It's not as complicated as it looks; it would be much easier to understand if COMPARE didn't use $L as an LoL reference, and %L as a hash.

The main part of the program builds up 2 LOL (lists of lists, see perldsc). @R contains lists where the 5th element isn't --, and @L contains those where the 5th element IS and 2nd element isn't.

Note that there's no error checking for cases where both the 2nd and 5th elements are --, I don't know if that matters to you.

The 2 LOL's are then passed into the COMPARE routine. By the way, the 

use Data::Dumper;
looks like a red herring; I don't see anything there that needs that module.

In compare, $L and $R are references to the LOLs built up in the main routine. %L and %R are hashes that map all of the indexes in @$L and @$R respectively to (anything, since the key existence is all that matters), to keep a set of the rows that still need to be processed.

The nested for loops go over every possible combination of items in @$R and @$L, checking if the 4th element in the @$R entry matches the 2nd in @$L. In that case, and if that entry in @$L hasn't already been matched (that's the next unless part), those entries are deleted from %R and %L (so they don't get printed out later), and the "match" line is printed out. The "last;" call ends the $J loop, and moves processing on to the next item in @$R.

After the 2 loops, all the unmatched entries are printed out.

Is it clearer now, or did I make it worse? :)
--
Mike

In reply to Re: Enlightenment by RMGir
in thread Enlightenment by johnirl

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