numperm: Get the nth Permutation Vector by Periodic Placement

Description

numperm implicitly numbers all permutations of length olength, returning the permnumth of these.

Usage

numperm(olength, permnum)

Arguments

olength

The number of items to permute

permnum

The number of the permutation to use (in 1:olength!)

Value

A permutation vector

Details

The n! permutations on n items can be deterministically ordered via a factorization process in which there are n slots for the first element, n-1 for the second, and n-i for the ith. This fact is quite handy if you want to visit each permutation in turn, or if you wish to sample without replacement from the set of permutations on some number of elements: one just enumerates or samples from the integers on [1,n!], and then find the associated permutation. numperm performs exactly this last operation, returning the permnumth permutation on olength items.

# NOT RUN {#Draw a graphg<-rgraph(5)
#Permute the rows and columnsp.1<-numperm(5,1)
p.2<-numperm(5,2)
p.3<-numperm(5,3)
g[p.1,p.1]
g[p.2,p.2]
g[p.3,p.3]
# }