如何在Scheme中使用计数器

Question

I'm trying to write a program where a doctor sees only 5 patients and then the program ends. Right now it's not ending and it keeps asking for the next patient. Ignore the else in the doctor-driver-loop procedure. I add one to count in that procedure but I guess it keeps going back to zero every time. How do I fix this?

(define count 0)

(define (new-patient counter)
  (if (= counter 5) (write-line 'doctor has seen 5 patients today so the day is now over)
    (visit-doctor (ask-patient-name))))

(define (doctor-driver-loop name earlier-responses)
  (newline)
 (write '**)
  (let ((user-response (read)))
   (cond ((equal? user-response '(goodbye))
         (write-line (list 'goodbye name))
         (write-line '(see you next week))
         (new-patient (+ 1 count)))
      (else (write-line (reply (responses-list earlier-responses user-response) user-response))
            (doctor-driver-loop name (responses-list earlier-responses user-response))))))

(define (visit-doctor name)
  (write-line (list 'hello name))
  (write-line '(what seems to be the trouble?))
  (doctor-driver-loop name initial-earlier-response))

Answer 1

Try replacing this line:

(new-patient (+ 1 count))

With these two lines:

(set! count (+ 1 count))
(new-patient count)

In your current code, count will always have a value of zero for each iteration of doctor-driver-loop , because its value was never updated - the (+ 1 count) part adds one to count without changing count , and the next time doctor-driver-loop get called, count will be zero again.

Be aware that this is a quick fix, but not the ideal solution. For starters, count should not be defined as a global variable, instead it should be a parameter to the driver loop that gets passed with an initial value of zero and incremented when calling new-patient with each patient that gets processed.