SICP sicp 108: 3.21 exercises 1083.21 and

SICP sicp 108: 3.21 exercises 1083.21 and
Exercise 3-21 original

Exercise 3.21. Ben Bitdiddle decides to test the queue implementation described above. He types in the procedures to the Lisp interpreter and proceeds to try them out:

(define q1 (make-queue))(insert-queue! q1 'a)((a) a) (insert-queue! q1 'b) ((a b) b) (delete-queue! q1)((b) b)(delete-queue! q1)(() b)

"It's all wrong !" He complains. "The interpreter's response shows that the last item is inserted into the queue twice. and when I delete both items, the second B is still there, so the queue isn't empty, even though it's supposed to be. "Eva Lu Ator suggests that Ben has misunderstood what is happening. "It's not that the items are going into the queue twice," she explains. "It's just that the standard Lisp printer doesn't know how to make sense of the queue representation. if you want to see the queue printed correctly, you'll have to define your own print procedure for queues. "Explain what Eva Lu is talking about. in particle, show why Ben's examples produce the printed results that they do. define a procedure print-queue that takes a queue as input and prints the sequence of items in the queue.

Analysis

Ben's code is fine. The problem is that the standard output function of Lisp is different from the desired output method. The question requires defining a process print-queue to output what Ben thinks.

Code

(define (print-queue queue)   (car queue));Value: print-queue

Test

(define q2 (make-queue));Value: q2(print-queue q2);Value: ()(insert-queue! q2 'a);Value 16: ((a) a)(print-queue q2);Value 17: (a)(insert-queue! q2 'b);Value 16: ((a b) b)(print-queue q2);Value 17: (a b)(delete-queue! q2);Value 16: ((b) b)(print-queue q2);Value 18: (b)(delete-queue! q2);Value 16: (() b)(print-queue q2);Value: ()

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