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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