autovivication gonna make me lose my mind

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

Apparently, I have been doing something wrong for years, or I missed a change to Perl recently. I saw this code in an answer today:

#!/usr/local/bin/perl

use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent   = 1;

my @array1 = qw{rose orange green};
my @array2 = qw{blue red};

my %hash;
push @{$hash{first}}, \@array1, \@array2;
print Dumper \%hash;
[download]

I was under the impression that this code will not run. Specifically, because of the array deref. $hash{first} should return undef, and I did not think you could dereference an undefined value and magically get an array reference, but the code runs. I had always used:

$hash{first} ||= [];
push @{ $hash{first} },'foo';
[download]

I am sure (in my own mind, at least) that I would get run-time errors if I left out the initialization.
Can someone enlighten me as to where my cognitive breakdown lies?

-pete
"Worry is like a rocking chair. It gives you something to do, but it doesn't get you anywhere."

Comment on autovivication gonna make me lose my mind Select or Download Code

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Re: autovivication gonna make me lose my mind by kyle (Abbot) on Sep 16, 2008 at 15:52 UTC
If you're dereferencing an element as an lvalue (i.e., in a kind of assignment), it will autovivify. If you're dereferencing otherwise, it will fail. For example: `$ perl -le 'use strict;my %h;push @{$h{first}}, "hello";print $h{first +}' ARRAY(0x8153d78) $ perl -e 'use strict;my %h;print @{$h{first}}, "hello";' Can't use an undefined value as an ARRAY reference at -e line 1.` [download]	[reply] [d/l]
Re^2: autovivication gonna make me lose my mind by dreadpiratepeter (Priest) on Sep 16, 2008 at 16:06 UTC
A ha, that's my disconnect. I got the error for an rvalue at some point and drew the wrong conclusion (I generalized it to lvalues too). So I have cargo-culted myself for the last few years. Time to simplify some code. Thank you so much for dispelling my confusion -pete "Worry is like a rocking chair. It gives you something to do, but it doesn't get you anywhere."	[reply]
Re^2: autovivication gonna make me lose my mind by ig (Vicar) on Nov 04, 2008 at 14:06 UTC
I think autovivification can happen in contexts other than dereferencing an element as an lvalue. Consider the following script. `#!/usr/bin/perl use strict; use warnings; use Data::Dumper; my $hashref; print "exists\n" if(exists($hashref->{foo}->{bar})); print Dumper($hashref);` [download] It prints `$VAR1 = { 'foo' => {} };` [download] So, unless the argument to exists is dereferenced as an lvalue (I don't know how it is implemented) there are other contexts. Explaining Autovivication, Accidentally creating hash elements with grep { defined } on a hash slice and The Bad, the Ugly, and the Good of autovivification have more info.	[reply] [d/l] [select]
Re: autovivication gonna make me lose my mind by Fletch (Bishop) on Sep 16, 2008 at 15:42 UTC
Dereferencing an undefined value to (magically) get an appropriate reference is kind of the definition of autovivification; wherein lies the confusion? The cake is a lie. The cake is a lie. The cake is a lie.	[reply]
Re: autovivication gonna make me lose my mind by tilly (Archbishop) on Sep 16, 2008 at 16:51 UTC
As far as I know, Perl's autovivification behaviour last significantly changed in Perl 5.004. So Perl has had its current behaviour for a decade or so. However if you use Perl and any other language, then your belief was probably a carryover from other languages. Autovivification is a fairly rare feature, for instance it is not found in any of Ruby, Python, JavaScript or Java.	[reply]