1 (define (test-case actual expected)
2   (newline)
3   (display "Actual:   ")
4   (display actual)
5   (newline)
6   (display "Expected: ")
7   (display expected)
8   (newline))
9 
10 (define (front-ptr queue) (car queue))
11 (define (rear-ptr queue) (cdr queue))
12 (define (set-front-ptr! queue item) (set-car! queue item))
13 (define (set-rear-ptr! queue item) (set-cdr! queue item))
14 (define (empty-queue? queue) (null? (front-ptr queue)))
15 (define (make-queue) (cons '() '()))
16 (define (front-queue queue)
17   (if (empty-queue? queue)
18       (error "FRONT called with an empty queue" queue)
19       (car (front-ptr queue))))
20 (define (insert-queue! queue item)
21   (let ((new-pair (cons item '())))
22     (cond ((empty-queue? queue)
23 	   (set-front-ptr! queue new-pair)
24 	   (set-rear-ptr! queue new-pair)
25 	   queue)
26 	  (else
27 	   (set-cdr! (rear-ptr queue) new-pair)
28 	   (set-rear-ptr! queue new-pair)
29 	   queue))))
30 (define (delete-queue! queue)
31   (cond ((empty-queue? queue)
32 	 (error "DELETE! called with an empty queue" queue))
33 	(else
34 	 (set-front-ptr! queue (cdr (front-ptr queue)))
35 	 queue)))
36 
37 ;; Exercise 3.21
38 
39 ;; Define a procedure print-queue that takes a queue as input and prints the sequence of items in the queue. 
40 
41 (define (print-queue queue)
42   (newline)
43   (newline)
44   (display (front-ptr queue))
45   (newline)
46   (front-ptr queue))
47 
48 (define q (make-queue))
49 (insert-queue! q 'a)
50 (test-case (print-queue q) '(a))
51 (insert-queue! q 'b)
52 (test-case (print-queue q) '(a b))
53 (insert-queue! q 'c)
54 (test-case (print-queue q) '(a b c))
55 (insert-queue! q 'd)
56 (test-case (print-queue q) '(a b c d))
57 (insert-queue! q 'e)
58 (test-case (print-queue q) '(a b c d e))
59 (delete-queue! q)
60 (test-case (print-queue q) '(b c d e))
61 (delete-queue! q)
62 (test-case (print-queue q) '(c d e))
63