Chapter 3, Modularity, Objects, and State
Exercise 3.22
This version of code looked simple as the operations like insert-queue
, delete-queue
need not to be passed with the queue parameter. It is just available to them!
For using it, I think it is individual choice whether one likes calling the operation by passing the object/context. Or by sending a message/operation to the object.
Note that I returned front-ptr(inside insert and delete operations) instead of dispatch, as it simplified printing. Perhaps it is better to return dispatch because it can help in combining multiple operations in a single statement.
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#lang sicp
(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")
(car 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-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"))
(else
(set-front-ptr! (cdr front-ptr))
front-ptr)))
(define (dispatch m)
(cond ((eq? m 'empty-queue?) (empty-queue?))
((eq? m 'front-queue) (front-queue))
((eq? m 'insert-queue!) insert-queue!)
((eq? m 'delete-queue!) (delete-queue!))
(else (error "Invalid operation: " m))))
dispatch))
Example/Output:
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> (define q1 (make-queue))
> (display ((q1 'insert-queue!) 'a))
(a)
> (display ((q1 'insert-queue!) 'b))
(a b)
> (display (q1 'front-queue))
a
> (display (q1 'delete-queue!))
(b)
> (display (q1 'delete-queue!))
()
> (display ((q1 'insert-queue!) 'hello))
(hello)
> (display ((q1 'insert-queue!) 'world))
(hello world)
>