Thread: how to come out of double loop

Some languages allow you to label loops (eg, "inner" and "outer") and break out of nests that way. C kind of does that via goto, but it is generally discouraged as bad style. You can accomplish much the same thing if you place the nest in a function by itself and use return, but that is not always desirable.

So a normative way is via a control variable or "flag":

Code:

	int i, j, flag = 0;
	for (i = 0; i < 3; i++) {
		for (j = 0; j < 3; j++) {
			printf("%d %d\n", i, j);
			if (i + j == 3) {
				flag = 1;
				break;
			}
		}
		if (flag) break;
	}

Or you could just reuse your if statement in the outer loop:

Code:

for(i=0; i<3; i++)
   {
     for(j=0; j<3; j++)
     {
       if ( check_rows(i,j) == 0 ) 
         printf("<<%d, %d>>",i,j);
         break;
     }
   if( check_rows(i,j)==0)
        break; 
}

Or perhaps

Code:

for ( i = 0 ; i < 3 && !flag ; i++ ) {
  for ( j = 0 ; j < 3 && !flag ; j++ ) {
    // something sets flag to true, both loops will then exit
    // but the side effect is i and j will be incremented anyway (unlike say break or goto)
  }
}

@Matticus - Thanks for catching that, didn't see it when I was typing up the answer.

@OP - These are all valid suggestions, as MK27 said I would do what you can to stay away from goto. It is a poor programming practice.

seems like I got many suggestions here. Thanks, I'll check them and reply again.

The best option is to put it in its own function and use a return.

Failing that, the next most efficient option (excluding goto because I would not stoop that low) is to do this:

Code:

   for(i=0; i<3; i++)
   {
     for(j=0; j<3; j++)
     {
        if ( check_rows(i,j) == 0 ) 
        {
            i = 3;
            break;
        }
     }
   }

Originally Posted by nonoob

Bad idea, AndrewHunter. If the test fails in the j loop, that is, the loop completes, then the check outside will have j exceed array bounds.
I'd say this is a perfect example of where a goto is acceptable. Any other solution is ugly and uses more variables.

Thanks for pointing that out; I didn't even consider that.

Originally Posted by nonoob

and uses more variables.

Code:

for( x = 0; x < this; x++ )
{
    for( y = 0; y < that; y++ )
    {
        if( something )
        {
            x = this;
            y = that;
        }
    }
}

As long as you don't require the loop control values outside of the loop, you don't need more variables. You don't even need break.


Quzah.

Personally I prefer to use a flag as Salem has suggested. If side effects are an issue you should be able to get around it with a few extra break statements or "if(flag) break" statements. That's just a general rule, of course you should be flexible depending on exactly what the loop is doing . . . .

Setting the loop control variables to their upper bounds seems like a bad idea to me though. First, it might require more maintenance, especially if the "this/that" boundary expressions are complicated. And a compiler optimizer which would otherwise see a perfect loop with nicely striding loop variables could get confused by your meddling with the control variables. You might get less optimized code as a result.

Finally: using a goto to break out of the outer loop works as well. It's the most socially acceptable use of a goto, but still, it's a goto. I'd use a return statement in a heartbeat but probably not a goto. Although either of these would probably be good in terms of compiler optimization.