Question

I am receiving the following error in my Lisp code:

value (PROBLEM1 (+ N 1)) is not of the expected type NUMBER.

The intent of the code is to sum all numbers up to 1000 that are divisible by 3 or 5.

(defun problem1 (n)
  (if (< n 1000)
      (if (or (= 0 (mod n 3)) (= 0 (mod n 5))) 
      (apply '+ '(n (problem1 (+ n 1)))) 
      (apply '+ '(0 (problem1 (+ n 1)))))
   0))

I realize that the error is probably because problem1 is returning a list – but when I trace the values in my head the function should work correctly. Therefore, can someone explain to me if I am misusing (or missing) an apostrophe?

Answer 1

Rather than missing a ', you have too many. When you write

(apply '+ '(n (problem1 (+ n 1))))

You are trying to apply + to a list containing the symbol n and the unevaluated list (problem1 (+ n 1)). This is because you have quoted the argument list, and ' prevents evaluation. What you actually want is to use list, e.g.

(apply '+ (list n (problem1 (1+ n))))