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

..or do I have to give

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

or

P.nk <- ch开发者_StackOverflow中文版oose(n,k) * factorial(k)

Thank you.

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.

I think the gregmisc package provides these functions.

library(gregmisc)
permutations(n=4,r=4)

Mailing list reference: [R] permutation

Check out nsamp(n,k,ordered=T) in the 'prob' package

Another way for doing that, from Base R is

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

that is a simple implementation of the following formula

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

package gtools

# R version 3.5.3
install.packages("gtools")
library(gtools)

base::nrow(gtools::permutations(500,2))

result:

[1] 249500

also see combinations-and-permutations-in-r, permutation_with_replacement.R

another package prob:

base::ncol(prob::permsn(500,2))

[1] 249500