defined of a matrix

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

Hi,
I was a little surprised that

my @test = ( 0 );
if ( defined @{ $test[1] }[1] ){}
my $l = @test;
print $l;
[download]

produced a "2".
Sure it is easy to extend the if clause as

my @test = ( 0 );
if ( defined $test[1] && defined @{ $test[1] }[1] ){}
my $l = @test;
print $l;
[download]

and all is fine again, but where can I find any documentation of this behaviour? It was unexpected to me and I would like to read more about it. Neither perlref nor the perlreftut seemed to document that an element is appended to @test as soon as it is accessed.
Cheers,
Hans

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Re: defined of a matrix by pelagic (Priest) on Jun 15, 2004 at 12:43 UTC
> I would like to read more about it Search for autovivification. addon: merlyn has also written a couple of articles on that topic. pelagic	[reply]
Re^2: defined of a matrix by PerlingTheUK (Hermit) on Jun 15, 2004 at 12:55 UTC
Thanks, that makes it much clearer.	[reply]
Re: defined of a matrix by rnahi (Curate) on Jun 15, 2004 at 13:03 UTC
Perhaps your code would look better with exists rather than defined. `#!/usr/bin/perl -w use strict; my @test = ( 0 ); my $len = 0; if ( exists $test[1]->[1] ) { $len = @test; } print $len;` [download]	[reply] [d/l]
Re^2: defined of a matrix by pelagic (Priest) on Jun 15, 2004 at 13:28 UTC
this does also autovivification ... pelagic	[reply]
Re: defined of a matrix by Roy Johnson (Monsignor) on Jun 15, 2004 at 14:50 UTC
Your 2nd test would be better as `if (ref $test[1] and exists $test[1][1]) {}` [download] It occurs to me that a sub that returns the passed value iff it is a ref might provide some shorthand for this idiom: `sub iff_ref { return ref $_[0] ? $_[0] : undef; } # Then your test becomes (if exists doesn't mind the notation) if (exists iff_ref($test[1])->[1]) {} # or it might have to be if (exists ${iff_ref($test[1])}[1]) {}` [download] I don't have perl5 where I am today, so this is absolutely untested. We're not really tightening our belts, it just feels that way because we're getting fatter.	[reply] [d/l] [select]