Breaking out of a nested loop

Question

If I have a for loop which is nested within another, how can I efficiently come out of both loops (inner and outer) in the quickest possible way?

I don't want to have to use a boolean and then have to say go to another method, but rather just to execute the first line of code after the outer loop.

What is a quick and nice way of going about this?

Thanks

Answer 1

I think unless you want to do the "boolean thing" the only solution is actually to throw. Which you obviously shouldn't do..!

Answer 2

Well, "goto", but that is ugly... and not always possible. You can also place the loops into a method (or an anon-method) and use "return" to exit back to the main code.

        // goto
        for (int i = 0; i < 100; i++)
        {
            for (int j = 0; j < 100; j++)
            {
                goto Foo; // yeuck!
            }
        }
    Foo:
        Console.WriteLine("Hi");

        // anon-method
        Action work = delegate
        {
            for (int x = 0; x < 100; x++)
            {
                for (int y = 0; y < 100; y++)
                {
                    return; // exits anon-method
                }
            }
        };
        work(); // execute anon-method
        Console.WriteLine("Hi");

Answer 3

Is it possible to refactor the nested for loop into a private method? That way you could simply 'return' out of the method to exit the loop.

Answer 4

Don't quote me on this, but you could use goto as suggested in the MSDN. There are other solutions, as including a flag that is checked in each iteration of both loops. Finally you could use an exception as a really heavyweight solution to your problem.

GOTO:

for ( int i = 0; i < 10; ++i ) {
   for ( int j = 0; j < 10; ++j ) {
      // code
      if ( break_condition ) goto End;
      // more code
   }
}
End:

Condition:

bool exit = false;
for ( int i = 0; i < 10 && !exit; ++i ) {
   for ( int j = 0; j < 10 && !exit; ++j ) {
      // code
      if ( break_condition ) {
         exit = true;
         break; // or continue
      }
      // more code
   }
}

Exception:

try {
    for ( int i = 0; i < 10 && !exit; ++i ) {
       for ( int j = 0; j < 10 && !exit; ++j ) {
          // code
          if ( break_condition ) {
             throw new Exception()
          }
          // more code
       }
    }
catch ( Exception e ) {}

Answer 5

Use a suitable guard in the outer loop. Set the guard in the inner loop before you break.

bool exitedInner = false;

for (int i = 0; i < N && !exitedInner; ++i) {

    .... some outer loop stuff

    for (int j = 0; j < M; ++j) {

        if (sometest) {
            exitedInner = true;
            break;
        }
    }
    if (!exitedInner) {
       ... more outer loop stuff
    }
}

Or better yet, abstract the inner loop into a method and exit the outer loop when it returns false.

for (int i = 0; i < N; ++i) {

    .... some outer loop stuff

    if (!doInner(i, N, M)) {
       break;
    }

    ... more outer loop stuff
}

Answer 6

factor into a function/method and use early return, or rearrange your loops into a while-clause. goto/exceptions/whatever are certainly not appropriate here.

def do_until_equal():
  foreach a:
    foreach b:
      if a==b: return

Answer 7

I was thinking that exceptions aren't cheap/should only be thrown in a truly exceptional condition etc. Hence I don't think this solution would be good from a performance perspective.

I don't feel it it is right to take advantage of the newer features in .NET (anon methods) to do something which is pretty fundamental.

Because of that, tvon (sorry can't spell full username!) has a nice solution.

Marc: Nice use of anon methods, and this too is great but because I could be in a job where we don't use a version of .NET/C# that supports anon methods, I need to know a traditional approach too.

Answer 8

You asked for a combination of quick, nice, no use of a boolean, no use of goto, and C#. You've ruled out all possible ways of doing what you want.

The most quick and least ugly way is to use a goto.

Answer 9

Don't know if it works in C#, but in C I often do this:

    for (int i = 0; i < 100; i++)
    {
        for (int j = 0; j < 100; j++)
        {
            if (exit_condition)
            {
                // cause the outer loop to break:
                i = INT_MAX;
                Console.WriteLine("Hi");
                // break the inner loop
                break;
            }
        }
    }

Answer 10

Since I first saw break in C a couple of decades back, this problem has vexed me. I was hoping some language enhancement would have an extension to break which would work thus:

break; // our trusty friend, breaks out of current looping construct.
break 2; // breaks out of the current and it's parent looping construct.
break 3; // breaks out of 3 looping constructs.
break all; // totally decimates any looping constructs in force.

Answer 11

Depending on your situation, you may be able to do this, but only if your not executing code AFTER the inner loop.

for (int i = 0; i < 100; i++)
{
    for (int j = 0; j < 100; j++)
    {
        i = 100;
        break;
    }
}

It's not elegent, but it may be the easiest solution depending on your problem.

Answer 12

I've seen a lot of examples that use "break" but none that use "continue".

It still would require a flag of some sort in the inner loop:

while( some_condition )
{
    // outer loop stuff
    ...

    bool get_out = false;
    for(...)
    {
        // inner loop stuff
        ...

        get_out = true;
        break;
    }

    if( get_out )
    {
        some_condition=false;
        continue;
    }

    // more out loop stuff
    ...

}

Answer 13

For people who found this question via other languages, Javascript, Java, and D allows labeled breaks and continues:

outer: while(fn1())
{
   while(fn2())
   {
     if(fn3()) continue outer;
     if(fn4()) break outer;
   }
}

Answer 14

Hi guys,

Thanks for the suggestions. Saying a boolean is not acceptable is just being stubborn, and the suggestions with a boolean are fine.

Thanks again everyone

Answer 15

---

how terminate nesed loop

---

int temp=1;
for(i=1;i<n&&temp;i++)
{
    for(j=1;j<m&&temp;j++)
    {
    if(n<0)
        flag=0;
    else
        printf("not");
    }
}