Question

What I mean by my question is:

Lets say there is a list (a b c d)

and I want to make sure there are 4 elements in there before proceeding with my code.

if the list says (a b c)

i want it to return nil, because it’s not what I want.

Can anyone help =)?

Answer 1

Use the LENGTH function:

(when (>= (length list) 4)
  ;; rest of your code
)

If you need a method that doesn’t scan the entire list just to see if there are N element, you can use this function:

(defun length-at-least (list n)
  (cond ((zerop n) t)
        (list (length-at-least (cdr list) (1- n)))
        (t nil)))

Then you can do:

(when (length-at-least list 4)
  ;; rest of your code
)