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 <[email protected]
<mailto:[email protected]>> 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 ⊥
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