#!/usr/bin/perl -l

use strict;
use warnings;

my @old=split ' ', <DATA>;
print "@old";

while (<DATA>) {
    my @cur=split;
    print "@cur[0,1]" eq "@old[0,1]" ?
      $cur[2] : "@cur";
    @old=@cur;
}

__END__
foo bar baz
foo fred  baz
foo  fred burz
bar baz burp

[download]

Additional checks and adaptation to current situation are left as an exercise to the reader.

Although it should be clear that this is meant as a minimal example, I'd like to stress that "@cur[0,1]" eq "@old[0,1]" is a simple minded equality test based on joining on $", which happens to be a space by default. Of course this works here, but is bound to be unreliable in a realistic situation, and you may have to roll a more sophisticated test yourself.

In reply to Re: compare current row with previous row by blazar
in thread compare current row with previous row by Anonymous Monk

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