pvighneshmufc:

He meant that the following lines are inside of a loop, like this:

# read file1 into a %hash
... code to do that here ...

# inside a loop
while (my $line = <$file2>) {
    # read file2 line by line
    ... this was done in the loop condition above ...

    # split the $line to @fields at |

    # if the first $fields[0] exists in the %hash,
        # print the whole $line to file 3
}
[download]

This is a relatively common question, so LanX gave you the outline of a good solution to the problem.

A frequent mistake is to try to read *both* files inside the loop, giving one of two bad outcomes:

• Either the first file is completely read in the first pass of the loop, so the code can only find a single match if it happens to be the first line in the second file, or
• the code re-opens the first file each time, and therefore can find all the matches, but runs extremely slowly⁽¹⁾ because it reads the first file completely for each line in the second file.

(1) Extremely slowly in the relative sense--for small files you may not notice it. But if your files get large enough, you'll wonder why such a fast computer is so freakin' slow.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

The file has around 7 lakh lines so yeah :p

Perl read file line by line led me to

http://perlmaven.com/open-and-read-from-files

The first code example shows it already, though $line is named $row here.

I'm not going to give you complete code, because I want to help you to learn Perl. :)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Wikisyntax for the Monastery}

Thanks a lot , suggest me some tutorials to start with , i am very new to this so suggest like that :p

See Files and I/O in perlintro.

You have at least one bug of forgetting sigil $ in line once, but yes this was the basic idea.

And dumping inside the loop is costly.

Furthrmore you might want to use <code> tags next time. :)

Minor nitpick: when setting value 1, you don't need exists anymore. :)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Wikisyntax for the Monastery}

It is me who posted anonymously since we aren't allowed to log in at the university.
I didn't get where I missed "sigil $ in line once".
Furthermore, i am getting errors such as Use of uninitialized value in line 34 at variables $row and $fields at the penultimate line.
So when i place the  print $row if exists $hash{$fields[0]}; after the loop it gives an error "Perl requires explicit package name"
i am really a novice at this if you could guide me in this I would learn more.

use strict; <br>
use warnings; <br>
use Data::Dumper; <br>

my $file1 = 'file1';<br>
my $file2 = 'file2'; <br>
#reading file1 into a hash<br>

my %hash;<br>

open (my $fh,'<',$file2) or die $!;<br>
while(my $line=<$fh>)<br>
{<br>
chomp $line;<br>
$hash{$line}=1;<br>

print Dumper %hash;<br>
}<br>
close $fh;<br>


#reading file2 line by line
<br>
open (my ($fh2),'<',$file1) or die $!;
<br>
while (my ($row) = <$fh2>) {<br>
  chomp $row;<br>
# next if $row =~ /^\s*$/;<br>
my (@fields) = split(/\|/, $row);<br>
print $row if exists $hash{$fields[0]};<br>
}<br>

close $fh2;<br>
[download]


I aint getting any O/P plus when i print Dumper %hash i get loads of $VAR
What am i doing wrong?

#!/nairvigv/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $file1 = 'BBIDs.txt';
my $file2 = 'fixedincomeTransparency.out.px.derived.updates';
#reading file1 into a hash

my %hash;
my @fields;
open (my $fh,'<',$file1) or die $!;
while(my $line=<$fh>)
{
chomp $line;
 next if $line =~ /^\s*$/;
 $hash{$line}=1;
}
#print Dumper(\%hash);

close $fh;



open (my ($fh2),$file2) or die $!;

while (my $row = <$fh2>)
 {
         chomp $row;

       print "$row\n";
         next if $row =~ /^\s*$/;
        my (@fields) = split(/\|/, $row);



print "$fields[0]\n ";




if (exists $hash{$fields[0]})
{
print "$row\n";
}
}


close $fh2;
~
[download]

Due to broken indentation it's hard to read and I won't go into details.

Have a look at http://perl.plover.com/FAQs/Buffering.html and basic debugging checklist

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Wikisyntax for the Monastery}

Sorry would indentation next time onwards. Basic what's happening is the big files have whitespace before and after each entry So I have to remove those while I store them to a key .

open (my ($fh2),$file2) or die $!;

while (my $row = <$fh2>)
 {
         chomp $row;

       print "$row\n";
         next if $row =~ /^\s*$/;
        my (@fields) = split(/\|/, $row);
[download]

so i have to add $row =~ s/\s*$// after  next if $row =~ /^\s*$/; and it will work? i cant really check from here since i am at home and would have to wait another 12 hours