CSC151 2007S, Class 43: Deep Recursion, Revisited

Admin:
* Are there questions about Assignment 15?
  Biologists and Biological Chemists everywhere say "It's fun and it's
  extra credit, give it a try."
* Are there questions about The Project?
* Perhaps not so suprisingly, there are no Thursday extras this week.
* Reading for tomorrow: Association Lists.
* Reading for Monday: Higher-Order Procedures.

Overview:
* Lists, revisited.
* Trees, introduced.
* Deep recursion, considered.

Review of Friday:
* Talked about Pears (or maybe pairs)
* Pairs are used to make lists

BAD CODE FOR FIND A COLOR IN A TREE
(define ctree.contains?
  (lambda (ctree color)
    ; Is the car of the first node color?
    (cond 
      ; If the ctree is itself a color and equals
      ; the color, return true
      ((and (rgb? ctree) (equal? ctree color))
       #t)
      ((and (not (pair? ctree)) (not (equal? ctree color)))
       #f)
      ((equal? (car ctree) color)
       #t)
      ((pair? (car ctree))
       (ctree.contains? (car ctree) color))
      (else (ctree.contains? (cdr ctree) color)))))
          a
;;; Purpose:
;;;   Determine if ctree contains color.
(define ctree.contains?
  (lambda (ctree color)
    (if (not (pair? ctree))
        (equal? ctree color)
        (or (ctree.contains? (car ctree) color)
            (ctree.contains? (cdr ctree) color)))))

; Simplest trees: One color
(define ctree1 color.red)

; Two element tree
(define ctree2 (cons color.blue color.black))

; Three element tree
(define ctree3 (cons color.grey (cons color.black color.white)))

; Four element tree
(define ctree4 (cons (cons color.cyan color.semi-sweet-chocolate)
                     (cons color.midnight-blue color.dark-brown)))