easier to comprehend
  • subjectively higher ratio of readability

So it is just handling the case via a different approach.

Basically a question to the author of this question: what do you consider of this approach?

JourneyMan
24#
JourneyMan Reply to 2016-03-31 18:33:14Z

I feel using labels makes the code seem very much like a goto statement. This is just a thought. Why don't we throw an exception at the inner for loop and encapsulate the two for loops with a try catch block. Something like

try {
  ...
  for(Object outerForLoop : objectsOuter) {
     ...
     for (Object innerForLoop : objectsInner) {
        ...
        if (isConditionTrue) 
             throw new WrappedException("With some useful message. Probably some logging as well.");
     }
  } catch (WrappedException) {
        // do something awesome or just don't do anything swallow the exception.    
  }

Just a thought. I prefer this code since it gives me better loggability (Like that's a word) for me when this is being run in production or something.

L Joey
25#
L Joey Reply to 2016-11-15 08:04:47Z

for (int j = 0; j < 5; j++) //inner loop should be replaced with for (int j = 0; j < 5 && !exitloops; j++).

Here, in this case complete nested loops should be exit if condition is True . But if we use exitloops only to the upper loop

 for (int i = 0; i < 5 && !exitloops; i++) //upper loop

Then inner loop will continues, because there is no extra flag that notify this inner loop to exit.

Example : if i = 3 and j=2 then condition is false. But in next iteration of inner loop j=3 then condition (i*j) become 9 which is true but inner loop will be continue till j become 5.

So, it must use exitloops to the inner loops too.

    boolean exitloops= false;
    for (int i = 0; i < 5 && !exitloops; i++) { //here should exitloops as a Conditional Statement to get out from the loops if exitloops become true. 
        for (int j = 0; j < 5 && !exitloops; j++) { //here should also use exitloops as a Conditional Statement. 
            if (i * j > 6) {
                exitloops = true;
                System.out.println("Inner loop still Continues For i * j is => "+i*j);
                break;
            }
            System.out.println(i*j);
        }
    }
Chit Khine
26#
Chit Khine Reply to 2016-08-25 11:25:58Z

If it is inside some function why don't you just return it:

for (Type type : types) {
    for (Type t : types2) {
         if (some condition) {
            return value;
         }
    }
}
Dharmik Patel
27#
Dharmik Patel Reply to 2017-03-24 01:43:41Z

For some cases, We can use while loop effectively here.

  Random rand = new Random();
  //Just an example
  for (int k = 0; k < 10; ++k) {
    int count = 0;
    while (!(rand.nextInt(200) == 100)) {
       count++;
    }

    results[k] = count;
  }
Community
28#
Community Reply to 2017-05-23 12:02:50Z

I wanted to answer this question but was marked as a duplicate which prevents me from posting as well. So posting it here instead !

If its a new implementation you can try re-writing the logic as if-else_if-else statements.

while(keep_going) {

    if(keep_going && condition_one_holds) {
        // code
    }
    if(keep_going && condition_two_holds) {
        // code
    }
    if(keep_going && condition_three_holds) {
        // code
    }
    if(keep_going && something_goes_really_bad) {
        keep_going=false;
    }
    if(keep_going && condition_four_holds) {
        // code
    }
    if(keep_going && condition_five_holds) {
        // code
    }   
}

Otherwise you can try setting a flag when that special condition has occured and check for that flag in each of your loop-conditions.

something_bad_has_happened = false;
while(something is true && !something_bad_has_happened){
    // code, things happen
    while(something else && !something_bad_has_happened){
        // lots of code, things happens
        if(something happened){
            -> Then control should be returned ->
            something_bad_has_happened=true;
            continue;
        }
    }   
    if(something_bad_has_happened) { // things below will not be executed
        continue;
    }

    // other things may happen here as well but will not be executed
    //  once control is returned from the inner cycle
}

HERE! So, while a simple break will not work, it can be made to work using continue.

If you are simply porting the logic from one programming language to java and just want to get the thing working you can try using labels

Gholamali-Irani
29#
Gholamali-Irani Reply to 2018-01-12 10:43:31Z

Demo for break, continue, label.

So java keywords break and continue have default value, it's the "Nearest Loop", Toady a few years after use Java, I just got it !

It's seem used rare, but useful.

import org.junit.Test;

/**
 * Created by cui on 17-5-4.
 */

public class BranchLabel {
    @Test
    public void test() {
        System.out.println("testBreak");
        testBreak();

        System.out.println("testBreakLabel");
        testBreakLabel();

        System.out.println("testContinue");
        testContinue();

        System.out.println("testContinueLabel");
        testContinueLabel();

    }

    /**
     testBreak
     a=0,b=0
     a=0,b=1
     a=1,b=0
     a=1,b=1
     a=2,b=0
     a=2,b=1
     a=3,b=0
     a=3,b=1
     a=4,b=0
     a=4,b=1
     */
    public void testBreak() {
        for (int a = 0; a < 5; a++) {
            for (int b = 0; b < 5; b++) {
                if (b == 2) {
                    break;
                }
                System.out.println("a=" + a + ",b=" + b);
            }
        }
    }

    /**
     testContinue
     a=0,b=0
     a=0,b=1
     a=0,b=3
     a=0,b=4
     a=1,b=0
     a=1,b=1
     a=1,b=3
     a=1,b=4
     a=2,b=0
     a=2,b=1
     a=2,b=3
     a=2,b=4
     a=3,b=0
     a=3,b=1
     a=3,b=3
     a=3,b=4
     a=4,b=0
     a=4,b=1
     a=4,b=3
     a=4,b=4
     */
    public void testContinue() {
        for (int a = 0; a < 5; a++) {
            for (int b = 0; b < 5; b++) {
                if (b == 2) {
                    continue;
                }
                System.out.println("a=" + a + ",b=" + b);
            }
        }
    }

    /**
     testBreakLabel
     a=0,b=0,c=0
     a=0,b=0,c=1
     * */
    public void testBreakLabel() {
        anyName:
        for (int a = 0; a < 5; a++) {
            for (int b = 0; b < 5; b++) {
                for (int c = 0; c < 5; c++) {
                    if (c == 2) {
                        break anyName;
                    }
                    System.out.println("a=" + a + ",b=" + b + ",c=" + c);
                }
            }
        }
    }

    /**
     testContinueLabel
     a=0,b=0,c=0
     a=0,b=0,c=1
     a=1,b=0,c=0
     a=1,b=0,c=1
     a=2,b=0,c=0
     a=2,b=0,c=1
     a=3,b=0,c=0
     a=3,b=0,c=1
     a=4,b=0,c=0
     a=4,b=0,c=1
     */
    public void testContinueLabel() {
        anyName:
        for (int a = 0; a < 5; a++) {
            for (int b = 0; b < 5; b++) {
                for (int c = 0; c < 5; c++) {
                    if (c == 2) {
                        continue anyName;
                    }
                    System.out.println("a=" + a + ",b=" + b + ",c=" + c);
                }
            }
        }
    }

}
Igor Rybak
30#
Igor Rybak Reply to 2017-07-16 10:24:26Z

Java 8 Stream solution:

        List<Type> types1 = ...
        List<Type> types2 = ...

        types1.stream()
                .flatMap(type1 -> types2.stream().map(type2 -> new Type[]{type1, type2}))
                .filter(types -> /**some condition**/)
                .findFirst()
                .ifPresent(types -> /**do something**/);
Jason
31#
Jason Reply to 2017-12-03 11:47:49Z

You can do one thing

1.) set a local variable to false

2.) set that variable true in the first loop, when you want to break out

3.) then you can check in the outer loop, that whether the condition is set then break from the outer loop as well.

boolean isBreakNeeded = false;
for (int i = 0; i < some.length; i++) {
    for (int j = 0; j < some.lengthasWell; j++) {
        //want to set variable if (){
        isBreakNeeded = true;
        break;
    }

    if (isBreakNeeded) {
        break; //will make you break from the outer loop as well
    }
}

Well, it quite seemed a simple way for me to put it like this.

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