I would like to get an average value of an array.

Let's say, an Array R := (1 2 3 4 5 6 7 8 9 10)

One condition is, I don't know the total length of an array because it will
be changed every time. Maybe I could write down as (although it looks
complicated..),

(setf total-value 0)
(loop for i from 0 to arr-index
         do (setf total-value (+ total-value (aref arr i)))
         finally (setf ave-value (/ total-value (+ arr-index 1))))

However, I just wonder that is there any function or macro which can add
whole values in an array? I want use this function with one simple line.... I
couldn't find that kind of things, but maybe something exist? Thanks for
advance. =)

Sungwoo

(setf ave-value (/ (loop :for i :from 0 :below array-length
                         :summing (aref arr i))
                   array-length))

(/ (loop for x across <array> sum x)
   (length <array>))

cl-user(3): (defun average (arr)
              (/ (reduce #'+ arr)
                 (length arr)))
average
cl-user(4): (average #(1 2 3 4 5 6 7 8 9 10))
11/2
cl-user(5):

Watch out for zero-length arrays.

Sungwoo

Sungwoo

Sungwoo

because if it's really an array, you can get its dimension easily.
reduce #'+ can sum a vector.

(defun vector-average (one-dim-array)
  (let ((n (array-dimension vector 0)))
    (/ (reduce #'+ vector) n)))

this can be extended to handle more dimensions, but you get the idea.

Sungwoo

  I should give you the hints and the pointers, but I'm tired, so here's
  how you compute the average of a vector of numbers:

              (/ (reduce #'+ <vector>) (length <vector>))

  Incidentally, the standard deviation is also very useful if you ever
  work with averages:

(let* ((mean (/ (reduce #'+ <vector>) (length <vector>)))
       (sdev (sqrt (/ (reduce #'+ <vector> :key #'(lambda (elt) (expt (- elt mean) 2)))
                      (1- (length <vector>))))))
  ...)

  As you may have noticed, this is idiomatic Common Lisp for the sum
  operator.

Sungwoo