breaking multiple loops

[rozner]

Hi,

I would like to know if it is possible to break a loop from inside another loop. In Java this is possible like so:

myloop:
for(; ; )
  for(; ; )
    break myloop;

Is there a C equivalent of this

Of course I can just create another variable and set it to a particular variable in the inner loop, however this results in an extra comparison on every iteration of the outer loop.

Thanks

[Thantos]

You could use the evil GOTO command

[rozner]

well...from what I've heard and read about GOTO, and the fact that you call it EVIL, I'll probably stay away from it. Thanks anyways.

[major_blagger]

Does this work? Probably breaks a lot of style rules.

for (i = 1; i < I_MAX; i++)
 for (j = 1; j < J_MAX; j++)
    if (needToBreakBoth()) {
       i = I_MAX; 
       break;
    }

K&R says use a goto or use an extra variable. They say this case is one of the few exceptions where you might possibly consider using a goto.

[Thantos]

That would work major_blagger however then you are wasting the value of i.

I said evil goto because a lot of people think its horrible. The fact is that goto has it's uses. Granted most of the time their are ways around it but in cases like this it does make sense to use it.

[Edward]

The fact is that goto has it's uses. Granted most of the time their are ways around it but in cases like this it does make sense to use it.

This situation is one of those uses. The usual suggestion of using a status variable is both wasteful and clutters the code. So many people find this to be perfectly reasonable.

while (condition1) {
    while (condition2) {
        if (something)
            goto end;
    }
}
end:

I have no preference either way so that I might stay out of the line of fire, but a good solution that clarifies the code in some cases and removes the need for goto is to wrap the inner loop in a function.

int func()
{
    while (condition2) {
        if (something)
            return 1;
    }

    return 0;
}

...

while (condition1) {
    if (func())
        break;
}

Of course, setting up parameters and such can also be a bother at times. So aside from more careful design, the problem remains to be solved effectively for all cases. Remember that everything is relative. People who refuse to use goto "because it's evil" clearly don't have an open mind about programming. If a tool can solve the problem better than others in a certain case then there is nothing wrong with using that tool.

[c99]

Votes for,

while (condition1)
    while (condition2)
       if (cond)
            goto label;

label:

I've been programming C for approx 14 months and I've yet had
good reason to use the goto keyword but in this particular case
I would certainly consider it.

[Thantos]

Thank you for rehasing what I said Edward

[WaltP]

Not me. I'd still opt for:

while (condition1)
{
    while (condition2)
    {
       if (cond)
       {
            condition1 = false;
            break;
       }
    }
}

[rozner]

well, thanks for all the input on this one, I guess I'll try the GOTO, it'll be first for me in C.

[Hunter2]

WaltP:
But what happens when you have

while (condition1)
{
    while (condition2)
    {
       if (cond)
       {
            condition1 = false;
            break;
       }
    }
    cout << "I don't want to display this if !condition1\n";
    cout << "I don't want this either\n";
    cout << "Or this." << std::endl;
}

Then you'd have to do the other test case again (either another variable - or do something to make the original condition1 false, potentially causing other problems), and if you're really strapped for speed, you won't want that. Just as Edward said.

The usual suggestion of using a status variable is both wasteful and clutters the code.

[WaltP]

Originally posted by Hunter2
WaltP:
But what happens when you have

while (condition1)
{
    while (condition2)
    {
       if (cond)
       {
            condition1 = false;
            break;
       }
    }
    if (condition1)
    {
        cout << "I don't want to display this if !condition1\n";
        cout << "I don't want this either\n";
        cout << "Or this." << std::endl;
    }
}

It's not elegant, but what you are trying to do is nonelegant (inelegant?) in the first place.

Or as was mentioned before, put this loop in a function and return instead of break.

[Hunter2]

Yes, but then you're testing for a condition again, potentially killing your program if it's a really crucial speed-spot. Granted, if it isn't crucial, it probably won't make much of a difference. But I'm against putting the loop in a function. That would mean that if you need to access data from outside the loop, you'd either have to make it global or pass it as parameters, which would mean processing time setting up the stack; Besides which, it means you'd have to do the jump-to-other-section-of-code-then-jump-back-to-where-you-were thing when reading the code, potentially damaging readability. And besides which, it takes a lot more effort to do, as Edward said.