Re: hash ref mind blow

The following might help to make things a little more clear. Hash has 2 values, which are both hash references (to a anonymous hashes { drinks => 1 } and { drinks => 2 }). When you do the shallow copy, the hash references are copied, but they still refer to the same hashes in both %hash and %copy:

use strict;
my %hash = ();
$hash{a}{drinks}=1;
$hash{b}{drinks}=2;

my $p = \%hash;
my %copy = %{ $p };

print "ref hash: ", \%hash, "\nref copy: ", \%copy, "\n";
print "values hash: ",join(", ", values %hash), "\nvalues copy: ",join
+(", ", values %copy), "\n";
[download]

Prints:

copy: ab
hash: ab
ref hash: HASH(0x82ef7d8)
ref copy: HASH(0x82ef928)
values hash: HASH(0x82e0308), HASH(0x82ef788)
values copy: HASH(0x82e0308), HASH(0x82ef788)
[download]

Note that the values are identical, and thus the values of %hash and %copy refer to the same hash references.

Comment on Re: hash ref mind blow Select or Download Code

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Re^2: hash ref mind blow by Anonymous Monk on Sep 24, 2008 at 16:51 UTC
Yup, perfectly clear! Thank you! But I've just realized that I don't how to do one thing... how can I make %hash and %copy be in the same mem location? So that when I add a new key to %copy, %hash also changes. I'm just lazy about working with a ref directly as in `$$p{c}=3;` [download] or `$p->{c}=3;` [download] I just want `$copy{c}=3;` [download] To change the value of %hash. -rod	[reply] [d/l] [select]
Re^3: hash ref mind blow by Corion (Patriarch) on Sep 24, 2008 at 16:53 UTC
For global variables, just do a glob assignment: `*copy = \%hash;` [download] then, `%copy` and `%hash` are the same data structure. Otherwise, that is, for lexical variables, use Data::Alias.	[reply] [d/l] [select]
Re^4: hash ref mind blow by ikegami (Patriarch) on Sep 24, 2008 at 21:39 UTC
Make sure to localize package variables (such as `copy`). `our %copy; local copy = \%hash;` [download] or `use Data::Alias; alias my %alias = %hash;` [download]	[reply] [d/l] [select]
Re^3: hash ref mind blow by Anonymous Monk on Sep 25, 2008 at 08:55 UTC
Just so you know, Data::Dumper is part of every perl `#!/usr/bin/perl -- use strict; my %hash = (); $hash{a}{drinks}=1; $hash{b}{drinks}=2; my $p = \%hash; my %copy = %{ $p }; use Data::Dumper; print Dumper( \%hash, \%copy, $p ); __END__ $VAR1 = { 'a' => { 'drinks' => 1 }, 'b' => { 'drinks' => 2 } }; $VAR2 = { 'a' => $VAR1->{'a'}, 'b' => $VAR1->{'b'} }; $VAR3 = $VAR1;` [download]	[reply] [d/l]
Re^4: hash ref mind blow by JadeNB (Chaplain) on Dec 28, 2009 at 00:11 UTC
Just so you know, Data::Dumper is part of every perl Every modern Perl, of course: `$ corelist Data::Dumper Data::Dumper was first released with perl 5.005` [download]	[reply] [d/l]