問題3.22

問題3.22 – SICP（計算機プログラムの構造と解釈）その120

2009年03月06日

(define (make-queue)
  (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 (front-queue)
         (if (empty-queue?)
             (error "FRONT called to an empty queue")
             (car front-ptr)))
       (define (rear-queue)
         (if (empty-queue?)
             (error "REAR called to on empty queue")
             (car rear-ptr)))
       (define (insert-queue! item)
         (let ((new-pair (cons 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 to an empty queue"))
               (else
                 (set-front-ptr! (cdr front-ptr)))))
       (define (print-queue)
         front-ptr)
       (define (dispatch m)
         (cond ((eq? m 'insert-queue!) insert-queue!)
               ((eq? m 'delete-queue!) delete-queue!)
               ((eq? m 'front) front-queue)
               ((eq? m 'rear) rear-queue)
               ((eq? m 'print-queue) print-queue)
               (else
                 (error "Undefined operation -- MAKE-QUEUE" m))))

       dispatch))

(define q (make-queue))
((q 'insert-queue!) 'a)
gosh> (a)
((q 'insert-queue!) 'b)
gosh> (a b)
((q 'insert-queue!) 'c)
gosh> (a b c)
((q 'delete-queue!))
gosh> (b c)
((q 'delete-queue!))
gosh> (c)
((q 'insert-queue!) 'd)
gosh> (c d)
((q 'delete-queue!))
gosh> (d)
((q 'delete-queue!))
gosh> ()
((q 'delete-queue!))
gosh> *** ERROR: DELETE! called to an empty queue

計算機プログラムの構造と解釈

posted with amazlet at 08.11.07

ジェラルド・ジェイサスマンジュリーサスマンハロルドエイブルソン
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