Perl compare individual elements of two arrays - perl

Question

I have two files with two columns each:
FILE1
A B
1 #
2 #
3 !
4 %
5 %
FILE 2
A B
3 #
4 !
2 &
1 %
5 ^
The Perl script must compare column A in both both files, and only if they are equal, column B of FIlE 2 must be printed
So far I have the following code but all I get is an infinite loop with # from column B
use strict;
use warnings;
use 5.010;
print "enter site:"."\n";
chomp(my $s = <>);
print "enter protein:"."\n";
chomp(my $p = <>);
open( FILE, "< $s" ) or die;
open( OUT, "> PSP.txt" ) or die;
open( FILE2, "< $p" ) or die;
my #firstcol;
my #secondcol;
my #thirdcol;
while ( <FILE> )
{
next if $. <2;
chomp;
my #cols = split;
push #firstcol, $cols[0];
push #secondcol, $cols[1]."\t"."\t".$cols[3]."\t"."\t"."\t"."N\/A"."\n";
}
my #firstcol2;
my #secondcol2;
my #thirdcol2;
while ( <FILE2> )
{
next if $. <2;
my #cols2 = split(/\t/, $_);
push #firstcol2, $cols2[0];
push #secondcol2, $cols2[4]."\n";
}
my $size = #firstcol;
my $size2 = #firstcol2;
for (my $i = 0; $i <= #firstcol ; $i++) {
for (my $j = 0; $j <= #firstcol2; $j++) {
if ( $firstcol[$i] eq $firstcol2[$j] )
{
print $secondcol2[$i];
}
}
}

Answer 1

my (#first, #second);
while(<first>){
chomp;
my $foo = split / /, $_;
push #first , $foo;
}
while(<second>){
chomp;
my $bar = split / / , $_;
push #second, $bar;
}
my %first = #first;
my %second = #second;
Build a hash of the first file as %first and second file as %second with first column as key and second column as value.
for(keys %first)
{
print $second{$_} if exists $second{$_}
}
I couldn't check it as I am on mobile. hope that gives you an idea.

Answer 2

I assume that column A is ordered and that you actually want to compare the first entry in File 1 to the first entry in File 2, and so on.
If that's true, you have nested loop that you don't need. Simplify your last while as such:
for my $i (0..$#firstcol) {
if ( $firstcol[$i] eq $firstcol2[$i] )
{
print $secondcol2[$i];
}
}
Also, if you're at all concerned about the files being of different length, then you can adjust the loop:
use List::Util qw(min);
for my $i (0..min($#firstcol, $#firstcol2)) {
Additional Note: You aren't chomping your data in the second file loop while ( <FILE2> ). That might introduce a bug later.

Answer 3

If your files are called file1.txt and file2.txt the next:
use Modern::Perl;
use Path::Class;
my $files;
#{$files->{$_}} = map { [split /\s+/] } grep { !/^\s*$/ } file("file$_.txt")->slurp for (1..2);
for my $line1 (#{$files->{1}}) {
my $line2 = shift #{$files->{2}};
say $line2->[1] if ($line1->[0] eq $line2->[0]);
}
prints:
B
^
equals in column1 only the lines A and 5
without the CPAN modules - produces the same result
use strict;
use warnings;
my $files;
#{$files->{$_}} = map { [split /\s+/] } grep { !/^\s*$/ } do { local(#ARGV)="file$_.txt";<> } for (1..2);
for my $line1 (#{$files->{1}}) {
my $line2 = shift #{$files->{2}};
print $line2->[1],"\n" if ($line1->[0] eq $line2->[0]);
}