From: Suraj Kurapati <snk@...>
Date: 2009-03-31T08:15:53+09:00
Subject: Re: Is there an idiom for mass-populating Hash keys?

Shot (Piotr Szotkowski) wrote:
> The catch is I’ll be doing various things to this graph,
> like merging vertices together, grouping them by their labels (graph
> colouring), and doing similar operations on (labeled) edges.

Try providing a default_proc to Hash.new:

  Hash.new {|h,k| h[k] = nil }

Now previously unregistered vertices will be registered upon hash access 
(Hash#[]).  To illustrate this technique, consider this "dynamic 
programming" based Fibonacci sequence implementation:

  fib = Hash.new {|h,k| k < 2 ? 1 : h[k-1] + h[k-2] }

  p 0 => fib[0]
  p 1 => fib[1]
  p 5 => fib[5]
  p :cache => fib
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