Create a hash. The keys will be the reaction numbers. The values will be hash references with two keys: 'reactants' and 'products'. The values will be hash references again, use reactants and products respectively as keys. The values can stay undefined. After the hashes are filled, just go over the keys and print out all the tuples in nested loops.

Dear monk,

thank you for your reply.

can you please code this in perl.I am not a perfectionist programmer in perl.Please help me out.

waiting for your reply.

What is your problem ?

1. I can't open a file. open
2. I can't read lines in a file. perlop I/O Operators
3. I don't know what to do with a line. split
4. I don't know how to keep values somewhere. perldata perllol

Feel free to contact me to arrange payment...

#!/usr/bin/env perl
use Modern::Perl;

my @p;

open my $fd2, '<', 'INPUT_FILE2.TXT' or die $!;
while(<$fd2>){
    next if /#/ or ! /\w/;
    my( $p, $n ) = split;
    push @{$p[$n]}, $p;
}
close $fd2;

open my $fd1, '<', 'INPUT_FILE1.TXT' or die $!;
while(<$fd1>){
    next if /#/ or ! /\w/;
    my( $r, $n ) = split;
    say "reactant: $r, product: $_" for @{$p[$n]};
}
close $fd1;
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Aaron B.
My Woefully Neglected Blog, where I occasionally mention Perl.

Just create a HashOfArrays, with reaction number as key and an array of products as data. Fill it with the data from File2.txt. Then read in the File1.txt and for every reactant check the hash for the array of products it can be transformed into.

If I am reading your question correctly, you wish to join two files where they have common values in the second column (see join for the unix command line equivalent).