From: Stefano Crocco <stefano.crocco@...>
Date: 2009-01-13T06:02:53+09:00
Subject: Re: what's the rules re whether a Hash can use either a Symbol or String to reference the value???

Alle lunedì 12 gennaio 2009, Greg Hauptmann ha scritto:
> Hi,
> Getting confused with Hashes a bit.  It seems sometimes I can use both a
> Symbol or a String to get the value back, but other times the Symbol
> doesn't work.  Is there any rule re how to remember how Hash handles
> setting and getting values uses Symbol / String?
>
> e.g. hash_variable[:amount] versus hash_variable["amount"]
>
> e.g. hash_var = {:amount => 10}, versus hash_var = {"amount" => 10)

By itself, a hash distinguish between strings and symbols, so if an entry is 
stored under a string key, you'll only be able to retrieve it using a string 
key. If the entry has a symbol entry, then it can only be obtained using a 
symbol key:

h ={'a' => 1, :b => 2}
puts h['a'] # => 1
puts h[:a]  # => nil
puts h[:b]  # => 2
puts h['b'] # => nil

You can setup the hash so that it will work either with strings or symbols. 
For example:

hash = Hash.new do |h, k|
  if k.is_a?(String) and h.has_key? k.to_sym then h[k] = h[k.to_sym]
  elsif k.is_a?(Symbol) and h.has_key? k.to_s then h[k] = h[k.to_s]
  else nil
  end
end

hash['a'] = 1
hash[:b] = 2
puts hash['a'] # => 1
puts hash[:a]  # => 1
puts hash[:b]  # => 2
puts hash['b'] # => 2
puts hash[1]   # => nil

See the documentation for Hash.new to see how passing it a block works.

I hope this helps

Stefano