CSC151 2009F, Class 33: Geometric Art Through Numeric Recursion

Admin:
* Are there questions on Exam 2?
  * Yes, you already have five points of extra credit.  That's what I get
    for adding examples at the last minute.
* I will reserve time at the start of class for pre-quiz questions.
* My oldest son has a sequences problem that I can't answers
  * Consider the sequence 1, 15, 71, 295, 1191, 4775, ...
  * One defn: a[1] = 1, a[2] = 15, a[n] = 5*a[n-1] - 4*a[n-2]
  * Another: a[1] = 1, a[n] = 4*a[n-1] + 11
  * A third: a[1] = 1, a[n] = a[n-1] + 14*4^(n-2)
  * All three seem to define the same sequence.  Why are they equivalent?
* After you have finished the quiz, start the lab.

Overview:
* Parallel lines, Concentric Circles, and Beyond.

Questions on the Exam
* For problem 3, should we use our answers to problems 1 and 2? 
  * If at all possible.  (If you were not able to solve problems 1 and 2,
    you should still do your best to answer problem 3.)

Questions in preparation for the Quiz
* Is letrec just "let for recursive local procedures"?
  * Yes, it's fine to think of it that way
* And what is named let?
  * Another mechanism for local recursive procedures
  * Shorter than letrec
  * Some people find it clearer
  (let NAME ((PARAM1 INITIAL_VALUE1)
             (PARAM2 INITIAL_VALUE2))
    BODY)
  * Scheme thinks of (let ((NAME VAL) (NAME VAL)) ...)
                 and (let NAME ((PARAM VAL) (PARAM VAL)) ...)
    as being TWO DIFFERENT THINGS