# weima learns to program

my attempt to do the exercises in sicp.

## Friday, November 27, 2009

### sicp exercise 3.22

; Exercise 3.22.  Instead of representing a queue as a pair of pointers, we can build a queue as a procedure with local state. The local state will consist of pointers to the beginning and the end of an ordinary list. Thus, the make-queue procedure will have the form
;
; (define (make-queue)
;   (let ((front-ptr ...)
;        (rear-ptr ...))
;           <definitions of internal procedures>
;        (define (dispatch m) ...)
;              dispatch))
;
; Complete the definition of make-queue and provide implementations of the queue operations using this representation.

(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 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))
(else
(set-cdr! rear-ptr new-pair)
(set-rear-ptr! new-pair)))))
(define (delete-queue!)
(cond ((empty-queue?)
(error "DELETE! called on empty queue"))
(else
(set-front-ptr! (cdr front-ptr)))))
(define (print-queue)
(display front-ptr))
(define (dispatch m)
(cond ((eq? m 'empty-queue? )  empty-queue?)
((eq? m 'insert-queue!) insert-queue!)
((eq? m 'delete-queue!) delete-queue!)
((eq? m 'front-queue  )   front-queue)
((eq? m 'print-queue  )   print-queue)
(else (error "ERROR"))))
dispatch))

(define (empty-queue? queue) ((queue 'empty-queue?)))
(define (insert-queue! queue item) ((queue 'insert-queue!) item))
(define (delete-queue! queue) ((queue 'delete-queue!)))
(define (front-queue queue) ((queue 'front-queue)))
(define (print-queue queue) ((queue 'print-queue)))

(define q1 (make-queue))

(insert-queue! q1 'a)
(insert-queue! q1 'b)

(print-queue q1) (newline)

(delete-queue! q1)
(delete-queue! q1)

(print-queue q1) (newline)