SICP_3.21-3.22

(define (front-ptr queue) (car queue))
(define (rear-ptr queue) (cdr queue))

(define (set-front-ptr! queue item) (set-car! queue item))
(define (set-rear-ptr! queue item) (set-cdr! queue item))

(define (empty-queue? queue) (null? (front-ptr queue)))

(define (make-queue) (cons '() '()))

(define (front-queue queue)
  (if (empty-queue? queue)
      (error "FRONT called with an empty queue" queue)
      (car (front-ptr queue))))

(define (insert-queue! queue item)
  (let ((new-pair (cons item '())))
    (cond ((empty-queue? queue)
           (set-front-ptr! queue new-pair)
           (set-rear-ptr! queue new-pair)
           queue)
          (else (set-cdr! (rear-ptr queue) new-pair)
                (set-rear-ptr! queue new-pair)
                queue))))

(define (delete-queue! queue)
  (if (empty-queue? queue)
      (error "DELETE called with an empty queue" queue)
      (begin
        (set-front-ptr! queue (cdr (front-ptr queue)))
        queue)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define q1 (make-queue))

(insert-queue! q1 'q)
(insert-queue! q1 'b)
(insert-queue! q1 'c)
(delete-queue! q1)

;;;;;;;;;;;;;;;;3.21

(define (print-queue queue)
  (car queue))

(print-queue q1)
(rear-ptr q1)
(front-ptr q1)
(set-cdr! (rear-ptr q1) '(m))
;(set-cdr! q1 '(m))

(set-rear-ptr! q1 '(m))
q1

;;;;;;;;;;;;;;;;3.22
(define (make-queue2)
  (let ((front-ptr '())
        (rear-ptr '()))
    (define (set-front-ptr! item)
      (set! front-ptr item))
    (define (set-rear-ptr! item)
      (set! rear-ptr item))
    (define (empty-queue?)
      (null? front-ptr))
    (define (insert-queue! item)
      (let ((new-pair (list item)))
        (cond ((empty-queue?)
               (set-front-ptr! new-pair)
               (set-rear-ptr! new-pair)
               front-ptr)
              (else
               (set-cdr! rear-ptr new-pair)
               (set-rear-ptr! new-pair)
               front-ptr))))
    (define (delete-queue!)
            (cond ((empty-queue?)
                    (error "DELETE! called with an empty queue" front-ptr))
                  (else
                    (set-front-ptr! (cdr front-ptr))
                    front-ptr)))
    (define (dispatch m)
      (cond ((eq? m 'insert-queue!) insert-queue!)
            ((eq? m 'delete-queue!) delete-queue!)
            ((eq? m 'empty-queue?) empty-queue?)
            (else (error "Undefined operation" m))))
    dispatch))

(define (make-queue3)
    (let ((front-ptr '())
          (rear-ptr '()))
        (define (insert-queue! item)
            (cond ((empty-queue?)
                    (let ((init-list (list item)))
                        (set! front-ptr init-list)
                        (set! rear-ptr init-list)
                        front-ptr))
                  (else
                    (let ((new-item (list item)))
                        (set-cdr! rear-ptr new-item)
                        (set! rear-ptr new-item)
                        front-ptr))))
        (define (delete-queue!)
            (cond ((empty-queue?)
                    (error "DELETE! called with an empty queue" front-ptr))
                  (else
                    (set! front-ptr (cdr front-ptr))
                    front-ptr)))
        (define (empty-queue?)
            (null? front-ptr))
        (define (dispatch m)
            (cond ((eq? m 'insert-queue!)
                    insert-queue!)
                  ((eq? m 'delete-queue!)
                    (delete-queue!))
                  ((eq? m 'empty-queue?)
                    (empty-queue?))
                  (else
                    (error "Unknow operation -- DISPATCH" m))))
        dispatch))

;(define q2 (make-queue3))
;((q2 'insert-queue!) 3)
;((q2 'insert-queue!) 4)

;(define q2 '(1))
;(set-cdr! q2 (list 2))
;q2

这几题的理解花了几天，发现自己是想多了点，然后还有一个原因是理解错误

126-128 的结果为（1 2）不是 （1 （2））