[Numpy-discussion] Writing new ufuncs

Anne Archibald peridot.faceted@gmail....
Mon May 12 00:37:38 CDT 2008

2008/5/11 Robert Kern <robert.kern@gmail.com>:

> Perhaps, but ufuncs only allow 0 or 1 for this value, currently.

That's a shame, minus infinity is the identity for maximum too.

> Also, I was wrong about using PyUFunc_ff_f. Instead, use PyUFunc_ff_f_As_dd_d.

Hmm. Well, I tried implementing logsum(), and it turns out neither
PyUFunc_dd_d nor any of the other functions - mostly flagged as being
part of the ufunc API and described in the automatically-generated
ufunc_api.txt - are exported. All are static and not described in any
header I could find. That said, I know scipy defines its own ufuncs,
so it must either reimplement these or have some way I hadn't thought
of to get at them.

I've attached a patch to add the ufunc logsum to numpy. It's a bit
nasty for several reasons:
* I wasn't sure where to put it, so I put it in _compiled_base.c in
numpy/lib/src. I was very hesitant to touch umathmodule.c.src with its
sui generis macro language.
* I'm not sure what to do about log1p - it seems to be available in
spite of HAVE_LOG1P not being defined. In any case if it doesn't exist
it seems crazy to implement it again here.
* since PyUFunc_dd_d does not seem to be exported, I just cut-n-pasted
its code here. Obviously not a solution, but what are extension
writers supposed to do?

Do we want a ufunc version of logsum() in numpy? I got the impression
that in spite of some people's doubts as to its utility there was a
strong contingent of users who do want it. This plus a logdot() would
cover most of the operations one might want to do on numbers in log
representation. (Maybe subtraction?). It could be written in pure
python, for the most part, and reduce() would save a few exps and logs
at the cost of a temporary, but accumulate() remains a challenge. (I
don't expect many people feel the need for reduceat()).


P.S. it was surprisingly difficult to persuade numpy to find my tests.
I suppose that's what the proposed transition to nose is for? -A
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