Problem 4.   Your company keeps track of customers with a customer list. For each customer, your company records the customer name (a string) and the customer's current outstanding balance (a number).

It is your task to develop a program that identifies customers with high outstanding balances. Specifically, your program consumes a list of customers and a threshold amount th. It produces a list of customers whose outstanding balance is at least th.

Solution:

(define-struct customer (name balance))
;; Customer = (make-customer String Number)
;; [PT 2, one for struct definition and one for dd]

;; A LOC is one of:
;; -- empty
;; -- (cons Customer LOC)
;; [PT 1, for data def of list-of-customers]

;; toomuch : LOC Number -> LOC [PT 1]
;; extract those customer records
;; whose balance exceeds th [PT 1]
(define (toomuch th aloc) 
  (cond ;; [PT 2, for cond lines]
    [(empty? aloc) empty] ;; [PT 1, for empty]
    [else (cond
            ;; [PT 3, two for two cases, one for correctness]
            [(>= (customer-balance (first aloc)) th)
             (cons (first aloc) (toomuch th (rest aloc)))]
            [else (toomuch th (rest aloc))])]))

;; Examples/tests: [PT 1, for examples/tests]
(equal? (toomuch
          15
          (cons (make-customer "A" 10)
            (cons (make-customer "B" 20)
              (cons (make-customer "C" 15) empty))))
  (cons (make-customer "B" 20)
    (cons (make-customer "C" 15) empty)))