Re: Entering the land of Perl

[choroba]

Hello manbroski, welcome to the Monastery!
Prototypes are seldom used in Perl. It is usually enough to pass a reference as the argument directly:

sort_and_print_hash_keys(\%letters);

Moreover, counting characters in a string is idiomatically done via the tr operator. We know in advance that the possible characters are A, C, T and G only, so you can write:

#!/usr/bin/perl
use strict;
use warnings;
use feature qw(say);

my $s = shift;
say join " ", map eval "$s =~ tr/$_//", qw/A C G T/;

Which was exactly my solution to the DNA problem on Rosalind :-)

[manbroski]

That, sir, is a beauty.

Anything functional (in the map-reduce sense) makes me quite happy. You have my thanks.

EDIT: what version is this? 5.16 'use strict' returns all zeros for each count. Does that provide the function return code?

[choroba]

The shift requires the string as an argument of the script. If you want to read it from a file, you have to use

$s = <>;

instead and provide the input file as the argument.