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

I want to do a check like 

if (exists $h{A} and exists $h{A}{B} and exists $h{A}{B}{C}) {

}

[download]

Is there a way to do this more compactly? (Check for existence of a key without auto-vivifying the path?)

Replies are listed 'Best First'.
Re: recursive exists in perl
by ikegami (Patriarch) on Dec 09, 2010 at 23:02 UTC
[reply]
[d/l]

      Is that really the case? For example, while the following doesn't autovivify $h{A}{B}{C} it does autovivify $h{A}{B}.

      use Data::Dumper;

my %h;

if ( exists( $h{A}{B}{C} ) ) {
    print "Hi mom!\n";
}

print Dumper \%h;

      [download]
      $VAR1 = {
          'A' => {
                   'B' => {}
                 }
        };
[reply]
[d/l]
        use Data::Dumper;

my %h;
no autovivification;
if ( exists( $h{A}{B}{C} ) ) {
    print "Hi mom!\n";
}

print Dumper \%h;
__END__

$VAR1 = {};

        [download]
[reply]
[d/l]
Re: recursive exists in perl
by moritz (Cardinal) on Dec 09, 2010 at 22:20 UTC
    There are modules to do that, for example Data::Diver, or modules that disable autovivification.

    There was also a proposal (and patch) that adds an "exists-or" operator \\ to perl, to parallel the "or" || and "defined-or" // operator. I don't know if it got accepted in the end.

    Perl 6 - second systems done right
[reply]
[d/l]
[select]
Re: recursive exists in perl
by Anonymous Monk on Dec 09, 2010 at 22:49 UTC
    I'm not sure whether you'd consider this more compact, but it does make it easier to see the path of keys you're checking: 
    use List::Util 'reduce';
if (reduce { $a && exists $$a{$b} ? $$a{$b} : undef } 
    \%foo, 'bar', 'baz', 'troz')
{
}

    [download]
[reply]
[d/l]
Re: recursive exists in perl
by JavaFan (Canon) on Dec 09, 2010 at 22:16 UTC
    Short of using a subroutine that does the same thing, no.
[reply]

Back to Seekers of Perl Wisdom