trying to implement my own stack object in DrRacket

Question

Here is what I have so far, but I am unable to run it since there is an error so I don't know if it works, I am trying to create my own stack object using Object Oriented programming in DrRacket, I am using a dispatch method to call the different procedures I have if they are invoked properly. For example first I would create a stack and then I would push and pop to the stack and also be able to print it.

(define (make-stack)
  (define my-stack '())

(define (pop)
 (define (pop-helper my-stack)
  (let ((result (car  my-stack))))
   (set!  my-stack (cdr  my-stack))
  result)
 (pop-helper  my-stack))

(define (push)
(define (push-helper x  my-stack)
  (set!  my-stack (cons x  my-stack)))
(push-helper x  my-stack))
(define (empty?)
 (define (empty-helper  my-stack)
   (if (null?  my-stack) #t
      #f))
  (empty-helper my-stack))
(define (print)
(define (print-helper  my-stack)
  (if (empty?) '()
      (print (cdr  my-stack))))
  (print-helper  my-stack))

(define (dispatch method)
(cond 
  ((eq? method 'pop) pop)
  ((eq? method 'push) push)
  ((eq? method 'print) print)
  (else (lambda() (display "Unknown Request: ")(display method)(newline)))))
    dispatch)

thanks in advance!

What is the error you are getting? – ben rudgers Dec 06 '17 at 06:13 — ben rudgers, Dec 06 '17 at 06:13

Óscar López · Accepted Answer · 2017-12-06T07:33:11.820

0

You have a couple of syntax errors in two procedures, and you must not pass the stack as a parameter, it'll shadow the my-stack global variable. Here's the fix:

(define (pop)
  (let ((result (car my-stack))) ; `let` brackets were off
    (set! my-stack (cdr my-stack))
    result))

(define (push x) ; `x` parameter was missing
  (set! my-stack (cons x my-stack)))

edited Dec 06 '17 at 07:33

answered Dec 06 '17 at 06:29

Óscar López

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this did fix the errors that I was getting, but it seems like something is not working because to test it I made a stack and pushed items to it, and when I print, it prints an empty list, is there something wrong with my print procedure? – Dec 06 '17 at 06:42
@ScrappyMontana yup, it's broken. `empty?` requires an argument, and although you're recurring over the `cdr` part of the stack, you're not actually _displaying_ the `cdr`. Don't sweat over it, your stack is a list, so just `(display my-stack)` – Óscar López Dec 06 '17 at 06:46
@ Óscar López where do I (display my-stack) ? – Dec 06 '17 at 07:07
That'd be the whole body of `print` – Óscar López Dec 06 '17 at 07:20
I think the push is broken, nothing gets pushed to my stack – Dec 06 '17 at 07:23
@ScrappyMontana it's all those helpers. You're assigning a _parameter_ called `my-stack`, not the global variable `my-stack`. I updated my answer, and that's it for today, it's really late around here ;) – Óscar López Dec 06 '17 at 07:34
1

fixed it, silly me, I was thinking properly since it's late here too. Thank you so much and you have a good night! – Dec 06 '17 at 09:08

trying to implement my own stack object in DrRacket

1 Answers1