raulgh5 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have a doubt on Hash variable in Perl. In script, I see the below hash defined: 
my %_abc;

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and in some subroutines, I see this: 
$_abc{"info"}->{$env}->{"Generic"}->{"Cat A"}=10;
$_abc{"warning"}->{$env}->{"Generic"}->{"Cat B"}=20;
......

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As the first variable is a hash, I suppose the $_abc are values in hash, but what is the use of -> here? Can someone explain to me the meaning of "->" here? My understanding is the elements of a hash are key/value pairs, thus, why we have 3 "->" here?

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Re: doubts on perl Hash
by NetWallah (Canon) on Oct 12, 2015 at 05:37 UTC
    What you are dealing with is a structure that looks something like this: 
    my %_abc=( 
   info => { env=> 
               {Generic =>
                  {
                     "Cat A" => "Value A",
                     "Cat B" => "Value B"
                   }
               }
            }
);

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    Your code attempts to create or replace "Value A", and "Value B".

    Your ideas are correct .. One individual KEY has a single VALUE.

    However, that value can be a REFERENCE, which can contain an entire structure.

            Software efficiency halves every 18 months, thus compensating for Moore's Law.

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Re: doubts on perl Hash
by kcott (Archbishop) on Oct 12, 2015 at 05:24 UTC

    G'day raulgh5,

    Welcome to the Monastery.

    The arrows after the $_abc{"info"} part are not wrong; however, they are not actually required. See perlreftut: Arrow Rule.

    Please post your code (and data) within <code>...</code> tags. What you have here is very difficult to read!

    — Ken

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Re: doubts on perl Hash
by AppleFritter (Vicar) on Oct 12, 2015 at 09:21 UTC

    In addition to what's been said above, I'd like to throw some more links at you that may be useful in this and future scenarios:

    • perlop: The Arrow Operator
    • perldsc - Perl Data Structures Cookbook (introduction to/tutorial for nested data structures)
    • Data::Dumper - useful for debugging, to find out what structure your data has. For instance, in your case, you could do this:

      use feature qw/say/;
use Data::Dumper;

# ...

say Dumper \%hash;

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      ...and you'd immediately get a pretty good idea of what kinda structure %hash actually contains.

    Hope these help!

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Re: doubts on perl Hash
by Anonymous Monk on Jul 07, 2017 at 09:05 UTC 
    use strict;
use warnings;
use feature qw( say );

my @aa = ("A", "C", "D", "E", "F", "G", "H", "I", "K", "L", "M", "N", 
+"P", "Q", "R", "S", "T", "V", "W", "Y");

my $pair;
for my $n1 (@aa) {
   for my $n2 (@aa) {
     $pair = $n1 . $n2;    
               
   }
}

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    these are the 20 amino acids and i have to make 400 possible combination of two amino acid (di amino acid) like this: 
    AA AC AD ...... CA CC CD CE CF CG.....YA.....YY

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    I TRIED TO MODIFY THE CODE but the scope of "for" loop diminishes when it ends. why it is ending there only, since i made the variable $pair as global only giving two Y's as output
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      The code shown has no output: it simply overwrites the value of $pair with a new value on each iteration of the nested loops. So, if you add the statement say $pair; after both loops have finished, of course you see only the last value of $pair, which happens to be YY.

      To get all the values, you need to move the say statement into the inner loop:

      use strict;
use warnings;
use feature qw( say );

my @aa = qw(A C D E F G H I K L M N P Q R S T V W Y);
my $pair;

for my $n1 (@aa)
{
    for my $n2 (@aa)
    {
        $pair = $n1 . $n2;    
        say $pair;
    }
}

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      As an alternative, you could store the pairs in an array and output them after the loops have finished:

      use strict;
use warnings;
use feature qw( say );

my @aa = qw(A C D E F G H I K L M N P Q R S T V W Y);
my @pairs;

for my $n1 (@aa)
{
    for my $n2 (@aa)
    {
        push @pairs, $n1 . $n2;
    }
}

say for @pairs;

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      Hope that helps,

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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