R循环嵌套求助

data(cars)
dim(cars)
speed <- cars[, 1]
dist <- cars[, 2]
Ybar <- mean(dist)
Ns <- seq(100, 2000, by=100)
output <- rep(NA, length(Ns))
k <- 1
for (N in Ns) {
    ybar <- rep(0, N)
    for (i in 1:N) {
        ybar[i] <- mean(sample(dist, 10))
    }
    output[k] <- mean(ybar)
    k <- k + 1
}

恩 应该就是这样了，谢谢版主啊！