From: "Shot (Piotr Szotkowski)" <shot@...>
Date: 2009-03-30T05:31:19+09:00
Subject: Re: Is there an idiom for mass-populating Hash keys?

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Rick DeNatale:

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

>> class Graph
>>  def initialize enum
>>    @vertices =3D {}
>>    enum.each { |vertex| @vertices[vertex] =3D nil }
>>  end
>> end

> 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.

Right, I was thinking about this approach on and off all day today,=20
actually. :) The catch is I=E2=80=99ll be doing various things to this grap=
h,=20
like merging vertices together, grouping them by their labels (graph=20
colouring), and doing similar operations on (labeled) edges.

While I do like the cleanliness of the approach with @vertices being=20
just a Set, @edges being just a Set of two-element Sets, and there being=20
two additional Hashes for vertex and edge labels, I=E2=80=99m not sure I wa=
nt=20
the overhead of making sure there aren=E2=80=99t any leftover keys in the l=
abel=20
Hashes after the graph drops some of its edges/vertices =E2=80=93 in other=
=20
words, anytime I delete a vertex or an edge I=E2=80=99d have to delete it f=
rom=20
two places (either that, or anytime I query the labels I=E2=80=99d have to =
make=20
sure I=E2=80=99m filtering out no-longer-existing vertices/edges =E2=80=93 =
which is even=20
uglier).

Note: I do know about GRATR, but it seems unmaintained; I found=20
(granted, minor) bugs in it by simply requiring it with warnings=20
enabled, and it requires (again, minor) patching to get it working
with Ruby 1.9, but my patches to fix both are ignored (and there haven=E2=
=80=99t=20
been a relase for a year and a half). I would also use just a tiny=20
fraction of GRATR=E2=80=99s functionality, so it=E2=80=99s simpler for me t=
o have my
own Graph class (and be able to easily RubyInline parts of it if/when=20
I need the performance).

=E2=80=94 Shot
--=20
Is your keyboard jammed, or are you just writing Haskell? [Avdi Grimm]

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