perl: reverse values in a column - perl

Question

I have a tab-seperated table (table1) with 4 columns, looking like this
A + 1 1
A + 2 2
A + 3 3
B - 2 4
B - 3 5
B - 4 6
B - 5 7
C + 1 8
C + 2 9
C + 3 10
D - 1 11
D - 2 12
D - 3 13
The letters in col1 define groups. Now, if there is a "+" in col2, I just want to keep the line as it is. If there i a "-" in col2, I want to reverse the values in col3, for all lines belonging to the group defined by col1. For this example the output should look like this
A + 1 1
A + 2 2
A + 3 3
B - 5 4
B - 4 5
B - 3 6
B - 2 7
C + 1 8
C + 2 9
C + 3 10
D - 3 11
D - 2 12
D - 1 13
So for group A, everything stays the same. But for group B the original values in col3 are: 2, 3, 4, 5. They should become 5, 4, 3, 2. The rest of the columns stays the same.
How should I work this out in Perl?

Answer 1

Keep the lines of a - group in a buffer, output it when the group changes.
#!/usr/bin/perl
use warnings;
use strict;
sub output {
my $buffer = shift;
my #rev = map $_->[2], #$buffer;
$_->[2] = pop #rev for #$buffer; # Reverse the 3rd column.
print join("\t", #$_) for #$buffer;
#$buffer = ();
}
my #buffer;
my $group;
print scalar <>; # header
while (<>) {
my #cols = split /\t/;
if (#buffer and $cols[0] ne $group) {
output(\#buffer);
}
if ('+' eq $cols[1]) {
print;
} else {
$group = $cols[0];
push #buffer, \#cols;
}
}
output(\#buffer) if #buffer; # Don't forget to output the last buffer.

Answer 2

Here's an alternative solution that makes use of the advantage of random access if you read the data into memory
use strict;
use warnings;
my #data = map { chomp; [ split /\t/ ]; } <DATA>;
my %ranges;
for ( grep $_->[1] eq '-', #data ) {
push #{ $ranges{$_->[0]} }, $_;
}
for my $range ( values %ranges ) {
for ( my $i = 0; $i*2 < $#$range; ++$i ) {
my ($from, $to) = #{$range}[$i, $#$range-$i];
( $from->[2], $to->[2] ) = ( $to->[2], $from->[2] );
}
}
print join("\t", #$_), "\n" for #data;
__DATA__
col1 col2 col3 col4
A + 1 10
A + 2 20
A + 3 35
B - 2 5
B - 3 21
B - 4 23
B - 5 36
output
col1 col2 col3 col4
A + 1 10
A + 2 20
A + 3 35
B - 5 5
B - 4 21
B - 3 23
B - 2 36