CSC151 2009F, Class 21: Boolean Values and Predicate Procedures

Admin:
* EC for attending Rave (since students in the class are helping plan/sponsor
  it)
* Don't forget the cool programming languages talk today at noon.
* Exams not graded.  
  Barring continued unexpected circumstances, they will be returned Monday.
* Are there questions on assignment 5?
* Are there questions on problem 6 on Wednesday's lab?
* Reading for Monday: Conditionals.
* It's time for another Lab writeup.   Do problems 2-5.
* It's quiz day!  (There was a parenthesization issue that I think I've
  fixed on every exam.)
  * But there will be lecture (starting ten minutes after the start of
    the quiz, plus or minus)
  * Feel free to keep working
* Have a great Family Weekend!

Overview:
* What did you learn from the reading?
* What questions did you have from the reading?
* 'and' and 'or' as control structures.
* Lab.

Questions on Assignment 5
* None

Questions on Problem 6 on Wednesday's Lab
* Let vs Let*
* Both bind names to valuesa
  * In let, the computation of the values is *simultaneous*, followed by
    bindings
  * In let*, each evaluation/binding is done in sequence
  * Often not an issue which you use
  * But if one of the expressions uses a prior name, you need let*
  * Example

Student question, revisited

> (let ((x 20)
        (y 20))
    (+ x y))
40
> (let ((x 20)
        (y x))
    (+ x y))
reference to unidentified identifier: x

Just to make our lives more confusing:
> (define x 10)
> (let ((x 20)
        (y x))
    (+ x y))
30

Key Concepts from the Reading
* A new type, Boolean, true and false, represented as #t and #f
* Predicates exist
  Predicate ARE procedures that return true or false
  Example: integer?
  Usually have a name that end with a question mark
  Exception that proves the rule: = (test for equality)
* Procedures that work with Booleans: and and or
  * Okay, they're not quite procedures

 
Questions from the Reading
* Are the other operations, like xor?
  * No, but you can write them


Conditional Control Structures
* How do and and or work?