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Re: Pair of items

by PhiRatE (Monk)
on Jul 30, 2003 at 06:29 UTC ( #279086=note: print w/replies, xml ) Need Help??

in reply to Pair of items

What are your performance requirements? you said "some items in an array", which indicates to me that you're talking 10 or 20, which means that the solutions provided are fine. If you start talking about large sets however, you're going to get in real trouble iterating over the whole lot doing is_ok's, you want to cull out all the unavailable options as soon as possible so you can pick the first available one straight off. To do this you have a quick setup phase like this:
for ($ref=0; $ref<$#items; $ref++) { for (@{$items[$ref]}) { if (!$contains{$_}) { $contains{$_} = [$ref]; } else { push @{ +$contains{$_}}, $ref; } } }
This gives you a hash of arrays called %contains, which maps numbers to the index of the items array of an element containing that number. From this, you can determine all pairs which are acceptable partners like this:
sub get_pairs { my %invalids = (); my @valids = (); for $number (@_) { for $id (@{$contains{$number}}) { $invalids{$id} = 1; } } for ($id=0; $id < $#items; $id++) { push @valids, $items[$id]; } return @valids; } @all_matches = get_pairs(2,3,4)
You can of course get a single acceptable pair much faster by just scanning up the possible ids until you reach one that isn't invalid.

You gain significant improvements using this method in instances where you have a *lot* of comparisons, and when you need all available matches. For a smal number of comparisons the setup cost for this method is probably not worth it.

All code is example only, hasn't been tested etc and doesn't properly scope or anything. Hopefully the point is fairly obvious.

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