CSC151 2010S, Class 22: Boolean Values and Predicate Procedures

Overview:
* Questions and tips on Assignment 5.
* Review: Local Bindings
* Boolean values Q&A.
* Lab.

Admin:
* Reading for Wednesday: Conditionals.
* Advance warning: Exam 2 distributed next Tuesday; substitute here W-F.
* EC for Thursday's Thursday Extra: (not sure).
* EC for Friday's CS Table: Notes from an alum.
* Racial Bias Incident

Assignment 5: Questions and Tips
* Can you provide insight on resetting the turtle?
  * Problem 1 (polygon): Should be natural.
  * Problem 2 (rotated): 
    * Fortunately, the polygon procedure puts us back into the right place.
    * So our main problem is the angle.
      * Figure out how far it has turned.
      * Turn it back the inverse of that.
  * Problem 3 (scaled): Should be automatic
* Tips from your colleagues
  * Draw pictures by hand to figure out the "trig stuff"
  * Start early and stop, you'll get epiphanies
    * For example, you don't turn by the interior angle.
      * Pictures help on this one

Local Bindings
* We learned about let
* let works a lot like define, except that
  it lets us define values within a function
* If we have a series of definitions that
  depend on each other, we need to nest them
* Let expressions look like:
  (let (NAME-VALUE-PAIRS)
     ...)
  * Each name-value-pair has the form
    (NAME VALUE)
  * A typical let looks like
    (let ((NAME VALUE))
      ...)
* We can build interesting sequences of values
  E.g., let a be a color, let b be a redder version
  of a, let c be a redder version of b, etc.
* let* is a nice alternative to nested lets.

LAB
* Thoughts on "experimentally determine"
  * Intent: Some thought about other components
  * Some of you mathematically determined those values.  
    * An experiment is good to check your answer
  * Most of you said "I'm not going to try 0 1 2 3 4 ....".
    * Is there an efficient search strategy?
(define valid-component?
  (lambda (c)
    (<= 0 c 255)))