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

Replies are listed 'Best First'.
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]


go ahead... be a heretic
	PerlMonks