I use the SNAP (Stacked Nodes Affect Parent) method. I use it for my YAPE modules. The process is rather simple. Make two variables refer to the same reference, and you create an array for stacking. 
my $node = my $tree = [];
my @stack;

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You never modify $tree directly -- you do all the manipulation on $node, and when you need to recurse, you add a new "field" to $node; then you push the $node onto the @stack, and then you set $node equal to the field: 
if ($token eq '(') {
  push @{ $node }, [];
  push @stack, $node;
  $node = $node->[-1];  # the new empty array ref
}

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When you return from the recursion, you get the top value from the @stack: 
if ($token eq ')') {
  $node = pop @stack;
}

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Now, $node contains what it was before, PLUS any modifications to that empty array reference. And because $node and $tree refer to the same data, modifying the data in $node modifies $tree.

It's important to realize that saying: 

$a = $b = [];
$a = 2;

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does not affect $b in any way. It's the changing of the data referred to by $a that affects $b.

Without further ado, here is my implementation of a directory tree structure, using SNAP. 

#!/usr/bin/perl -w

use File::Find;
use Data::Dumper;
use strict;

$Data::Dumper::Indent = 1;

build_tree(my $tree, shift);
print Dumper $tree;


{ 
  sub build_tree {
    my $node = $_[0] = {};
    my @s;
    find( sub {
      # fixed 'if' => 'while' -- thanks Rudif
      $node = (pop @s)->[1] while @s and $File::Find::dir ne $s[-1][0]
+;
      return $node->{$_} = -s if -f;
      push @s, [ $File::Find::name, $node ];
      $node = $node->{$_} = {};
    }, $_[1]);
    $_[0]{$_[1]} = delete $_[0]{'.'};
  }
}

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The recursion is not handled by me, it's handled by find(), so I had to kludge my way into faking the start and end of a recursion. But it works.

japhy -- Perl and Regex Hacker

In reply to Re: How to map a directory tree to a perl hash tree by japhy
in thread How to map a directory tree to a perl hash tree by Rudif

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