#R version 3.3.2

u1000 <- runif(1000, 0, 1)

f <- function (n) {
   sum(u1000[1:n])
}

f2 <- function (n) {
   sum(u1000[1:n]) / n
}

f3 <- function (n) {
   sum(u1000[1:n]) - n/2
}

numbers <- (1:1000)
sums <- lapply(numbers, f3)

plot(numbers, sums, type="l")