Lisp Function that Returns a Sum

Question

I am trying to write a weird function, so bear with me here. This function should take a list L as a parameter and have a sum variable. If L is not a list, it should return nil. Otherwise, it should iterate through each element of the list and do the following:

• If the element is a number and less than zero, it should subtract 1 from the sum.
• If the element is a number and greater than zero, it should add 1 to the sum.
• if the element is 0 or not a number, then it should add 0 to the sum.

Here's the code I have, but it returns 0 regardless of arguments passed in:

(defun sigsum (L)
  (let ((sum 0))                   ;;variable sum
  (if (not (listp L))              ;;if L is not a list
      nil                          ;;return nil
      (dotimes (i (length L))      ;;otherwise loop through L
        (if (numberp (elt L i))    ;;if each element is a number
            (if (< (elt L i) 0)    ;;if is less than 0 subtract 1 from sum
                (- sum 1)
            (if (> (elt L i) 0)    ;;if greater than 0 add 1 to sum
                (+ sum 1))
            (+ sum 0))             ;;else add 0 to sum
          (+ sum 0)))              ;;not a number so add 0 to sum
  )
  sum)                             ;;return sum
)

As always, any help is greatly appreciated.

Answer 1

(defun discrete (lis)
 (cond
  ((and (listp lis) (not (equal lis nil)))
   (let ((sum 0))
    (loop for item in lis do
     (cond ((or (not (numberp item)) (equal 0 item)) t)
      ((and (numberp item) (> item 1)) (setf sum (+ 1 sum)))
      ((and (numberp item) (< item 1)) (setf sum (- sum 1)))))
    sum))))

Usage:(discrete '(-1 2 3 0 2 2 -1 2 34 0 -1))=> 3

(discrete '(-4 a b))=> -1

(discrete '()) => NIL

(discrete '(a s d f))=> 0