exam9.5.7

;; mht created on Nov 15, 2011

;; average-price : list-of-numbers -> number
(define (average-price a-list-of-nums)
  (cond
    [(empty? a-list-of-nums) 0]
    [else
     (/ (sum-price a-list-of-nums)
        (count-price a-list-of-nums))]))

;; sum-price : list-of-nums -> number
(define (sum-price a-list-of-nums)
  (cond
    [(empty? a-list-of-nums) 0]
    [else
     (+ (first a-list-of-nums)
        (sum-price (rest a-list-of-nums)))]))

;; count-price : list-of-nums -> number
(define (count-price a-list-of-nums)
  (cond
    [(empty? a-list-of-nums) 0]
    [else
     (+ (count-price (rest a-list-of-nums))
        1)]))

;; test
(average-price (cons 2 (cons 3 (cons 1 empty))))
(average-price (cons 2 empty))