in reply to descending a tree of hash references

Amoe,

It's not clear to me what you are trying to do. After you have traversed the hash refs as you have shown and $nodule has the hash ref of $structure->{stuff}{hierarchy}{album}, then changing $nodule->{$target} will have the effect of changing $structure->{stuff}{hierarchy}{album}{$target}.

Some more code or context would be useful.

YuckFoo

Update: Some code to traverse a list of hash keys and change the last one. HTH. 


#!/usr/bin/perl

   use strict;

   my ($hash);

   $hash->{one}{two}{three} = 'before';
   print "$hash->{one}{two}{three}\n";

   changehash($hash, [qw(one two three)], 'after');
   print "$hash->{one}{two}{three}\n";

#-----------------------------------------------------------
sub changehash {

   my ($node, $keys, $val) = @_;
   my $key;

   my $last = pop(@{$keys});

   for $key (@{$keys}) {
      if (defined ($node->{$key})) {
         $node = $node->{$key};
      }
      else {
         print STDERR "Unknown key: $key\n";
         return;
      }
   }

   if (defined ($node->{$last})) {
      $node->{$last} = $val
   }
   else {
      print STDERR "Unknown key: $last\n";
      return;
   }
}

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Re: Re: descending a tree of hash references
by Amoe (Friar) on Feb 20, 2002 at 12:35 UTC

    Hey YuckFoo. Ready for context? Here goes...

    I'm getting a hashref from a DB_File, and unfreezing the data structure contained within with Storable. Then I traverse a user-defined path in the hashref and set a key in it, and it'll eventually get frozen back to the DB_File. An added complication (one that I should have mentioned, with hindsight :() is that if the full path doesn't exist, I have to create all the hashrefs leading up to the end - whilst still preserving everything that was in the original data structure. Here's a literal chunk of code from what I have.

        # the first element of $path is always a key in the DB_File
    my ($root, $tip) = (shift(@path), pop(@path));
    
    # get old tree
    my $old = thaw($db{$root});
    
    # start off at the root
    $nodule = $old;
    
    while (my $next_key = shift @path) {
        unless (exists $nodule->{$next_key}) {
            $nodule->{$next_key} = {};    
        }
        $nodule = $nodule->{$next_key};
    }
    
    $nodule->{$tip} = $content;
    print Dumper($old);

    [download]

    The thing is, when I print $old with Data::Dumper, I would expect it to be a structure looking like this (given that @path = ('foo', 'bar'), $tip = 'baz' and $content = 'some arbitrary data'):

    $VAR1 = {foo =>
        {bar =>
            {baz => 'some arbitrary data'}
        }
    };

    [download]

    Instead it looks like:

    $VAR1 = undef;

    So that's what I want to achieve, and the example I gave in the question seems to be lose context in this environment, when by all rights it should work. Damn real life.



    --
    my one true love
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      Amoe,

      Thanks, problem well stated. Unfortunately I couldn't reproduce the problem. I think you are not getting what you expect in $old when thawing. Try print Dumper($old) immediately after thawing.

      YuckFoo

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