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Tied multidimensional hashes

by davis (Vicar)
on Oct 13, 2006 at 08:52 UTC ( #578086=perlquestion: print w/ replies, xml ) Need Help??
davis has asked for the wisdom of the Perl Monks concerning the following question:

I seem to be having a problem STOREing multidimensional values in a TIEd hash. I'm now going round in circles because even my minimal test case (which is typically all I need to produce to fix my own problem) isn't doing what I expect.

I've reviewed How to tie multilevel (multidimensional) hash? and the Tie::Hash documentation, and I've frankly run out of brain power.
I'm ultimately trying to concatenate keys of the subhashes together where the top-level key matches ignoring case. However, the code below doesn't even warn twice (it warns once with an empty hashref), suggesting it's broken in a stupid way I can't spot, or I'm not grokking Ties.

use warnings; use strict; use Data::Dumper; use Tie::Hash; package FlatHash; use Data::Dumper; our @ISA = qw(Tie::ExtraHash); sub STORE { my $me = $_[0]; my $key = $_[1]; my $value = $_[2]; $key = lc $key; $_[0][0]{$key} = $value; if(ref $value eq "HASH") { warn Dumper($value); } else { print "This isn't a hash\n"; } } package main; tie my %flat, "FlatHash"; $flat{Monkey}{habits} = "Eating"; $flat{monkey}{habits} = "Scratching"; warn Dumper(\%flat);
Ultimately I'd like the final datastructure to be something like:
$VAR1 = { 'monkey' => { 'habits' => ['Eating', 'Scratching'] } };
Any hints gratefully received.

davis
Kids, you tried your hardest, and you failed miserably. The lesson is: Never try.

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Re: Tied multidimensional hashes
by shmem (Canon) on Oct 13, 2006 at 11:33 UTC
    A bit debugging like so
    sub STORE { warn "STORE: ".join(', ',map{"'$_'"} @_)."\n"; ...

    reveals

    STORE: 'FlatHash=ARRAY(0x820fd58)', 'Monkey', 'HASH(0x820fd88)'

    at the first store attempt - your $_[2] isn't what you expect. What is happening here?

    Storing the value "Eating", a FETCH is done first, which you don't see, because Tie::Hash does this for you. Since FETCH doesn't return anything, an anonymous hash is autovivified and passed into STORE as the value. This value is stored under the (lowercased) key 'monkey'. Done with the tied hash. The autovivified anonymous hash is not tied. This hash is thereafter populated with the key/value pair (habits,Eating).

    At the second assignment, the value stored under the key 'monkey' is retrieved - the anonymous hash, which is not tied, and then a normal store operation is done: the value 'Eating' in that anonymous hash is overwritten with 'Scratching'. The tied hash is involved only at FETCH.

    Did you read notice the 'FETCH' sub in Re: How to tie multilevel (multidimensional) hash?? It takes care that the value retrieved is of the right type (a tied hash), so that autovivification does not happen.

    Ultimately I'd like the final datastructure to be something like:
    $VAR1 = { 'monkey' => { 'habits' => ['Eating', 'Scratching'] } };

    This is not what you said at the beginning:

    I'm ultimately trying to concatenate keys of the subhashes together where the top-level key matches ignoring case.

    That would be

    $VAR1 = { 'monkeyhabits' => 'Scratching' }

    What are you really trying to do? And why?

    --shmem

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                                  /\_¯/(q    /
    ----------------------------  \__(m.====·.(_("always off the crowd"))."·
    ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
Re: Tied multidimensional hashes
by xdg (Monsignor) on Oct 13, 2006 at 12:42 UTC

    Unless your goal is to practice reimplementing multi-dimensional tied hashes, I'd try one of the existing modules on CPAN instead. The "classic" is MLDBM. But dragonchild has done some tremendous work on DBM::Deep and I'd suggest trying that one first.

    -xdg

    Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

      xdg,
      I am afraid I have no idea how either of the modules you suggested would come close to accomplishing what davis is wanting to do.
      tie my %flat, "FlatHash"; $flat{Monkey}{habits} = "Eating"; $flat{monkey}{habits} = "Scratching"; warn Dumper(\%flat); __END__ Ultimately I'd like the final datastructure to be something like: $VAR1 = { 'monkey' => { 'habits' => ['Eating', 'Scratching'] } };

      To me it sounds like a variation on Data::MultiValuedHash or less likely Hash::MultiKey except that the top level is also case insensitive.

      Cheers - L~R

        I am afraid I have no idea how either of the modules you suggested would come close to accomplishing what davis is wanting to do.

        Perhaps something in DBM::Deep using filter_(fetch|store)_(key|value) would work. Severly untested, and I see the smoke from dragons rising through the fog, but I think it would be possible.

        --MidLifeXis

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