From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-01-26T18:01:14+09:00
Subject: Re: Nested hash with arrays for default value

On Mon, Jan 25, 2010 at 10:49 PM, Glen Holcomb <damnbigman@gmail.com> wrote:

> Exactly, infinite depth would be nice as it would make a more temporally
> portable solution.

With it, you have infinite depth, until in a branch you decide to stop
by appending (<<) a value.
Then you fix the depth of that branch.

> The proxy looks to be working great.  I am a bit
> confused as to why the << method in the proxy doesn't overwrite a leaf with
> a new array though.  I'm not complaining as it works the way I want it to,
> I'm just perplexed.

When you access h[1][2][3], a proxy object is inserted in the hash for
that key. The proxy object remembers the hash and the key. When you
call << on the proxy object, it replaces itself in the hash with an
empty array, to which it appends the value. So further calls to
h[1][2][3] will return that array and no proxy objects anymore. It
works the same for the upper levels: calling h[1] inserts a proxy in
the hash. When you call [] on it (for example h[1][2]) it replaces
h[1] with a hash.

Maybe this clarifies a bit more:


/temp$ cat nested_hash_array.rb && ruby nested_hash_array.rb
class ProxyDefault
	def initialize hash, key
		@hash = hash
		@key = key
	end
	
	def [](key)
		puts "the hash is: #{@hash.inspect} when calling [] on the proxy object"
		@hash[@key] = Hash.new {|hash,key| ProxyDefault.new(hash, key)}
		puts "the hash is: #{@hash.inspect} after replacing the proxy with a hash"
		@hash[@key][key]
	end
	
	def << value
		puts "the hash is: #{@hash.inspect} when calling << on the proxy object"
		@hash[@key] = [value]
		puts "the hash is: #{@hash.inspect} after replacing the proxy with an array"
		@hash[@key]		
	end
end
		
h = Hash.new {|hash,value| ProxyDefault.new(hash, value)}

h[1][2] << "value"

p h
p h[1][2]

the hash is: {} when calling [] on the proxy object
the hash is: {1=>{}} after replacing the proxy with a hash
the hash is: {} when calling << on the proxy object
the hash is: {2=>["value"]} after replacing the proxy with an array
{1=>{2=>["value"]}}
["value"]

Jesus.