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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: [Math] Issue 348
Date Fri, 05 Mar 2010 22:31:09 GMT
Well, the exponential, chi^2 and Maxwell-Boltzman distributions are all
specializations of the gamma distribution.

If you working on a Monte-carlo estimate where the parameters of your chi^2
distribution vary according to a hyper-distribution, then it would be nice
to implement the chi^2 distribution by changing the getter's for the two
parameters of the Gamma distribution to get the specialized values.  Then it
would be nice to implement the generalized hyper-distributed chi^2 by
over-riding the getters for the parameters of the chi^2 distribution to
sample from the hyper-distribution.

If you use getters throughout, then chi^2 is nearly a one-liner, the
hyper-distributed chi^2 is another one-liner.  The JIT will optimize away
all of the abstractions and give a result that is as fast as any other
implementation.

You can complain that this example is contrived, but an over dispersed
exponential is a common concept and is best described in this same fashion.

It might be that you could do all of this eagerly, having the hyper-chi^2
instantiate a new chi^2 which instantiates a new gamma for ever sample, but
then everybody has to be injecting a random number generator up and down the
chain (because those *are* often heavyweight and can't be instantiated
cheaply).  Over-riding the getters is way cleaner when you need to
implement.

On Fri, Mar 5, 2010 at 1:56 PM, Gilles Sadowski <
gilles@harfang.homelinux.org> wrote:

> Can you imagine a not contrived example? I.e. why would one inherit from a
> class while throwing away the implementation?
>



-- 
Ted Dunning, CTO
DeepDyve

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