SICP sicp 109: 3.22 exercises 1093.22 and

SICP sicp 109: 3.22 exercises 1093.22 and
Exercise 3-22 Original

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.

Analysis

The local state in this question is composed of the start and end pointers pointing to a regular table. And insert-queue! And delete-queue! Nested in the entire process. Use dispatch to call these functions.

(define (make-queue)    (let ((front-ptr '())          (rear-ptr '()))   (define (empty-queue?)      (null? front-ptr))   (define (insert-queue! item)       (cond ((empty-queue?)              (let ((init-list (list item)))                  (set! front-ptr init-list)                  (set! rear-ptr init-list)                  front-ptr))             (else              (let ((new-item (list item)))                  (set-cdr! rear-ptr new-item)                  (set! rear-ptr new-item)                  front-ptr))))   (define (delete-queue!)       (cond ((empty-queue?)              (error "DELETE! called with an empty queue" queue))             (else              (set! front-ptr (cdr front-ptr))              front-ptr)))   (define (dispatch m)      (cond ((eq? m 'insert-queue!)              insert-queue!)            ((eq? m 'delete-queue!)              (delete-queue!))            ((eq? m 'empty-queue?)              (empty-queue?))            (else             (error "Unknown operation -- DISPATCH" m))))    dispatch))

Test

(define q3 (make-queue))        ;Value: q3((q3 'insert-queue!) 'a)                ;Value 15: (a)((q3 'insert-queue!) 'b);Value 16: (a b)(q3 'delete-queue!)                  ;Value 17: (b)(q3 'delete-queue!);Value: ()(q3 'empty-queue?)                   ;Value: #t

Supplement

Because insert-queue! There are parameters, so no parentheses need to be added in dispatch; otherwise, an error is reported.

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