Question

Explain this lisp recursive function that gets the length of a list.

; a recursive function to find the lenght of the list
(defun len (list)
(if list
(1+ (len (cdr list)))
0))

What does the 1+ and the 0 at the end do?

Answer 1

(defun len (list)
(if list
(1+ (len (cdr list))) ; Here if list is not empty we add 1 and call for next element of list cdr go for next element
0)) ; if list is empty we return 0

Example :

; a recursive function to find the lenght of the list
(defun len (list)
(if list
(1+ (len (cdr list)))
0))
(write (len (list 1 2 3)))

/* OUTPUT */

3

Now, len(list 3) become 1

so, 1 + len(list 3) = 2

which returns to len(list 2,3) becomes 2

so,

1 + len(list 2,3) becomes 3

now , len(list 1 2 3) becomes 3 and it returns from where it called

/* PLEASE UPVOTE */