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

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

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Re: I just want to understand why ... by Athanasius (Archbishop) 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 http://sysarch.com/Perl/autoviv.txt. Hope that helps, Athanasius <°(((>< contra mundum	[reply]
Re: I just want to understand why ... by 2teez (Vicar) on Oct 08, 2012 at 08:19 UTC
... Why? Autovivification check this Understand autovivification If you tell me, I'll forget. If you show me, I'll remember. if you involve me, I'll understand. --- Author unknown to me	[reply]
Re: I just want to understand why ... by Anonymous Monk on Oct 08, 2012 at 08:35 UTC
From the other day see Re: How do you check if a key exists without creating it ? no autovivification; `if( exists $foo->{bar}{anything}{you}{want} ){ }` [download] Or use Data::Diver 'Dive'; `if( my $want = Dive( $foo, qw/ bar anything you want / ) ){ ... }` [download]	[reply] [d/l] [select]
Re^2: I just want to understand why ... by Athanasius (Archbishop) on Oct 08, 2012 at 09:15 UTC
No autovivification. Huh? `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 >` [download] Looks like 3 levels of autovivification to me. ??? Update: See below. Athanasius <°(((>< contra mundum	[reply] [d/l]
Re^3: I just want to understand why ... by choroba (Cardinal) on Oct 08, 2012 at 09:17 UTC
And where is `no autovivification;` in your code? لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^4: I just want to understand why ... by Athanasius (Archbishop) on Oct 08, 2012 at 09:24 UTC
Re^2: I just want to understand why ... by Anonymous Monk on Oct 08, 2012 at 08:53 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 }` [download]	[reply] [d/l]
Re: I just want to understand why ... by parv (Parson) on Oct 09, 2012 at 07:47 UTC
Could you change the title from being useless to "Understanding creation of missing keys" or similar?	[reply]
Re^2: I just want to understand why ... by fshrewsb (Acolyte) on Oct 09, 2012 at 10:15 UTC
Point taken about the title ! Thanks all. I had not encountered the term "autovivification" before. `no autovivification qw(exists);` [download] should solve my problems without creating new ones !	[reply] [d/l]
Re^3: I just want to understand why ... by parv (Parson) on Oct 10, 2012 at 07:17 UTC
Much appreciate the changed title.	[reply]


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