list,sum,lisp,iteration,clisp , Lisp Function that Returns a Sum

## Question:

Tag: list,sum,lisp,iteration,clisp

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.

``````(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

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