# Function LIST-LENGTH

### Syntax

• list-length listlength

### 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

```

None.

None.

### Exceptional Situations

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

### 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))))

```

None.