# WARNING! Contains deliberate bug, for instructional purposes.
# Calculate rank correlation between two vectors
# Inputs: two data vectors (x,y)
# Outputs: rank correlation coefficient
rankcor <- function(x,y) {
  n <- length(x)
  stopifnot(length(y)==n)
  x.ranks <- rank(x)
  y.ranks <- rank(y)
  mean.x <- mean(x.ranks)
  mean.y <- mean(y.ranks)
  covariance.term <- cov(x.ranks-mean.x,y-mean.y)
  sd.x <- sd(x.ranks)
  sd.y <- sd(y.ranks)
  rank.cor <- covariance.term/(sd.x*sd.y)
  return(rank.cor)
}