How to use perl to parse a text file - perl

Question

I am new to perl,
I have text file contains 2 columns:
lib1 cell1
lib1 cell2
lib2 cell3
lib2 cell1
I would like to use perl to find there is duplicated in name in column 2 then print the name of column 1
In this text cell1 is repeated 2 times.
I would like to have a report something like:
cell1 found in lib1 lib2
I use the code below to read and open the file
#!/usr/bin/env perl
use strict;
use warnings;
for my $file ( #ARGV ){
open my$in_fh, '<', $file or die "could not open $file: $!\n";
while( my $line = <$in_fh> ){
chomp( $line );
print "$line\n"
}
}
But I don't know how to find the duplicated name in second column and print the 1st column

Answer 1

There are a few things here that Perl can do for you.
First, Perl will handle opening and reading the files you specify on the command line this the empty deadline operator (and here I'm using the safer double diamond version introduced in v5.22):
use v5.22
while( <<>> ) {
...
}
Then, you can track what you've seen with a hash. Extract the columns, and use the interesting column as the key in the hash. Here I post-increment it's value. On the first go around, the post increment returns 0 (then increases the value by 1), so the conditional is false the first time. The next time it sees that same key, the value is true, so it warns:
use v5.22
my %Seen;
while( <<>> ) {
chomp;
my( $first, $second ) = split;
if( $Seen{$second}++ ) {
warn "Duplicated second column! Line $.\n";
}
}
The hash is a great way to track things that are strings instead of positions.
Now, you want to know which values in the first column appear with each value in the second. You could get a bit more fancy with that hash and make another level in the hash to store the first column. Perl automatically takes care of the details for you (and we have extended examples of this in Intermediate Perl.
First, accumulate the data in the hash:
use v5.22
my %Seen;
while( <<>> ) {
chomp;
my( $first, $second ) = split;
$Seen{$second}{$first}++;
}
Once you have the hash, you move on to the second step of reporting the data. All the values of the second column are the top level keys for the hash. With that key, get the second level of the hash, and get those keys, which are the first column:
foreach my $second ( keys %Seen ) {
my #firsts = keys %{ $Seen{$second} };
say "$second found in #firsts";
}
With v5.24's postfix dereferencing, that's slightly cleaner since the dereference reads left to right rather than inside out:
use v5.24;
foreach my $second ( keys %Seen ) {
my #firsts = keys $Seen{$second}->%*;
say "$second found in #firsts";
}
And, since the hash keys in the second level only appear once per value, you don't have duplicates.