99 lisp problems

99.lisp

;; 99 lisp problems
;; http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html

;; P01
;; * (my-last '(a b c d))
;; (D)
(defun my-last (lst)
  (cond
    ((null lst) nil)
    ((null (cdr lst)) (list (car lst)))
    (t (my-last (cdr lst)))))

;; P02.
;; * (my-but-last '(a b c d))
;; (C D)
(defun my-but-last (lst)
  (cond
    ((null lst) nil)
    ((<= (length lst) 2) lst)
    (t (my-but-last (cdr lst)))))

;; P03.
;; * (element-at '(a b c d) 3)
;; C
(defun element-at (lst n)
  (cond
    ((null lst) nil)
    ((= n 1) (car lst))
    (t (element-at (cdr lst) (1- n)))))

;; P04
(defun number-elements (lst)
  (if (null lst)
    0
    (1+ (number-elements (cdr lst)))))

;; P05