Generating all distinct permutations of a list in R

Question

I'm trying to create a list of permutations of a list, such that, for example, perms(list("a", "b", "c")) returns

list(list("a", "b", "c"), list("a", "c", "b"), list("b", "a", "c"),      list("b", "c", "a"), list("c", "a", "b"), list("c", "b", "a")) 

I'm not sure how to proceed, any help would be greatly appreciated.

Answer 1

A while back I had to do this in base R without loading any packages.

permutations <- function(n){     if(n==1){         return(matrix(1))     } else {         sp <- permutations(n-1)         p <- nrow(sp)         A <- matrix(nrow=n*p,ncol=n)         for(i in 1:n){             A[(i-1)*p+1:p,] <- cbind(i,sp+(sp>=i))         }         return(A)     } } 

Usage:

> matrix(letters[permutations(3)],ncol=3)      [,1] [,2] [,3] [1,] "a"  "b"  "c"  [2,] "a"  "c"  "b"  [3,] "b"  "a"  "c"  [4,] "b"  "c"  "a"  [5,] "c"  "a"  "b"  [6,] "c"  "b"  "a"  

Answer 2

combinat::permn will do that work:

> library(combinat) > permn(letters[1:3]) [[1]] [1] "a" "b" "c"  [[2]] [1] "a" "c" "b"  [[3]] [1] "c" "a" "b"  [[4]] [1] "c" "b" "a"  [[5]] [1] "b" "c" "a"  [[6]] [1] "b" "a" "c" 

Note that calculation is huge if the element is large.