P17 - Split a list into two parts; the length of the first part is given

lisp

;;; A function that receives a list and a position and returns ;;;
;;; another lists two lists: the first containing the ;;;
;;; elements up to and including the position indicated, and the ;;;
;;; second containing the remainder.

(defun split (pos list)
  (list (split-up-to pos list) (split-after pos list))
  )

;;; never taking the first part of the LIST, up to' POS
;;; Number of positions starts from 1
;;
;;; Extreme cases:
;;; if POS <= 0, returns empty list
;;; if POS > list length, returns the entire list

(defun split-up-to (pos list)
    (cond
     ((<= pos 0) ())
     ((null list) list)
     (t (cons (car list) (split-up-to (cdr list) (1-pos)))
     )
    )

;;; A function that receives a list and a position and returns ;;;
;;; another list containing the elements after position ;;;
;;; indicated (positions numbered from 1) ;;
;;
;;; Extreme cases:
;;; if POS <= 0, returns the entire list
;;; if POS > list length, returns empty list

(defun split-after (pos list)
    (cond
     ((<= pos 0) list)
     ((null list) ())
     (t (split-after (cdr list) (1-pos))
     )
    )