How to count the number of keys that exist in a hash? - arrays

Question

I am working with an input file that contains tab delimitated sequences. Groups of sequences are separated by line breaks. The file looks like:
TAGC TAGC TAGC HELP
TAGC TAGC TAGC
TAGC HELP
TAGC
Here is the code I have:
use strict;
use warnings;
open(INFILE, "<", "/path/to/infile.txt") or die $!;
my %hash = (
TAGC => 'THIS_EXISTS',
GCTA => 'THIS_DOESNT_EXIST',
);
while (my $line = <INFILE>){
chomp $line;
my $hash;
my #elements = split "\t", $line;
open my $out, '>', "/path/to/outfile.txt" or die $!;
foreach my $sequence(#elements){
if (exists $hash{$sequence}){
print $out ">$sequence\n$hash{$sequence}\n";
}
else
}
$count++;
print "Doesn't exist ", $count, "\n";
}
}
}
How can I tell how many sequences exist before I print? I need to put that information into the name of the output file.
Ideally, I would have a variable that I could include in the name of the file. Unfortunately, I can't just take the scalar of #elements because there are some sequences that won't get printed out. When I try to push the keys that exist into an array and then print the scalar of that array, I still don't get the results I need. Here is what I tried (all variables that need to be global are):
open my $out, '>', "/path/to/file.$number.txt" or die $!;
foreach my $sequence(#elements){
if (exists $hash{$sequence}){
push(#Array, $hash{$sequence}, "\n");
my $number = #Array;
print $out ">$sequence\n$hash{$sequence}\n";
#....
Thanks for the help. Really appreciate it.

Answer 1

my $sequences = grep exists $hash{$_}, #elements;
open my $out, '>', "/path/to/outfile_containing_$sequences.txt" or die $!;
In list context, grep filters a list by a criterion; in scalar context, it returns a count of elements that met the criterion.

Answer 2

The easiest way would be to keep track of how many keys you are printing in a variable and once your loop finish, just rename the file with the number you calculated. Perl comes with a built-in function to do this. The code would be something like this:
use strict;
use warnings;
open(INFILE, "<", "/path/to/infile.txt") or die $!;
my %hash = (
TAGC => 'THIS_EXISTS',
GCTA => 'THIS_DOESNT_EXIST',
);
my $ammt;
while (my $line = <INFILE>){
chomp $line;
my $hash;
my #elements = split "\t", $line;
open my $out, '>', "/path/to/outfile.txt" or die $!;
foreach my $sequence(#elements){
if (exists $hash{$sequence}){
print $out ">$sequence\n$hash{$sequence}\n";
$ammt++;
}
else
}
print "Doesn't exist ", $count, "\n";
}
}
}
rename "/path/to/outfile.txt", "/path/to/outfile${ammt}.txt" or die $!;
I removed the $count variable, since it's not declared in your code (strict would complain about that). Here's the official doc for rename. Since it returns True or False, you can check that it was successful or not.
By the way, be aware that:
push(#Array, $hash{$sequence}, "\n");
is storing two items ($hash{$sequence} and \n), so that count would be twice as it should be.