I did a google search for log-gamma and then browsed around the documentation. I think the issue is I was poking through the "Special Functions" and statistics functions sections, but the binomial stuff is under the "flonum" section, even though fllog-gamma can be found in "Special Functions." Maybe cross-linking them would help? And perhaps including the word "combinations" or "n choose k" somewhere in the docs for fllog-binomial would make it a bit more searchable for people who are used to different names.
On Wed, Feb 20, 2013 at 10:51 AM, Neil Toronto <neil.toro...@gmail.com>wrote: > You're welcome! > > A user not finding a documented function is excellent feedback. It means > we need to communicate better. Do you remember how you searched for a > combinations function? > > Neil ⊥ > > > On 02/20/2013 08:45 AM, Luke Vilnis wrote: > >> Ha! Sorry for not reading the documentation more thoroughly - I hope >> this was at least a bit educational to someone besides me :) Fantastic >> library and docs, by the way. >> >> On Wed, Feb 20, 2013 at 10:38 AM, Neil Toronto <neil.toro...@gmail.com >> <mailto:neil.toro...@gmail.com**>> wrote: >> >> On 02/20/2013 06:42 AM, Luke Vilnis wrote: >> >> No problem. They should be faster even for fairly small numbers >> since >> they usually require the evaluation of a polynomial (an >> approximation of >> (log)gamma) versus repeated multiplication/division. From memory >> the >> code should be something like: >> >> (exp (fllog-gamma (+ 1.0 n)) - (fllog-gamma (+ 1.0 r)) - >> (fllog-gamma (+ >> 1.0 (- n r)))) >> >> fllog-gamma should also be faster than bflog-gamma or log-gamma >> if you >> don't need arbitrary precision. You're also right that this >> won't always >> give completely exact results - the Racket manual says that the >> only >> exact values are for log gamma of 1 and 2, but this usually is >> not a >> problem. >> >> PS. It looks like Racket's math collection has a built-in >> log-factorial >> function too, to avoid all the +1's, so you could try that. >> >> >> There's also `fllog-binomial', which computes the log number of >> combinations directly. IIRC, its maximum observed error is 2 ulps. >> >> Neil ⊥ >> >> >> ____________________ >> Racket Users list: >> >> http://lists.racket-lang.org/_**_users<http://lists.racket-lang.org/__users> >> <http://lists.racket-lang.org/**users<http://lists.racket-lang.org/users> >> > >> >> >> >
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