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Function LIST-LENGTH

Syntax:

list-length list => length

Arguments and Values:

list---a proper list or a circular list.

length---a non-negative integer, or nil.

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)))
 (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:

length

Notes:

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))))