47 lines
1.2 KiB
Racket
47 lines
1.2 KiB
Racket
|
#lang sicp
|
||
|
|
||
|
(define (make-queue)
|
||
|
(let ((front-ptr '())
|
||
|
(rear-ptr '()))
|
||
|
(define (front)
|
||
|
(cond ((empty?)
|
||
|
(error "FRONT called with empty queue"))
|
||
|
(else
|
||
|
(car front-ptr))))
|
||
|
(define (insert! item)
|
||
|
(let ((new-pair (cons item '())))
|
||
|
(cond ((empty?)
|
||
|
(set! front-ptr new-pair)
|
||
|
(set! rear-ptr new-pair))
|
||
|
(else
|
||
|
(set-cdr! rear-ptr new-pair)
|
||
|
(set! rear-ptr new-pair)))))
|
||
|
(define (delete!)
|
||
|
(cond ((empty?)
|
||
|
(error "DELETE! called with empty queue"))
|
||
|
(else
|
||
|
(set! front-ptr (cdr front-ptr)))))
|
||
|
(define (dispatch m)
|
||
|
(cond ((eq? m 'empty?)
|
||
|
(null? front-ptr))
|
||
|
((eq? m 'front) (front))
|
||
|
((eq? m 'insert!) insert!)
|
||
|
((eq? m 'delete!) (delete!))
|
||
|
((eq? m 'print)
|
||
|
(display front-ptr)
|
||
|
(newline))
|
||
|
(else
|
||
|
(error "Unknown method: " m))))
|
||
|
dispatch))
|
||
|
|
||
|
(define q1 (make-queue))
|
||
|
(q1 'print)
|
||
|
((q1 'insert!) 'a)
|
||
|
(q1 'print)
|
||
|
((q1 'insert!) 'b)
|
||
|
(q1 'print)
|
||
|
(q1 'front)
|
||
|
(q1 'print)
|
||
|
(q1 'delete!)
|
||
|
(q1 'print)
|