Re^5: Keys() required to autovivify?

A hash can't have a key without a value (that's why I refer to key/value pairs). If you reference $h{x} in some fashion (except as a target for exists) then a x/undef pair is created (autovivified) for %h if it didn't exist already. The undef returned by $h{x} is the value associated with the key 'x'.

"Adding" an undef value to %h when $h{x} is used is exactly what autovivification does.

Perl is environmentally friendly - it saves trees

Comment on Re^5: Keys() required to autovivify?

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Re^6: Keys() required to autovivify? by jrw (Monk) on Dec 30, 2007 at 23:59 UTC
Autovivification (in the case of a hash) is when a key is added to a hash where previously that key wasn't there. Just using $h{x}, as in $val = $h{x} does not add a new key to the hash. It simply assigns the default undef value to $val. Referencing through a key does autovivify the all but the last key: `#!/usr/bin/perl my %h; my $val = $h{x}{y}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = { 'x' => {} };` [download] Apparently, not only does $h{x} not autovivify $h{x}, but even %{$h{x}} does not autovivify $h{x} (you have to turn off strict and -w first): `#!/usr/bin/perl my %h; %{$h{x}}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = {};` [download] But, using keys() causes %{$h[x}} to autovivify $h{x}. `#!/usr/bin/perl -w use strict; my %h; keys %{$h{x}}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = { 'x' => {} };` [download]	[reply] [d/l] [select]

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Re^6: Keys() required to autovivify?
by jrw (Monk) on Dec 30, 2007 at 23:59 UTC

not

through

does

#!/usr/bin/perl
my %h;
my $val = $h{x}{y};
use Data::Dumper;
print Dumper \%h;
[download]

$VAR1 = {
          'x' => {}
        };
[download]

not

#!/usr/bin/perl
my %h;
%{$h{x}};
use Data::Dumper;
print Dumper \%h;
[download]

$VAR1 = {};
[download]

#!/usr/bin/perl -w 
use strict;
my %h;
keys %{$h{x}};
use Data::Dumper;
print Dumper \%h;
[download]

$VAR1 = {
          'x' => {}
        };
[download]

[reply]
[d/l]
[select]