From: Martin DeMello <martindemello@...>
Date: 2005-12-01T22:12:31+09:00
Subject: Re: Help constructing interesting hash?

mental@rydia.net wrote:
> 
>  def build2( values, keys )
>    hash = {}
>    values.zip( keys ) do |value, key|
>      ( hash[key] ||= [] ).push value
>    end
>    hash
>  end
> 
>  build2(*[[A, B, C], [1, 2, 2]]) #=> {1=>[A], 2=>[B, C]}

The other interesting  way is 

def build3(values, keys)
  hash = Hash.new {|h,k| h[k] = []}
  values.zip(keys) do |value, key|
    hash[key].push value
  end
  hash
end

i.e. pushing the responsibility of generating a [] the first time a key
is seen off onto the hash rather than the insertion code.

martin