in reply to Test if a subhash in a referenced hash exists

if (not exists $coordinates->{$group}{$id}{$stage}{"coords"}) { ...}

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(untested)

Perl 6 - links to (nearly) everything that is Perl 6.

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Re^2: Test if a subhash in a referenced hash exists
by DrHyde (Prior) on May 28, 2010 at 10:43 UTC
    If $group, $id, $stage etc don't exist in $coordinates, then that will auto-vivify them. You need to check the existence of each level in the hash if magically creating them merely by looking at them is a problem for your application. Something like this (ignoring your application's logic for the sake of making my example code clearer): 
    if(exists($coordinates->{$group})) {
  if(exists($coordinates->{$group}->{$id})) {
    if(exists($coordinates->{$group}->{$id}->{$stage})) {
      ...
    }
  }
}

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    or to generalise (untested. haven't even tried to compile it) ... 
    my $result = do_stuff_in_hash_without_autovivifying(
  sub {
    my $hash_to_work_on = shift;
    # do stuff here
  },
  $coordinates,                 # initial hash
  $group, $id, $stage, 'coords' # list of keys to traverse
);

sub do_stuff_in_hash_without_autovivifying {
  my($sub, $hash, @keys) = @_;
  if(@keys && exists($hash->{$keys[0]}) {
    return do_stuff_in_hash_without_autovivifying(
      $sub,
      $hash->{$keys[0]},
      @keys[1 .. $#keys]
    );
  } else {
    return $sub->($hash);
  }
}

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    If you're *really* paranoid about auto-vivification, then you can subvert Tie::Hash::Vivify to turn it into a fatal error instead of silent data corruption: 
    use Tie::Hash::Vivify;
use Data::Dumper;

my $hash = Tie::Hash::Vivify->new(sub {
  die("No auto-vivifying! Bad programmer! No bikkit!\n".Dumper(\@_))
});

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[d/l]
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      Another way to prevent autovivification is to use Data::Diver.
      Perl 6 - links to (nearly) everything that is Perl 6.
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      Thanks for pointing out that autovivificated hash parts stay around. Reading about autovivification I always thought that those parts would appear on the fly and be gone again after they have been looked at. Since I actually might test hashes several times, I will be on the outlook for this situation.
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Re^2: Test if a subhash in a referenced hash exists
by Henri (Novice) on May 28, 2010 at 10:22 UTC
    Moritz, your reply helped me find the problem. After your suggestion did not work either I rechecked the subroutine and found that I had not passed $coordinates into it. Doing so, now your suggestion and my original line both work. Also, I thought about exist, defined and true and think I should use exist, because it is a prerequisite to defined and true. Thanks a lot for your help! Henri
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      ...but note that exists does not test what you asked for in the subject line, i.e. if a subhash exists — it just tests whether the respective hash key exists, or more precisely, whether the respective hash element has ever been initialized (with whatever value, including undef). 

      $data->{foo} = "";

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Can't use string ("") as a HASH ref
+...'
}

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      or with undef as value (in which case the missing hash would be autovivfied) 

      $data->{foo} = undef;

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Use of uninitialized value in print
+...'
}

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[d/l]
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        If I understand you right, with “exits” I am testing if keys at the top level of the anonymous subhash exist. If I wanted to test if the hash reference exists I would need to use “ref”. Correct?
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