How can I break out of two nested for loops in objective-c?

Question

I have two for loops nested like this:

for(...) {
    for(...) {

    }
}

I know that there is a break; statement. But I am confused about if it breaks both loops or just the one in which it was called? I need to break both ones as soon as I see that it doesn't make sense to iterate more times over.

Answer 1

break breaks out of one loop, but you can add a check to the outer loop which breaks when the inner breaks.

bool dobreak = false;
for ( ..; !dobreak && ..; .. ) {
   for ( ... ) {
      if (...) {
         dobreak = true;
         break;
      }
   }
}

Answer 2

If a break is executed from within a set of nested loops, only the innermost loop in which the break is executed is terminated. (Just like standard C)

Answer 3

The break statement will only break out of the loop in scope, which is the parent loop. If you want to break out of the second loop as well you could use a boolean variable which is in scope of both loops

bool isTerminated = false;

for (...)
{
    if (!isTerminated)
    {
        for(...)
        {
            ...

            isTerminated = true;
            break;
        }
    }
    else
    {
        break;
    }
}

Answer 4

The break statement breaks out of the innermost loop. An additional test and break statement would be needed to break out of the outer loop.

Answer 5

Probably the easiest way is to use a "flag" variable

for(i=0; i<10 && (done==false); i++)
  for(j=0;j< 10; j++){
     ..
     ..
     if(...){done=true; break;}
  }

Answer 6

Change top loop's counter before break

for(i=0; i<10 ; i++)
  for(j=0;j< 10; j++){
     ..
     ..
     i = 10; 
     break;
  }

Answer 7

The break statement only gets you out of the innermost loop. If you don't want the added overhead in code, memory and performance of a dedicated state variable, I recommend refactoring the code out into a function or method of its own, and using return to get out of all the loops:

void do_lots_of_work(void)
{
  int i, j;

  for(i=0; i<10 ; i++)
  {
    for(j=0;j< 10; j++)
    {
     ..
     ..
     if(disaster_struck())
      return; /* Gets us out of the loops, and the function too. */
    }
  }
}

Answer 8

If using goto simplifies the code, then it would be appropriate.

for (;;) 
{
    for (;;) 
    {
        break; /* breaks inner loop */
    } 
    for (;;) 
    {
        goto outer; /* breaks outer loop */
    }
} 
outer:;

Answer 9

Others have mentioned how you can set a flag or use a goto, but I'd recommend refactoring your code so that the inner loop is turned into a separate method. That method can then return some flag to indicate that the outer loop should break. If you name your methods appropriately, this is much more readable.

for (int i = 0; i < 10; i++) {
   if (timeToStop(i)) break;
}

-(bool) timeToStop: (int) i {
    for (int j = 0; j < 10; j++) {
        if (somethingBadHappens) return true;
    }

    return false;
}

Pseudocode, not tested, but you get the idea.

Answer 10

Other than the already mentioned flag variable or goto you could throw an objective C exception

@try {
  for() {
    for() {
       @throw ...
    }
  }
}
@catch{
  ...
}

Answer 11

another solution is to factor out the second loop in a function

int i;

for(i=0; i<10 ; i++){
  if !innerLoop(i) {
    break;
  }
}


bool innerLoop(int i)
  int j;
  for(j=0;j< 10; j++){
    doSomthing(i,j);
    if(endcondtion){
      return false;
    }
  }
}