Accessing uninitialized arrays. Black magic

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

Hello monks,

can someone explain to me why this bit of code works and also if it is legal to code like this:

my @array_rev=();
@{$array_rev[1]}=(1,2,3);
print "@{$array_rev[1]}";
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This code prints 1 2 3. But I don't understand how the second line can work since the array is uninitialized but @{} expects some valid reference!

Thank you :)

Comment on Accessing uninitialized arrays. Black magic Download Code

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Re: Accessing uninitialized arrays. Black magic
by Athanasius (Archbishop) on May 27, 2013 at 02:32 UTC

Yes, it’s perfectly legal: it’s a special Perl feature called autovivification that brings array or hash elements into existence when they’re referenced. See Autovivification and Autovivification in Perl: An In-Depth Tutorial.

Hope that helps,

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

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Re: Accessing uninitialized arrays. Black magic
by 2teez (Vicar) on May 27, 2013 at 04:28 UTC

Hi Anonymous Monk,
Just a reminder, that the array @array_rev has undef as it's first element, since array index start from "0". So, what you are doing with this @{$array_rev[1]}=(1,2,3); is to assign an array reference to the second element of the array @array_rev.
Using Data::Dumper like so can give some insight:

use Data::Dumper;

my @array_rev;
@{$array_rev[1]}=(1,2,3);
print Dumper \@array_rev;
__END__
$VAR1 = [
          undef,
          [
            1,
            2,
            3
          ]
        ];
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If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

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