#!/usr/bin/perl
# The Missing Textutils, Ondrej Bojar, obo@cuni.cz
# http://www.cuni.cz/~obo/textutils
#
# Given a column number, 'split_at_colchange' adds an empty line whenever the
# value in the column changes. Useful as a preprocessing before 'blockwise'.
#
# $Id: split_at_colchange,v 1.2 2006/06/16 08:36:05 bojar Exp $
#

use strict;

my $col = shift;
die "split_at_colchange <colnumber>  < input > blocks" if !$col;

$col--;

my $lastval;
while (<>) {
  my @line = split /\t/, $_;
  # print STDERR "E: $lastval vs. $line[$col]; $col\n";
  if (defined $lastval && $lastval ne $line[$col]) {
    print "\n";
  }
  $lastval = $line[$col];
  print;
}