Re: combining 2 files with a comon field

[anotherstevew]

<perl newbie puts head above parapet for first time - cautiously - to humbly offer an approach that doesn't use arrays (so it can handle BIG files) and ignores keys only present in one of the input files>

#!/usr/bin/perl -w
use strict;

open ONE, "1.txt"  or die "Cannot open 1.txt to read\n $!";
open TWO, "2.txt"  or die "Cannot open 2.txt to read\n $!";
open TRE, ">3.txt" or die "Cannot open 2.txt to write\n $!";

while (<ONE>) {
  chomp;
  (my $onea, my $oneb) = split(/\|/);

  my $twoa = undef;
  my $twob = undef;
  while (! eof(TWO))
  {
    my $two = <TWO>;
    chomp $two;
    ($twoa, $twob) = split(/\|/, $two);
    last if ($twoa ge $onea);
  }

  if ($onea lt $twoa) {
    next;
  }
    else {
    print TRE "$onea\|$oneb\|$twob\|\n" if ($onea eq $twoa);
  }
}

[merzy]

Yea, I'm a big fan of this method. Memory-gentle, constant time, easy to understand. We use it a lot at work for files with tens of millions of lines. Note that it assumes sorted input files, but that's what sort(1) is for. :-)