R/vec-inv.R

In corybrunson/tautable: Tables of critical values for Kendall's tau-a

#' Count the inversions in a vector.
#' 
#' @param vec A numerical vector (theoretically, with no repeated values).
#' @return The number of pairs of entries i and j of `vec` for which
#'   i > j.
#' @export
#' @examples
#' vec_inv(1:5)
#' vec_inv(5:1)
vec_inv <- function(vec) {
    sum(unlist(sapply(1:(length(vec) - 1), function(i) {
        sapply((i + 1):length(vec), function(j) vec[i] > vec[j])
    })))
}