I don't know if one UR key may appear many times in one input file, each time with the same or different CI keys, or vice versa.

If so, you the hash key must be the whole (chomped) record of one input files, and proceed just like grizzley said, but reversing the fields of the records from the second file when testing for existence in the loop.

Also, if there are many records with exact the same keys on the second file, you can delete the full key from the hash each time you match it, just to get unique records in the output file.

Bonus track (not tested):

#!perl
use strict;
use warnings;

# ...

open $IN1, "<", $infile1 or die "cannot open $infile1: $!\n";
open $IN2, "<", $infile2 or die "cannot open $infile2: $!\n";
open $OUT, ">", $outfile or die "cannot open $outfile: $!\n";

my %pair = ();

while (<$IN1>) {
  chomp;
  s/^(\w+)\s+(\w+)$/$1 $2/; # just one space between keys
  $pair{$_} = 1;
}

while (<$IN2>) {
  chomp;
  s/^(\w+)\s+(\w+)$/$2 $1/; # swap keys
  if (exists $pair{$_}) {
    print $OUT "$_\n";
    delete $pair{$_};
  }
}

close $IN1;
close $IN2;
close $OUT;

[download]

Update: Initialized and changed name of the hash and newline added on output records. Still not tested!

In reply to Re: Hash Comparisions by vitoco
in thread Hash Comparisions by perl_n00b

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