Question

Using Perl, I want to print only the repeating (duplicate) values in each list. The values should appear at least 2 times (2 or more times). Each list (row) should be considered separately.

For example, given the input:

abc 51082 6457 31072 5575 5488 4842 16567 151

cde 5575 3674 8150  5575 3674 8150

fgh 5737 6887 48278 3674 34399 3674 8150

I want the following output:

abc 
cde 5575 3674 8150
fgh 3674

I wrote the following source code, but it’s not giving the correct output:

#!/usr/bin/perl -w

open FH, "input.txt";
@a=<FH>;

my %count_of;

foreach $tmp (@a) 
{
    foreach $word (split /\s/, $tmp) 
    {
        $count_of{$word}++;

        if ($count_of{$word} >=2)
        {
            print "$word\n";
        }
    }
}

exit;

Could someone please guide me on what changes need to be made to the code?
Thank you!

Answer 1

Here a working version. Look at the comments in the code to understand the corrections

#!/usr/bin/perl

# always use strict and warnings: this will help you to find errors
use warnings;
use strict;

open FH, 'input.txt';

# always declare variables
my @a=<FH>;

# you should close file handles
close FH;

# declare $tmp
foreach my $tmp (@a) {

    # you need to process the first element differently since you
    # want to always print it
    my @row = split /\s/, $tmp;

    # you should check for empty rows
    if (@row == 0) {
        # empty row
        next;
    }

    # removes the first element
    my $header = shift @row;

    # the first element is always printed
    print $header;

    # this should be local otherwise you are counting globally
    # a new declaration will give you a new hash
    my %count_of;

    # declare $word
    foreach my $word ( @row ) {

        # you should not increment an undefined variable
        if ( $count_of{$word} ) {
            $count_of{$word} = 1;
        } else {
            $count_of{$word}++;
        }

        # just print the second time you see the word
        if ( $count_of{$word} == 2) {
            print " $word";
        }

    }

    print "\n";

}

# it is good practice to return true at the end of a script (no need to exit)
1;

This produces:

abc
cde 5575 3674 8150
fgh 3674