Find doublet data in csv file - perl

Question

I'm trying to write a Perl script that can check if a csv file has doublet data in the two last columns. If doublet data is found, an additional column with the word "doublet" should be added:
Example, the original file looks like this:
cat,111,dog,555
cat,444,dog,222
mouse,333,dog,555
mouse,555,cat,555
The final output file should look like this:
cat,111,dog,555,doublet
cat,444,dog,222
mouse,333,dog,555,doublet
mouse,555,cat,555
I'm very much a newbie to Perl scripting, so I won't expose myself with what i've written so far. I tried to read through the file splitting the data into two arrays, one with the first two columns, and the other with the last two columns
The idea was then to check for doublets in the second array, and add (push?) the additional column with the "doublets" information to that array, and then afterwards merge to two array back together again(?)
Unfortunately my brain has now collapsed, and I need help from someone smarter than me, to guide me in the right direction.
Any help would be very much appreciated, thanks.

Answer 1

This is not the most efficient way but here is something to get you started. Script assumes that your input data is comma separated and can contain any number of columns.
#!/usr/bin/env perl
use strict;
use warnings;
my %h;
my #lines;
while (<>) {
chomp;
push (#lines,$_); # save each line
my #fields = split(/,/,$_);
if(#fields > 1) {
$h{join("",#fields[-2,-1])}++; # keep track of how many times a doublet appears.
}
}
# go back through the lines. If doublet appears 2 or more times, append ',doublet' to the output.
foreach (#lines) {
my $d = "";
my #fields = split(/,/,$_);
if (#fields > 1 && $h{join("",#fields[-2,-1])} >= 2) {
$d = ",doublet";
}
print $_,$d,$/;
}
The syntax #fields[-2,-1] is an array slice that returns an array with the last two column values. Then, join("",...) concatenates them together and this becomes the key to the hash. $/ is the input record separator which is newline by default and is quicker to write than "\n"
cat,111,dog,555,doublet
cat,444,dog,222
mouse,333,dog,555,doublet
mouse,555,cat,555