Function to calculate AUC

auc.R

# x = a vector for scores
# y = a vector of labels

function (x, y, decreasing = TRUE, top = 1) 
{
    if (length(x) != length(y)) {
        stop(paste("Length of scores does not match with labels."))
    }
    N <- length(y)
    n <- sum(y == 1)
    x_p <- -Inf
    area <- 0
    fp = tp = fp_p = tp_p = 0
    ord <- order(x, decreasing = decreasing)
    for (i in seq_along(ord)) {
        j <- ord[i]
        if (x[j] != x_p) {
            if (fp >= (N - n) * top) {
                rat <- ((N - n) * top - fp_p)/(fp - fp_p)
                area <- area + rat * (fp - fp_p) * (tp + tp_p)/2
                return(area/(n * (N - n) * top))
            }
            area <- area + (fp - fp_p) * (tp + tp_p)/2
            x_p <- x[j]
            fp_p <- fp
            tp_p <- tp
        }
        if (y[j] == 1) {
            tp <- tp + 1
        }
        else {
            fp <- fp + 1
        }
    }
    area <- area + (fp - fp_p) * (tp + tp_p)/2
    return(area/(n * (N - n)))
}