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

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?
>
>
>
> ____________________
>    Racket Users list:
>    http://lists.racket-lang.org/users
>