How to get out of this deeply-nested loop without using "goto"?

Question

In a situation like the loop below, what are the ways other than goto to get out of the nested loop at once?

#include<stdio.h>

int main(void)
{
    int  i,j,k ;

    for(i=1;i<=3;i++)
    {
        for(j=1;j<=3;j++)
        {
            for(k=1;k<=3;k++)
            {
                if(i==3&&j==3&&k==3)
                    goto out ;
                else
                    printf("%d%d%d\n",i,j,k) ;
            }
        }
    }
out :
    printf("Out of the loop at last!") ;
}

With that specific condition and those loop bounds (i.e. when only skipping the very last iteration), both `break` and `continue` work. Might wanna alter that a bit. — , May 15 '13 at 20:47
One options is to isolate just the loop itself in a function of its own and then use `return`. — 500 - Internal Server Error, May 15 '13 at 20:48
@delnan But don't `break` and `continue` are limit themselves to one loop only? — Thokchom, May 15 '13 at 20:49
@RobertHarvey How could be a C question be a duplicate of a C# question? — Thokchom, May 15 '13 at 20:52
@Thokchom They do, but in your example it doesn't matter, because that condition only triggers in the very last iteration, so after skipping the inner loop body the outer loops end too. — , May 15 '13 at 20:56
Nothing to be sorry about. If I need to exit early from a nested loop, I generally refactor the code so that I can just `return`, as @Inspired suggests, or set a condition, like DrummerB suggests. I'll reopen the question, since a different programming language makes this not an exact duplicate. — Robert Harvey, May 15 '13 at 20:58
This is just not constructive. You should have read from many sources also that `goto` can make sense under certain circumstances, e.g http://stackoverflow.com/questions/1586932/what-is-a-neat-way-of-breaking-out-of-many-for-loops-at-once/1586943#1586943. We have had these discussion `goto-or-not` so many times. — Jens Gustedt, May 15 '13 at 21:31
possible duplicate of [GOTO still considered harmful?](http://stackoverflow.com/questions/46586/goto-still-considered-harmful) — Alexander Shukaev, May 15 '13 at 21:31
@JensGustedt: I removed the "Not Constructive" parts of the question. — Robert Harvey, May 15 '13 at 21:35

score 5 · Answer 1 · answered May 15 '13 at 20:48

goto is not bad when used reasonably.

If you need to exit from many loops at once, it often means that your code should be redesigned. Such a 'cascade break is something like emergency exit. Maybe it's the right time to move your code into a separate function and use return there.

score 2 · Answer 2 · answered May 15 '13 at 20:50

One solution might be an additional condition.

#include<stdio.h>

int main(void)
{
    int  i,j,k ;
    int abort = 0;
    for(i=1; i<=3 && !abort; i++)
    {
        for(j=1; j<=3 && !abort; j++)
        {
            for(k=1; k<=3 && !abort; k++)
            {
                if(i==3&&j==3&&k==3)
                    abort = 0;
                else
                    printf("%d%d%d\n",i,j,k) ;
            }
        }
    }
    printf("Out of the loop at last!") ;
}

I just find such a thing horrible and much less readable (= error prone) than a plain and simple `goto`. — Jens Gustedt, May 15 '13 at 21:41

score 1 · Answer 3 · answered May 15 '13 at 20:49

1

Forget those "sources" - there ARE valid reasons for goto. Besides, break is not quite different. However, if you really want, you could move the loops into their own subfunction and leave this with return.

answered May 15 '13 at 20:49

JeffRSon

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score 1 · Answer 4 · answered May 15 '13 at 21:31

This is one of the few cases where using a goto would be acceptable, although it would be better to re-think that loop.

The main problem with goto is that it can destroy the ability to debug code by simple inspection.

Consider the following code snippet:

i = 10;
label: printf("%d\n", i);

What value gets printed for i? Does the line i = 10; even get executed? I can't know until I account for every instance of goto label; in the function, meaning I have to manually trace the execution path. Depending on the size of the function, this can go from a minor annoyance to a major pain in the ass. Bonus points if there are additional gotos in the same function.

If you follow a few simple rules, it isn't so bad: branch forward only, don't branch into a control structure (if, while, for, etc.), don't branch over declarations or initializations, etc. Basically treat it like a break statement. But honestly, if you can avoid using it, do so.

How to get out of this deeply-nested loop without using "goto"?

4 Answers4