R/permutation.R

In wiep/introToR17: intro to r 17 pkg

#' calculate all permutations of a vector x
#' @details
#' this is a naiive recursive implementation
#' @param x a vector
#' @return matrix; each column is one unqiue permutation of x
#' @export
permute_vector <- function(x) {
  n <- length(x)
  num_permutation <- factorial(n)

  if (n == 1) { # base case
    return(as.matrix(x))
  } else {
    num_sub_permutation <- factorial(n - 1)
    res <- matrix(0.0, nrow = n, ncol = num_permutation)
    for (i in 1:n) {
      y <- x
      # swap first entry in y with i-th
      tmp <- y[1]
      y[1] <- y[i]
      y[i] <- tmp

      # calculate all permutations of y[2:n]
      col_idx <- (i-1) * num_sub_permutation + seq(1:num_sub_permutation)
      res[1, col_idx] <- y[1]
      res[2:n, col_idx] <- permute_vector(y[2:n])
    }
    return(res)
  }
}