# [racket] Math library kudos

 From: Neil Toronto (neil.toronto at gmail.com) Date: Wed Feb 20 10:51:00 EST 2013 Previous message: [racket] Math library kudos Next message: [racket] Math library kudos Messages sorted by: [date] [thread] [subject] [author]

```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.toronto at gmail.com
> <mailto:neil.toronto at 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 ⊥
>
>
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>

```

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