From: Dave Burt <dave@...>
Date: 2005-08-29T14:16:22+09:00
Subject: Re: Hash.new(confusion)

transfire@gmail.com wrote:
> ...
> Please explain how r is empty but r[:b] exists.  Thank You.

# To summarize:
h = Hash.new({})
h[1][2] = 3
h #=> {}
h.default #=> {2=>3}

So you've added to the default object. It might be helpful to note another 
wrong way to do this:
h = Hash.new { {} }
h[1][2] = 3
h #=> {}
h.default #=> {}

Different, because default now returns a new hash every time.

But, to answer the implied question, "what is the code that will do what I 
intend?" there are a few ways to do this:

# Just one level of magic new hashes
h = Hash.new {|hash, key| hash[key] = {} }
h[1][2] = 3
h #=> {1=>{2=>3}}
h[1][2][3] = 4  # Error (calling []= on nil)

# Auto-vivifying hash: using proc
auto_vivivy_hash = proc {|hash, key| hash[key] = Hash.new 
&auto_vivivy_hash }
h = Hash.new &auto_vivify_hash
h[1][2][3] = 4  #=> {1=>{2=>{3=>4}}}

# Auto-vivifying hash: using Hash subclass
class X < Hash
  def default(key = nil)
    self[key] = self.class.new
  end
end
h = X.new  #=> {}
h[1][2][3] = 4
h  #=> {1=>{2=>{3=>4}}}

Cheers,
Dave