From: Sebastian Hungerecker <sepp2k@...>
Date: 2009-01-05T00:01:09+09:00
Subject: Re: Default values of hashes

Glenn wrote:
> In other words, if I need 3 levels, it creates a hash within hash object 3
> levels deep (like in the line of code above) and if I need 5, it creates
> one 5 levels deep.  I do not know how to do this (or even if it is
> possible).  Any suggestions on this one?

def nested_hash(levels, def_value=nil, &def_proc)
  inner_hash = def_proc ? Hash.new(&def_proc) : Hash.new(def_value)
  if levels == 1
    inner_hash
  else
    l = [lambda {|h,k| h[k] = inner_hash}]
    (levels-2).times do |i|
      l[i+1] = lambda {|h,k| h[k] = Hash.new( &l[i] ) }
    end
    Hash.new(&l[-1])
  end
end

This creates a nested hash levels levels deep, where the innermost hash can 
optionally have a default value or proc.

Usage:
h = nested_hash(1)
# Same as h = Hash.new

h = nested_hash(1) {|h,k| h[k] = Array.new}
# Same as h = Hash.new {|h,k| h[k] = Array.new}

h = nested_hash(3)
# Same as h = Hash.new {|h,k| h[k] = Hash.new {|h,k| h[k] = Hash.new}}

h = nested_hash(3, :foo)
# Same as h = Hash.new {|h,k| h[k] = Hash.new {|h,k| h[k] = Hash.new(:foo)}}

HTH,
Sebastian
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