Question

I want to create a function in lisp that receives a number and a list of pairs and iterates through the list of pairs and removes the ones in which the result of the division between the first element of the pair, and the second element of the same pair is different from the number passed as an argument. In the end it returns a list with only the ones in which the result of the division is the same.

I have the following code so far:

(defun retira-terco(num l1)
(cond ((null l1) ())
    ((not (equal num (/ (car(first l1)) (cdr(first l1)))))
                        (retira-terco num (rest l1)))
    (t (cons (first l1) (retira-terco num (rest l1))))))

When I try to run this example with a real example I get the following error:

Error: `(1)' is not of the expected type `NUMBER'

What am I doing wrong?

Answer 1

The problem with your code is in this line:

(/ (car(first l1)) (cdr(first l1)))

(car (first l1)) evaluates to a number, but (cdr (first l1)) evaluates to a list. You probably meant (cadr (first l1)).

That said, this code isn’t that great from a lispiness point of view. You have a condition you want to filter on. Use higher order programming to express that more like this:

(defun foo (num lst)
   (remove-if (lambda (item) 
                 (equal num 
                        (/ (car item) 
                           (cadr item)))) 
              lst)))