Clean way to break outer for loop from inner for loop

Question

Say you have a double-nested for, like when you are trying to see if an item in one unsorted array is inside another unsorted array. So for example say you have 2 lists candidates and corruptPeople, you're going through the candidates and skipping those that are listed in corruptPeople.

for( int i = 0 ; i < lenCandidates ; i++ )
{
    // veto candidates[i] if candidates[i] is in corruptPeople at all
    for( int j = 0 ; j < lenCorruptPeople ; j++ )
        if( candidates[i] == corruptPeople[j] )
             break_this_loop_and_then_continue_outer_loop ;

    // initiate the eligible candidates
    initiate( candidates[i] ) ;
}

do any of those answer your question? – Miles Alden Dec 07 '12 at 04:21 — Miles Alden, Dec 07 '12 at 04:21

score 3 · Answer 1 · answered Dec 07 '12 at 04:02

3

One way to do this (which I'm open to peer review for!) is using goto:

for( int i = 0 ; i < lenCandidates ; i++ )
{
    // veto candidates[i] if candidates[i] is in corruptPeople at all
    for( int j = 0 ; j < lenCorruptPeople ; j++ )
        if( candidates[i] == corruptPeople[j] )
             goto END_FOR ;

    // initiate the eligible candidates
    initiate( candidates[i] ) ;

    END_FOR:
    ; // seem to need an empty statement to make it compile
}

I'm curious what others have to say of the use of goto in this context. A dogmatic disagreeal with goto means you're going to have a dogmatic application of structured programming.. which looked pretty bad when I tried it.

answered Dec 07 '12 at 04:02

bobobobo

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I don't think using a goto could ever be considered a clean solution -- even if in the end it was necessary. – goji Dec 07 '12 at 04:11
1

Personally, this is one of the few cases where I _prefer_ `goto`. As maligned as it is, and as much as it can be abused, I find it simpler in cases like this than other solutions. – Joshua Green Dec 07 '12 at 04:38
It may not be clean, but unfortunately C doesn't have anything quite as nice as, say, Python's `for ... else` construct. – Joshua Green Dec 07 '12 at 04:42
1

The reason you need an empty statement to make it compile is that `;` is a statement but an empty sequence of tokens isn't. Therefore `END_FOR:;` is a labelled statement but `END_FOR:` isn't. – Steve Jessop Dec 07 '12 at 04:51
Some info regarding the need for `;`... http://stackoverflow.com/questions/8384388/variable-declaration-after-goto-label-c – Krishnabhadra Dec 07 '12 at 04:53

score 2 · Answer 2 · answered Dec 07 '12 at 04:08

2

Much better to use an additional variable than to resort to a goto:

for( int i = 0 ; i < lenCandidates ; i++ )
{
    // veto candidates[i] if candidates[i] is in corruptPeople at all
    int corrupt = 0;
    for( int j = 0 ; j < lenCorruptPeople ; j++ )
        if( candidates[i] == corruptPeople[j] )
        {
            corrupt = 1;
            break;
        }

    // initiate the eligible candidates
    if (!corrupt) initiate( candidates[i] ) ;
}

answered Dec 07 '12 at 04:08

goji

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Sorry I deleted my answer.. Didn't see this.. Mine was same – Krishnabhadra Dec 07 '12 at 04:15
This is the cleanest way.. easy to understand too – Krishnabhadra Dec 07 '12 at 04:17
I don't see why creating an additional variable and adding extra code -- including a check that should be redundant since we know its value along _every_ codepath -- is "much better" than resorting to a `goto`. – Joshua Green Dec 07 '12 at 04:39

score 0 · Answer 3 · answered Dec 07 '12 at 04:13

0

How about the below code? Am I missing something?

for( int i = 0 ; i < lenCandidates ; i++ )
{
    // veto candidates[i] if candidates[i] is in corruptPeople at all
    int j;
    for( j = 0 ; j < lenCorruptPeople ; j++ )
    {
        if( candidates[i] == corruptPeople[j] )
          break;
    }

    //j will be equal to lenCorruptPeople only when candidate is not in corrupt list   
    if(j==lenCorruptPeople)
    {
        initiate( candidates[i] ) ;
    }

}

answered Dec 07 '12 at 04:13

Jay

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Ya, j is out of scope there with `if(j!=lenCorruptPeople)` – Miles Alden Dec 07 '12 at 04:14
@Krishnabhadra, changed it a little. Now, will it be Ok? – Jay Dec 07 '12 at 04:15
Ok, now it is correct.. But I still prefer Troy's answer, which is far more easier to understand and direct. – Krishnabhadra Dec 07 '12 at 04:17
I agree. Use break over goto for sure. – Miles Alden Dec 07 '12 at 04:18
`goto` allows us to keep the variable local to the loop while not duplicating the end-of-loop check on `j`. Is it so clear that this `break` method is better? As long as `goto` is limited to cases like this, I don't see any problems. – Joshua Green Dec 07 '12 at 04:48

score 0 · Answer 4 · answered Dec 07 '12 at 04:43

0

Moving the inner loop to another function often helps:

typedef int Person; // or whatever it is.

Person *findPerson(Person *first, Person *last, Person val)
{
    while (first != last)
    {
        if (val == *first) return first;
        ++first;
    }
    return 0;
}

...

for( int i = 0 ; i < lenCandidates ; i++ )
{
    // veto candidates[i] if candidates[i] is in corruptPeople at all
    if (!findPerson(corruptPeople, corruptPeople+lenCorruptPeople, candidates[i]))
    {
        // initiate the eligible candidates
        initiate( candidates[i] ) ;
    }
}

answered Dec 07 '12 at 04:43

Steve Jessop

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I think I would go crazy if I had to define a new function every time I had a situation like this. – Joshua Green Dec 07 '12 at 04:49
@JoshuaGreen: Fortunately you don't have to do it every time, but it often helps. In this case "find" is a simple abstraction that might even be re-usable, so I think it's a fairly good candidate. If you don't like short functions that do one thing then of course `findPerson` is a nightmare ;-p – Steve Jessop Dec 07 '12 at 04:54

Clean way to break outer for loop from inner for loop

4 Answers4