[racket] a function for a generalization of permutations for floats without going to +inf.0?

Thanks that fixed it!

On Jan 25, 2015, at 8:34 PM, Neil Toronto <neil.toronto at gmail.com> wrote:

> You can compute in log space using `log-gamma`:
> 
>  (define (my-permutations* n k)
>    (exp (- (log-gamma (+ n 1)) (log-gamma (+ (- n k) 1)))))
> 
> I wouldn't trust the last four digits or so of large results, but that may be accurate enough for what you're doing.
> 
> If not, look at "math/private/flonum/flonum-factorial.rkt". I'm pretty sure you can just copy the code, remove the integer checks, and have something that works. (Test it, though.) Error should be <= 3 ulps if you do that.
> 
> Neil ⊥
> 
> On 01/24/2015 01:18 PM, Alexander D. Knauth wrote:
>> Is there a way to define a function for a generalization of permutations for flonums using gamma?
>> This doesn’t work because if n is >=171 and k is a float it just returns +inf.0:
>> #lang racket
>> (require math/special-functions)
>> (define (my-factorial x) (gamma (+ x 1)))
>> (define (my-permutations n k) (/ (my-factorial n) (my-factorial (- n k))))
>> 
>> Is there a better way to do this without it returning +inf.0, or is there a library function anywhere that does this?
>> 
>> 
>> 
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