Re: Figuring out some data structure

What's happening is "autovivification" of an array reference.

A quote from the book:
Programming Perl, 3rd Ed., p.251
Chapter 8: References
"Implicit Creation of References"

References of an appropriate type simply spring into existence if you dereference them in an lvalue context that assumes they exist.

So your statement:

    push @{$labels{$tablename}}, $_;
[download]

is the same as:

    $labels{$tablename} = [] if !exists $labels{$tablename};
    push @{$labels{$tablename}}, $_;
[download]

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Re^2: Figuring out some data structure by ikegami (Patriarch) on Nov 01, 2009 at 18:29 UTC
Small nit: It's more like `$labels{$tablename} = [] if !defined $labels{$tablename}; push @{$labels{$tablename}}, $_;` [download]	[reply] [d/l]
Re^3: Figuring out some data structure by gmargo (Hermit) on Nov 01, 2009 at 20:32 UTC
I stand corrected. If the `$labels{$tablename}` already exists but has an undefined value, then that "undef" is replaced with a reference to new anonymous array. I wish this was documented better.	[reply] [d/l]