[Numpy-discussion] New ufuncs
Anne Archibald
aarchiba@physics.mcgill...
Wed Nov 5 00:41:56 CST 2008
On 05/11/2008, Charles R Harris <charlesr.harris@gmail.com> wrote:
>
>
> On Tue, Nov 4, 2008 at 11:05 PM, T J <tjhnson@gmail.com> wrote:
> > On Tue, Nov 4, 2008 at 9:37 PM, Anne Archibald
> >
> > <aarchiba@physics.mcgill.ca> wrote:
> >
> > > 2008/11/5 Charles R Harris <charlesr.harris@gmail.com>:
> > >> Hi All,
> > >>
> > >> I'm thinking of adding some new ufuncs. Some possibilities are
> > >>
> > >> expadd(a,b) = exp(a) + exp(b) -- For numbers stored as logs:
> > >
> > > Surely this should be log(exp(a)+exp(b))? That would be extremely
> useful, yes.
> > >
> >
> > +1
> >
> > But shouldn't it be called 'logadd', for adding values which are stored as
> logs?
> >
>
> Hmm... but I'm thinking one has to be clever here because the main reason I
> heard for using logs was that normal floating point numbers had insufficient
> range. So maybe something like
>
> logadd(a,b) = a + log(1 + exp(b - a))
>
> where a > b ?
That's the usual way to do it, yes. I'd use log1p(exp(b-a)) for a
little extra accuracy, though it probably doesn't matter. And yes,
using logadd.reduce() is not the most efficient way to get a logsum();
no reason it can't be a separate function. As T J says, a logdot()
would come in handy too. A python implementation is a decent first
pass, but logdot() in particular would benefit from a C
implementation.
Anne
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