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##(link2sicp "book-Z-H-22.html#%_thm_3.22" "Exercise 3.22")
解答例
(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 with an empty queue")
front-ptr))
(define (insert-queue! item)
(let ((new-pair (cons item '())))
(cond ((empty-queue?)
(set-front-ptr! new-pair)
(set-rear-ptr! new-pair)
(front-queue))
(else
(set-cdr! rear-ptr new-pair)
(set-rear-ptr! new-pair)
(front-queue)))))
(define (delete-queue!)
(cond ((empty-queue?)
(error "DELETE! called with an empty queue"))
(else
(set-front-ptr! (cdr front-ptr))
(if (empty-queue?)
'empty-queue
(front-queue)))))
(define (dispatch m)
(cond ((eq? m 'insert!) insert-queue!)
((eq? m 'delete!) (delete-queue!))
((eq? m 'print) (front-queue))
(else
(error "Undefined operation -- DISPATCH" m))))
dispatch))
(define (insert-queue! queue item) ((queue 'insert!) item))
(define (delete-queue! queue) (queue 'delete!))
(define (print-queue queue) (queue 'print))
実行例
gosh> (define q (make-queue)) q gosh> (print-queue q) *** ERROR: FRONT called with an empty queue gosh> (insert-queue! q 'a) (a) gosh> (print-queue q) (a) gosh> (insert-queue! q 'b) (a b) gosh> (print-queue q) (a b) gosh> (delete-queue! q) (b) gosh> (print-queue q) (b) gosh> (delete-queue! q) empty-queue gosh> (print-queue q) *** ERROR: FRONT called with an empty queue
--hidenao
コード
##(sicp-answer-code "ex-3.22.scm")
