;
; examples from class 9/7/99
;

;
; flattening a list
;
(define (flatten l)
  (if (null? l)
      '()
      (append (if (list? (car l))
		  (flatten (car l))
		  (list (car l)))
	      (flatten (cdr l)))))

;
; expression tree evaluation
;
(define tree1 '(add (times 3 8) (divide 10 (minus 8 3))))

(define (eval-tree t)
  (if (number? t) 
      t
      (case (car t)
	((add) (+ (eval-tree (cadr t)) (eval-tree (caddr t))))
	((times) (* (eval-tree (cadr t)) (eval-tree (caddr t))))
	((minus) (- (eval-tree (cadr t)) (eval-tree (caddr t))))
	((divide) (/ (eval-tree (cadr t)) (eval-tree (caddr t)))))))

;
; exercise 6.34 from grillmeyer
;
; takes a list and returns the last positive number in the list or #f if
; none exist
;
(define (pos-finder l)
  (pos-finder-helper l #f))

(define (pos-finder-helper l result)
  (if (null? l)
      result
      (pos-finder-helper (cdr l)
			 (if (and (number? (car l)) 
				  (positive? (car l)))
			     (car l)
			     result))))

;
; the other version we saw in class (find the first positive number
; starting from the back)
;
(define (pos-finder2 l)
  (pos-finder2-helper (reverse l)))

((define (pos-finder2-helper l)
  (if (null? l)
      #f
      (if (and (number? (car l)) (positive? (car l)))
	  (car l)
	  (pos-finder2-helper (cdr l)))))