Computing Weighted Averages

It's easy to compute a weighted average incorrectly if you're not careful. A weighted average should have the property that if an assignment has a weight of, say 10% of the final grade, it should have that weight whether the assignment is graded on a scale of 100 or a scale of 20.

The following bit of code does the weighted-average algorithm correctly:

(define weighted-average
  (lambda (scores maxes weights)
    (let ((weighted-sum
            (sum
              (map
                (lambda (score max weight) (* (/ score max) weight))
                scores maxes weights)))
          (total-weight (sum weights)))
      (/ weighted-sum total-weight))))

(define sum
  (lambda (l)
    (if (null? l) 0 (+ (car l) (sum (cdr l))))))

The actual average is computed in Excel using this algorithm.

Mitchell Wand
College of Computer and Information Science, Northeastern University
360 Huntington Avenue #202 WVH,
Boston, MA 02115
Internet: wand@ccs.neu.edu
Phone: (617) 373-2072 / Fax: (617) 373-5121