Defining a hash value using another hash value.

Question

Is there a way to do the following while using only one data structure?

my %hash = (
  "key1" => "value1",
  "key2" => "value2",
  "key3" => $hash{key1},
);

So basically, I want to set a hash's key value to another key value. I've tried the above syntax but get the following warning:

Global symbol "%hash" requires explicit package name at ...

I'm assuming this is because I'm trying to reference the hash before it's actually been created.

Answer 1

Why not make your own function:

use strict;
use warnings;

sub make_hash (@) { 
    my %h;
    my @unresolved;
    while ( @_ ) {
        my ( $key, $value ) = splice( @_, 0, 2 );
        next unless defined $value;
        if (   not ref( $value ) 
           and my ( $ref ) = $value =~ /^ref:\s*(.*\S)\s*$/ 
           ) {
            if ( defined( my $v = $h{ $ref } )) {
                $h{ $key } = $v;
            }
            else {
                push @unresolved, [ $key, $ref ];
            }
        }
        else {
            $value =~ s/^lit://;
            $h{ $key } = $value;
        }
    }
    $h{ $_->[0] } = $h{ $_->[1] } foreach grep { exists $h{ $_->[0] }; } @unresolved;
    return wantarray ? %h : \%h;
} 

To demonstrate some of the power:

my %hash 
    = make_hash(
      'key1' => 'value1'
    , 'key2' => 'value2'
    , 'key3' => 'ref:key1'
    , 'key4' => 'lit:ref:key2'
    , 'key5' => 'lit:lit:ref:key3'
    );

The lit: prefix covers the case of "What if I really wanted to pass a value that is a non-reference as 'ref:so-and-so'? It also is recursive in answering, "What if I direly need to make a value 'lit:xzy'.

I've done this and I've also blessed a reference to a passed piece of data to a Lit class or something along those lines.

sub Ref ($) { bless \shift, 'Ref' }

And then in the make_hash routine you'd just check for ref( $value ) eq 'Ref'. And specify it like the following:

my %hash 
    = make_hash(
      'key1' => 'value1'
    , 'key2' => 'value2'
    , 'key3' => Ref 'key1'
    );

There are many ways to make Perl act like you wish it did.