Perl: Read columns and convert to array - perl

Question

I am new to perl, trying to read a file with columns and creating an array.
I am having a file with following columns.
file.txt
A 15
A 20
A 33
B 20
B 45
C 32
C 78
I wanted to create an array for each unique item present in A with its values assigned from second column.
eg:
#A = (15,20,33)
#B = (20,45)
#C = (32,78)
Tried following code, only for printing 2 columns
use strict;
use warnings;
my $filename = $ARGV[0];
open(FILE, $filename) or die "Could not open file '$filename' $!";
my %seen;
while (<FILE>)
{
chomp;
my $line = $_;
my #elements = split (" ", $line);
my $row_name = join "\t", #elements[0,1];
print $row_name . "\n" if ! $seen{$row_name}++;
}
close FILE;
Thanks

Answer 1

Firstly some general Perl advice. These days, we like to use lexical variables as filehandles and pass three arguments to open().
open(my $fh, '<', $filename) or die "Could not open file '$filename' $!";
And then...
while (<$fh>) { ... }
But, given that you have your filename in $ARGV[0], another tip is to use an empty file input operator (<>) which will return data from the files named in #ARGV without you having to open them. So you can remove your open() line completely and replace the while with:
while (<>) { ... }
Second piece of advice - don't store this data in individual arrays. Far better to store it in a more complex data structure. I'd suggest a hash where the key is the letter and the value is an array containing all of the numbers matching that letter. This is surprisingly easy to build:
use strict;
use warnings;
use feature 'say';
my %data; # I'd give this a better name if I knew what your data was
while (<>) {
chomp;
my ($letter, $number) = split; # splits $_ on whitespace by default
push #{ $data{$letter} }, $number;
}
# Walk the hash to see what we've got
for (sort keys %data) {
say "$_ : #{ $data{$_ } }";
}

Answer 2

Change the loop to be something like:
while (my $line = <FILE>)
{
chomp($line);
my #elements = split (" ", $line);
push(#{$seen{$elements[0]}}, $elements[1]);
}
This will create/append a list of each item as it is found, and result in a hash where the keys are the left items, and the values are lists of the right items. You can then process or reassign the values as you wish.