Problem Statement

Write a program to find the mean (arithmetic average) of a numeric vector. In case of a zero-length input, since the mean of an empty set of numbers is ill-defined, the program may choose to behave in any way it deems appropriate, though if the programming language has an established convention for conveying math errors or undefined values, it's preferable to follow it.

Let's start with the solution:

(defun mean (&rest sequence)
  (when sequence
    (/ (reduce #'+ sequence) (length sequence))))


(defun mean (list)
  (when list
    (/ (loop for i in list sum i)
       (length list))))