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Why does testing for the existence of hash keys create the keys ?

by fshrewsb (Acolyte)
on Oct 08, 2012 at 08:00 UTC ( #997763=perlquestion: print w/replies, xml ) Need Help??
fshrewsb has asked for the wisdom of the Perl Monks concerning the following question:

This is very simple, but it happens often and I have to write extra code to handle it :

use Data::Dumper; my $hashref; print Dumper $hashref; if(exists $hashref->{a}->{b}) { # Do something } print Dumper($hashref);

Of course, at the first print $hashref is undef, but after the exists statement $hashref is now: { 'a' => {} } Why ?

Replies are listed 'Best First'.
Re: I just want to understand why ...
by Athanasius (Chancellor) on Oct 08, 2012 at 08:18 UTC

    From exists:

    Although the most deeply nested array or hash element will not spring into existence just because its existence was tested, any intervening ones will.

    This is called autovivification. There is a good explanation of autovivification at

    Hope that helps,

    Athanasius <°(((><contra mundum

Re: I just want to understand why ...
by 2teez (Vicar) on Oct 08, 2012 at 08:19 UTC
Re: I just want to understand why ...
by Anonymous Monk on Oct 08, 2012 at 08:35 UTC

      And then there's the third way which requires some extra typing, but is fine for two-level or three-level hashes:

      my $r = $hashref; # better to use a short variable name here if($r->{a} and $r->{a}->{b} and exists $r->{a}->{b}->{c}) { # Do something }
      No autovivification.


      18:58 >perl -w -Mstrict -MData::Dumper -E "my $foo; say Dumper($foo); +if (exists $foo->{bar}{anything}{you}{want} ) { say 'hi' } say Dumper +($foo);" $VAR1 = undef; $VAR1 = { 'bar' => { 'anything' => { 'you' => {} } } }; 19:00 >

      Looks like 3 levels of autovivification to me.


      Update: See below.

      Athanasius <°(((><contra mundum

        And where is no autovivification; in your code?
        لսႽ ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
Re: I just want to understand why ...
by parv (Priest) on Oct 09, 2012 at 07:47 UTC
    Could you change the title from being useless to "Understanding creation of missing keys" or similar?

      Point taken about the title ! Thanks all. I had not encountered the term "autovivification" before.

      no autovivification qw(exists);

      should solve my problems without creating new ones !

        Much appreciate the changed title.

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