On 05/11/2008, Charles R Harris <[EMAIL PROTECTED]> wrote: > > > On Tue, Nov 4, 2008 at 11:05 PM, T J <[EMAIL PROTECTED]> wrote: > > On Tue, Nov 4, 2008 at 9:37 PM, Anne Archibald > > > > <[EMAIL PROTECTED]> wrote: > > > > > 2008/11/5 Charles R Harris <[EMAIL PROTECTED]>: > > >> 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 _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
