Question

How do I find, in Lisp, how many different elements does a list have?

Answer 1

(length (remove-duplicates <your list>))

The shortest way of doing it, but it is also possible to do it in a single pass, like so:

(defun count-distinct (list)
  (let ((table (make-hash-table)))
    (dolist (i list (hash-table-count table))
      (setf (gethash i table) t))))