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Function LIST-LENGTH

Syntax

  • list-length listlength

Arguments and Values

Description

Returns the length of list if list is a proper list. Returns nil if list is a circular list.

Examples

(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)))

CIRCULAR-LIST

(list-length (circular-list 'a 'b))

NIL

(list-length (circular-list 'a))

NIL

(list-length (circular-list))

0

Side Effects

None.

Affected By

None.

Exceptional Situations

Should signal an error of type type-error if list is not a proper list or a circular list.

See Also

Example Implementation

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. () ;; If fast pointer hits the end, return the count. (when (endp fast) (return n)) (when (endp (cdr fast)) (return (1+ n))) ;; 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))))

Notes

None.