list—a proper list or a circular list.
length—a non-negative integer, or nil
.
Returns the length of list if list is a proper list.
Returns nil
if list is a circular list.
(list-length '(a b c d)) → 4 (list-length '(a (b c) d)) → 3 (list-length '()) → 0 (list-length nil) → 0 (defun circular-list (&rest elements) (let ((cycle (copy-list elements))) (nconc cycle cycle))) (list-length (circular-list 'a 'b)) → NIL (list-length (circular-list 'a)) → NIL (list-length (circular-list)) → 0
Should signal an error of type type-error
if list is not a proper list or a circular list.
list-length
could be implemented as follows:
(defun list-length (x) (do ((n 0 (+ n 2)) ;Counter. (fast x (cddr fast)) ;Fast pointer: leaps by 2. (slow x (cdr slow))) ;Slow pointer: leaps by 1. (nil) ;; If fast pointer hits the end, return the count. (when (endp fast) (return n)) (when (endp (cdr fast)) (return (+ n 1))) ;; If fast pointer eventually equals slow pointer, ;; then we must be stuck in a circular list. ;; (A deeper property is the converse: if we are ;; stuck in a circular list, then eventually the ;; fast pointer will equal the slow pointer. ;; That fact justifies this implementation.) (when (and (eq fast slow) (> n 0)) (return nil))))