Autovivification Confusion

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

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Re: Autovivification Confusion by toolic (Bishop) on Aug 05, 2011 at 18:27 UTC
As perl tells you, it is a syntax error. Try this instead: `my %count = (abc => 0);` [download] See perldata for ways to initialize a hash.	[reply] [d/l]
Re: Autovivification Confusion by ikegami (Patriarch) on Aug 05, 2011 at 18:31 UTC
The error you're getting is not because `%count` doesn't exist. `$ perl -e'my $count{abc};' syntax error at -e line 1, near "$count{abc" Execution of -e aborted due to compilation errors. $ perl -e'my %count; my $count{abc};' syntax error at -e line 1, near "$count{abc" Execution of -e aborted due to compilation errors.` [download] Therefore, the issue isn't a lack of autovivification. If it somehow made sense to do `my $count{'abc'}`, you might be right that `%count` should be autovivified. What you are trying to achieve can be done using `my %count = ( abc => 0 );` [download]	[reply] [d/l] [select]
Re: Autovivification Confusion by Khen1950fx (Canon) on Aug 05, 2011 at 19:04 UTC
Your first example is a hash slice but no hash was declared. In your second example, you declared the hash, but that's not autovivification. Autovivification is the automatic creation of a variable reference when an undefined value is dereferenced. Maybe this example will help: `#!/usr/bin/perl use strict; use warnings; use Data::Dumper::Concise; my $hash = { 'foo' => { 'abc' => 0, }, 'bar' => { 'def' => 1, }, }; print Dumper($hash) unless exists $hash->{'baz'}{'ghi'},` [download] 'baz' doesn't exist, so perl creates it---autovivification.	[reply] [d/l]
Re^2: Autovivification Confusion by ikegami (Patriarch) on Aug 05, 2011 at 19:32 UTC
'baz' doesn't exist, so perl creates it---autovivification. In the docs, autovivification refers to the creation of `%{ $hash->{'baz'} }`, but it's definitely acceptable to consider the creation of `$hash->{'baz'}` autovivification as well. Both happen here.	[reply] [d/l] [select]
Re: Autovivification Confusion by Anonymous Monk on Aug 05, 2011 at 18:47 UTC
But shouldn't Perl have interpreted this `my $count{'abc'};` [download] as `my %count = ( 'abc' => undef );` [download] This is how I understand how autovivification works.	[reply] [d/l] [select]
Re^2: Autovivification Confusion by ikegami (Patriarch) on Aug 05, 2011 at 18:49 UTC
Yes, if `my $count{'abc'};` made sense, that could be an expected result. He had `my $count{'abc'} = 0;`, though, which is why he's expecting a different result. This is how I understand how autovivification works. autovivification is the automatic creation of variables when they are needed. No part of the OP's file contains code that uses `%count` (just `$count`), so it's never needed, so autovivification has nothing to do with the OP's problem.	[reply] [d/l] [select]