SICP_3.21-3.22

  1 (define (front-ptr queue) (car queue))
  2 (define (rear-ptr queue) (cdr queue))
  3 
  4 (define (set-front-ptr! queue item) (set-car! queue item))
  5 (define (set-rear-ptr! queue item) (set-cdr! queue item))
  6 
  7 (define (empty-queue? queue) (null? (front-ptr queue)))
  8 
  9 (define (make-queue) (cons '() '()))
 10 
 11 (define (front-queue queue)
 12   (if (empty-queue? queue)
 13       (error "FRONT called with an empty queue" queue)
 14       (car (front-ptr queue))))
 15 
 16 (define (insert-queue! queue item)
 17   (let ((new-pair (cons item '())))
 18     (cond ((empty-queue? queue)
 19            (set-front-ptr! queue new-pair)
 20            (set-rear-ptr! queue new-pair)
 21            queue)
 22           (else (set-cdr! (rear-ptr queue) new-pair)
 23                 (set-rear-ptr! queue new-pair)
 24                 queue))))
 25 
 26 (define (delete-queue! queue)
 27   (if (empty-queue? queue)
 28       (error "DELETE called with an empty queue" queue)
 29       (begin
 30         (set-front-ptr! queue (cdr (front-ptr queue)))
 31         queue)))
 32 
 33 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 34 
 35 (define q1 (make-queue))
 36 
 37 (insert-queue! q1 'q)
 38 (insert-queue! q1 'b)
 39 (insert-queue! q1 'c)
 40 (delete-queue! q1)
 41 
 42 ;;;;;;;;;;;;;;;;3.21
 43 
 44 (define (print-queue queue)
 45   (car queue))
 46 
 47 (print-queue q1)
 48 (rear-ptr q1)
 49 (front-ptr q1)
 50 (set-cdr! (rear-ptr q1) '(m))
 51 ;(set-cdr! q1 '(m))
 52 
 53 (set-rear-ptr! q1 '(m))
 54 q1
 55 
 56 ;;;;;;;;;;;;;;;;3.22
 57 (define (make-queue2)
 58   (let ((front-ptr '())
 59         (rear-ptr '()))
 60     (define (set-front-ptr! item)
 61       (set! front-ptr item))
 62     (define (set-rear-ptr! item)
 63       (set! rear-ptr item))
 64     (define (empty-queue?)
 65       (null? front-ptr))
 66     (define (insert-queue! item)
 67       (let ((new-pair (list item)))
 68         (cond ((empty-queue?)
 69                (set-front-ptr! new-pair)
 70                (set-rear-ptr! new-pair)
 71                front-ptr)
 72               (else
 73                (set-cdr! rear-ptr new-pair)
 74                (set-rear-ptr! new-pair)
 75                front-ptr))))
 76     (define (delete-queue!)
 77             (cond ((empty-queue?)
 78                     (error "DELETE! called with an empty queue" front-ptr))
 79                   (else
 80                     (set-front-ptr! (cdr front-ptr))
 81                     front-ptr)))
 82     (define (dispatch m)
 83       (cond ((eq? m 'insert-queue!) insert-queue!)
 84             ((eq? m 'delete-queue!) delete-queue!)
 85             ((eq? m 'empty-queue?) empty-queue?)
 86             (else (error "Undefined operation" m))))
 87     dispatch))
 88 
 89 (define (make-queue3)
 90     (let ((front-ptr '())
 91           (rear-ptr '()))
 92         (define (insert-queue! item)
 93             (cond ((empty-queue?)
 94                     (let ((init-list (list item)))
 95                         (set! front-ptr init-list)
 96                         (set! rear-ptr init-list)
 97                         front-ptr))
 98                   (else
 99                     (let ((new-item (list item)))
100                         (set-cdr! rear-ptr new-item)
101                         (set! rear-ptr new-item)
102                         front-ptr))))
103         (define (delete-queue!)
104             (cond ((empty-queue?)
105                     (error "DELETE! called with an empty queue" front-ptr))
106                   (else
107                     (set! front-ptr (cdr front-ptr))
108                     front-ptr)))
109         (define (empty-queue?)
110             (null? front-ptr))
111         (define (dispatch m)
112             (cond ((eq? m 'insert-queue!)
113                     insert-queue!)
114                   ((eq? m 'delete-queue!)
115                     (delete-queue!))
116                   ((eq? m 'empty-queue?)
117                     (empty-queue?))
118                   (else
119                     (error "Unknow operation -- DISPATCH" m))))
120         dispatch))
121 
122 ;(define q2 (make-queue3))
123 ;((q2 'insert-queue!) 3)
124 ;((q2 'insert-queue!) 4)
125 
126 ;(define q2 '(1))
127 ;(set-cdr! q2 (list 2))
128 ;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））

转载于:https://www.cnblogs.com/tclan126/p/6624561.html