;;
;; some simple lisp examples 
;;
;; a recursive factorial routine, 
;;    base case: factorial(1) = 1, 
;;    recursive case: factorial(N) = N * factorial(N-1)
;;
(defun myfact (num)
  (cond ((<= num 1) 1)
        (t (* num (myfact (- num 1))))
  )
)

; (myfact 4)
; (myfact 12)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; a recursive member function - is atm a member of lst
;;
(defun mymember (atm lst)
  (cond ((null lst) nil)
        ((eq atm (car lst)) lst)
        (t (mymember atm (cdr lst)))
  )
)

; (mymember 'd '(a b c d e f g))
; (mymember 'h '(a b c d e f g))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; a recursive list reversal - given (a b c) return (c b a)
;;
(defun myreverse (lst)
  (cond ((= 1 (length lst)) lst)
        (t (append (myreverse (cdr lst)) (list (car lst))))
  )
)

; (myreverse '(a b c d e f g))
; (myreverse '((a b c) (d e f g)))

;; another version of list reversal, using an accumulator
variable
;;
(defun myrev (lst &optional acc)
  (cond ((atom lst) acc)
        (t (myrev (cdr lst)  (cons (car lst) acc)))
  )
)

;;;;;;;the next routine is not tail recursion;;;;;;;;;;;;;;;;;;;;;;;

;; a recursive double-odd, double the odd values, leave the even
values alone
;;
(defun double-odd (lst)
  (cond ((null lst) nil)
        ((oddp (car lst)) (cons (* 2 (car lst)) (double-odd (cdr lst))))
        (t (cons (car lst) (double-odd (cdr lst))))
  )
)

; (double-odd '(1 2 3 4 5 6 7 8 9))

;;;;;;;mapping across lists;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;(mapcar 'evenp '(1 2 3 4 5 6 7 8 9))