Is there an R function to get the number of permutations of n objects take k P(n,k)?

Question

..or do I have to give

P.nk <- factorial(n) / factorial(n-k)

or

P.nk <- choose(n,k) * factorial(k)

Thank you.

Answer 1

Another way for doing that, from Base R is

permn <- prod( (n-(0:(k-1)))

that is a simple implementation of the following formula

$$p(n,k) = \prod_{j=0}^{k-1} n-j$$

Answer 2

I don't know of any existing function. Your first suggestion will fail with large n. Your second idea should work fine when written as a function:

perm <- function(n,k){choose(n,k) * factorial(k)}

Then perm(500,2) will give 249500 for example.