CSC151 2007S, Class 27: Preconditions, Revisited

* I will continue to reserve time at the start of class for topics you 
  would like to discuss.

Admin:
* Quiz 5 returned.  High scores count.  Low scores do not.
* Are there questions on Exam 2?  (Q&A appear below.)
* EC for Saturday's concert, the weekend play, and Mary B James.
  Russian house's International Women's day celebration on Sat. at 2pm
  1217 Park St.  
  Flute studio hosts amazing musical duo (and lots of other stuff) on
  Saturday in Bucksbaum
* Assignment 7 (due after break) is now available.  
  It does, however, require the next few readings/labs.
* I've added a link to the A-Z reference to every page.
* Monday's reading: Local Procedure Bindings.  (Not yet available.)

Overview:
* Verifying preconditions.
* Reporting errors with throw.
* Husk and Kernel programming.
* Quiz.
* Lab.

== Quiz 5 ===

Question one: Here is a color modifiying procedure.  Modify an image
* (image-variant portrait (compose rgb-lighter rgb-average-with-brown))

Question two: Compare two implementation
  (define rawb-1
    (let ((brown ..))
      (lambda (...)

vs
  (define rawb-2
    (lambda (...)
      (let ((brown ...))
         ...)))
== Exam Q&A ==

Q: Can you give me a hint about the structure for problem 3?
A: Sure

(image-compute-pixels!
 canvas 0 0 (- (image-width canvas) 1) (- (image-height canvas) 1)
 (lambda (pos)
   (cond
     ...
     (else 
      color-transparent))))
Q: Does the order of parmaeters to rgb-distance matter?
A: No (see analysis on real whiteboard0

Q: Does the order of the estimates to rgb-closest-estimate-of-2 matter?
A: Probably.  Consider
  (closest-estimate-of-2 color est1 est2)
  Suppose est1 and est2 are the same distance from color.  Which should
  you return?
  * It doesn't matter, so you can return either
  * Most people say "the first"