Scheme: Not a procedure (Dr. Racket)

Question

I'm running this program in Dr. Racket using R5RS scheme and am getting this error on the line (+ 1 IntDivide((- x y) y)):

"application: not a procedure; expected a procedure that can be applied to arguments given: 5 arguments...:"

The procedure is supposed to return the quotient of the division between two integers using subtraction. Since this is a homework problem, I'm not going to ask whether my solution is correct (I can debug that later), but rather what is causing this error. It seems to be commonly caused by excess brackets, but I can't seem to find them. Any help would be appreciated.

(define IntDivide (lambda (x y)
  (if (eqv? (integer? x) (integer? y))
    (begin
      (if (= y 0)
        (begin
          (write "Can't divide by zero") (newline)
          -1
        )
      )

      (if (= (- x y) 0)
        1
      )

      (if (< x y)
        0
      )

      (if (> x y)
        (+ 1 IntDivide((- x y) y))
      )
    )
  )
  (write "Please only input integers")
))

Thanks in advance!

Right you are! I suppose after staring for too long you get tunnel vision. — Knight Artorias, Apr 09 '17 at 20:38
I've been writing Clojure for long enough that as soon as I see `((`, I'm immediately suspicious. Valid code can contain 2 opening brackets in a row, but 90% of the time, it's a mistake. Watch out for that. — Carcigenicate, Apr 09 '17 at 21:05

score 3 · Answer 1 · answered Apr 09 '17 at 20:42

3

In addition to moving the operator inside the parens, you also need to replace the if with a cond:

(define IntDivide
  (lambda (x y)
    (if (eqv? (integer? x) (integer? y))
      (cond ((= y 0) (write "Can't divide by zero")
                     (newline)
                     -1)
            ((= x y) 1)
            ((< x y) 0)
            ((> x y) (+ 1 (IntDivide (- x y) y))))
      (write "Please only input integers"))))

The way you have it now, with the interior if expressions, won't work because they don't automatically return. They just evaluate and then the result gets thrown away.

answered Apr 09 '17 at 20:42

Brendan Cannell

679
4
11

Thanks for the response. I solved the problem yesterday. The ifs were remnants from an old attempt, but it's good to know about how the ifs work. – Knight Artorias Apr 10 '17 at 21:20
Why stop there? The one `if` could be the first term of the `cond` if you negate it. – Sylwester Apr 10 '17 at 22:57

score 2 · Accepted Answer · answered Apr 09 '17 at 20:31

2

Call IntDivide the same way you would any other function.

(+ 1 (IntDivide (- x y) y))

answered Apr 09 '17 at 20:31

Bill the Lizard

398,270
210
566
880

Major face palm moment there. Thanks for the quick response. – Knight Artorias Apr 09 '17 at 20:40
1

@KnightArtorias I can't tell you how many times I've done this when going back and forth between Scheme and other languages. – Bill the Lizard Apr 09 '17 at 20:41
@KnightArtorias Also, you do need to switch to a `cond` structure as mentioned in the answer by Brendan. I ran your code with just this fix, and it returns `""Please only input integers"` for most inputs. – Bill the Lizard Apr 09 '17 at 20:51
1

@BilltheLizard When I try to help someone with python, I always start everything with a `(`. It's hard to revert. – Carcigenicate Apr 09 '17 at 23:43

Scheme: Not a procedure (Dr. Racket)

2 Answers2