exam9.5.3

;; mht created on Nov 15, 2011

;; dollar-store? : list-of-numbers -> boolean
;; to check whether all of a-list-of-nums are < 1

(define (dollar-store? a-list-of-nums)
  (cond
    [(empty? a-list-of-nums) true]
    [else
     (and
      (< (first a-list-of-nums) 1)
      (dollar-store? (rest a-list-of-nums)))]))

;; test
(dollar-store? empty)
(not (dollar-store? (cons .75 (cons 1.95 (cons .25 empty)))))
(dollar-store? (cons .15 (cons .05 (cons .25 empty))))

;; generalized-dollar-store? : list-of-numbers number -> boolean
;; to check whether all of a-list-of-nums are below n

(define (generalized-dollar-store? a-list-of-nums n)
  (cond
    [(empty? a-list-of-nums) true]
    [else
     (and
      (< (first a-list-of-nums) n)
      (generalized-dollar-store? (rest a-list-of-nums) n))]))

;; test
(generalized-dollar-store? empty 1)
(not (generalized-dollar-store? (cons .75 (cons 1.95 (cons .25 empty))) 1))
(generalized-dollar-store? (cons .15 (cons .05 (cons .25 empty))) 1)