Thread: How to exit from nested loops?

Originally Posted by Bayint Naung

Use goto. It's the right place.

Code:

for (x = 0; x < lim0; x++){
    for (x1 = 0; x1 < lim1; x1++){
         for (x2 = 0; x2 < lim2; x2++){
               if (contidion)
                   goto end_nested_loop; // this exit the inner loop only - 
         }
     }
}
end_nested_loop:

What about the alternatives I was thinking about?

I don't see any alternative to do so in C99.
Since there's already something to solve it:goto.

Originally Posted by Bayint Naung

I don't see any alternative to do so in C99.
Since there's already something to solve it:goto.

What about the multiple break I implemented? Somebody
could even be happier I didn't use goto.

I prefer goto approach. It seems 'cleaner' to me.

You could put the loops into a function and then use return instead of goto.

Code:

int myLooper(int lim0, int lim1, int lim2)
{
   int x, x1, x2;

   for (x = 0; x < lim0; x++){
       for (x1 = 0; x1 < lim1; x1++){
           for (x2 = 0; x2 < lim2; x2++){
                if (contidion)
                    return(3);
          }
       }
   }
   return(0);
}

But don't forget you'll need to pass other parameters that are used in loops.

Code:

int foo;
char bar[100];
for (x = 0; x < lim0; x++){
    bar[x] = ...;
    for (x1 = 0; x1 < lim1; x1++){
        array[x][x1] += foo + ...;
        for (x2 = 0; x2 < lim2; x2++){
             if ( estimate ( result, THRESHOLD) > EPSILON ) 
                 break; // this exit the inner loop only - 
       }
    }
}

My first guess would be to not allow loops to get nested that deeply. I can't imagine why you would want to do that or that there isn't a better or alternative approach to nesting that deep.

Scientific application often involve models of 3D space that are usually represented by 3D arrays, so 3 nested loops is hardly unusual.

goto is the simplest way out, but if you are hard against goto the best way is to set flags in each loop to tell the next level loop to break if the inner one does.

Scientific application often involve models of 3D space that are usually represented by 3D arrays, so 3 nested loops is hardly unusual.

Regardless of the application or task at hand nested loops can always be unrolled or separated into smaller loops. Games and rendering apps use a lot of math and transformations as well and they do not require 3 nested loops.

You could rebuild your car's body with carbon fiber and exotic alloys too, but I'm not going to do it.

Nested loops may not be as elegant, or even quite as fast, but the logic is clear, and the code is easy to write and understand. Those who want to choke on a goto from inside a nested loop, probably have a waiting line of things they want to choke on.

To me, this is *THE* place to use a goto.

Originally Posted by Bubba

Regardless of the application or task at hand nested loops can always be unrolled or separated into smaller loops. Games and rendering apps use a lot of math and transformations as well and they do not require 3 nested loops.

If you want your software to be readable and possible to upkeep then why would you want to sacrifice the simplicity of nested loops? The use of goto is almost always discouraged because you lose readibility and it can become very hard to maintain, but here the opposite is true, whereas reducing the loops like you suggest does just that.