99 Lisp Problems

Ran across this website which has 99 Lisp problems, can't do them all, but I can do a few. They even include solutions to the problems so after you do it you can compare yours with theirs.

Ninety-Nine_Lisp_Problems


So here are my solutions to the first six.

1) Find the last box of a list.

(defun my-last (lst)
    (if (not (listp lst))
        nil
        (if (consp (cdr lst))
     (my-last (cdr lst))
     (car lst))))

2) Find the last but one box of a list.

(defun my-but-last (lst)
    (if (not (listp lst))
        nil
        (if (equal 2 (list-length (cdr lst)))
     (cdr lst)
     (my-but-last (cdr lst)))))

3) Find the K'th element of a list.

(defun element-at (lst num)
    (if (null lst)
        nil
        (if (eql 1 num)
     (car lst)
     (element-at (cdr lst) (- num 1)))))

4) Find the number of elements of a list.

(defun my-length (lst)
    (if (null lst)
        0
        (+ 1 (my-length (cdr lst)))))

5) Reverse a list.

(defun my-reverse (lst)
    (if (not (consp (cdr lst)))
        (append (cdr lst) (cons (car lst) ()))
        (append (my-reverse (cdr lst)) (cons (car lst) ()))))

6) Find out whether a list is a palindrome.

;; At first I had a big complicated way and then I realized...
(defun palindromep (lst)
    (equal (reverse lst) lst))