>: expects a real, given (void)

Question

I am currently struggling with creating a list that finds if all the numbers within the list are equal to each other:

(define (list-all-equal lon) (cond [(empty? lon) lon]
                                  [(empty? (rest lon)) (first lon)]
                                  [(empty? (rest (rest lon))) (first lon)]
                                  [(equal? (first lon) (list-all-equal(rest lon))) (write #true)]
                                  [(> (first lon) (list-all-equal(rest lon))) (write #false)]
                                  [(< (first lon) (list-all-equal(rest lon))) (write #false)]
                                  [else (list-all-equal (rest lon))]))

However, I keep on receiving the error: >: expects a real, given void. is there a way to solve the error?

Answer 1

The problem is that you're calling write as part of the recursion, and write "returns" void, which will lead to something like this:

(> some-number (void))

Never use write to return a value from your procedures! We want to return the results, not to display them on the console. And for this case you have to implement your procedure in such a way that, if it finds a different number, it stops comparing - the result is false and we shouldn't continue. A good strategy would be to pass along the first element and compare the rest against it:

(define (list-all-equal lst)
  (if (empty? lst)
      #t
      (list-all-equal-helper (first lst) (rest lst))))

(define (list-all-equal-helper ele lst)
  (cond ((empty? lst) #t)
        ((not (= (first lst) ele)) #f)
        (else (list-all-equal-helper ele (rest lst)))))        

Or, we could use apply :)

(define (list-all-equal lst)
  (apply = lst))

Either way, it works as expected:

(list-all-equal '(1 2 3))
=> #f
(list-all-equal '(1 1 1))
=> #t