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list-length list ⇒ length
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))))