Simple example calculating the gini in R

gistfile1.r

calc.gini <- function(y)
{
  num.pos <- sum(y==1)
  num.neg <- sum(y==0)
  tpr     <- cumsum(y==1)/num.pos
  fpr     <- cumsum(y==0)/num.neg
  area    <- (tpr[-1] + tpr[1:(length(tpr)-1)]) %*% (fpr[-1] - fpr[1:(length(fpr)-1)])
  1 - as.numeric(area)
}

d <- data.frame(gt = c(1,1,1,1,1,0,0,0,0), p = c(0.9, 0.7, 0.1, 0.2, 0.5, 0.01, 0.4, 0.23, 0.3))
calc.gini(d$gt[order(d$p)])