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Re: Is there an idiom for mass-populating Hash keys?

Rick DeNatale

3/29/2009 4:55:00 PM

On Sun, Mar 29, 2009 at 12:05 PM, Shot (Piotr Szotkowski) <shot@hot.pl>wrot=
e:

> Hello, good folk of ruby-talk.
>
> I was wondering whether there=92s a Ruby
> idiom to mass-populate keys of a Hash.
>
> I=92m coding a graph class supporting labeled vertices, and I want to
> represent this relationship as a @vertices Hash, with keys being the
> actual vertices and values being their labels =96 initially all nil:
>
> class Graph
> def initialize enum
> @vertices =3D {}
> enum.each { |vertex| @vertices[vertex] =3D nil }
> end
> end
>
> Is there a more elegant way to take an enum
> and turn it into nil-referencing Hash keys?
>

Why bother? The only reason I can thing of is that you want to use
@vertices.keys to get the collection of vertices, but why be indirect.

class Graph
def initialize(enum)
@vertices =3D enum
@labels =3D {}
end
end

@labels[anything] will return nil if anything isn't a key.


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Rick DeNatale

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2 Answers

Suraj Kurapati

3/30/2009 11:16:00 PM

0

Shot (Piotr Szotkowski) wrote:
> The catch is Iâ??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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Suraj Kurapati

3/30/2009 11:19:00 PM

0

Suraj Kurapati wrote:
> consider this "dynamic programming" based Fibonacci sequence implementation:
>
> fib = Hash.new {|h,k| k < 2 ? 1 : h[k-1] + h[k-2] }

Whoops, I forgot to store the computed value in the hash!

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

Now it should work. Sigh :-)
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