CSC151.01 2006S, Class 09: Boolean Values and Predicates

Admin:
* Absent: LaRue, McCardle, McFarlin, Zamora
  * Not a good day!
  * Warning!  Sam makes phone calls.
* Reading: Conditionals
* Due: HW4.
  * Available generally in the near future
* Assigned: HW5.
* EC: Tuesday Extra: Significance from Insignificance (two recent alums)

Overview:
* The Components of an Algorithm, Revisited.
* Questions on the Reading?
* Lab.

/The Components of an Algorithm, Revisited/

* Basic types and operations:
  * Numbers
  * Strings
  * Symbols
  * Lists
  * Boolean - and, or, not
* Naming 
  * define
* Sequencing
  * In definitions window
  * Implicit in nested expressions.
* Conditionals
  * Can tell if things are truish or false
  * And can make different decisions, using clever versions of and and or
  * NOT YET (note that it's the title of the reading for Monday)
* Loops
  * NOT YET
* Subroutines
  * Procedures.

/Questions on the Reading/

* If there's only (and), the value is #t
* If there's only (or), the value is #f
* Is there a logic to that result?  Yes.
* Will I tell you the logic?  No.  The lab brings up the same question and, after you've thought about it, the answer is at the end.

/Notes on the Lab/

* Q: Differences between 'eq?' and 'eqv?'
  * A: There are few such diffences.  The only one I know of is "large numbers"
    > (eq? (expt 3 100) (expt 3 100))
    #f
    > (eqv? (expt 3 100) (expt 3 100))
    #t
* Q: So why do you bring up eq? in this reading and lab?
  * A: For the sake of completeness
  * A: Just use eqv? or equal?
* Q: What are some ways to express "grade is between 0 and 100, inclusive?"
    (define valid0
      (lambda (grade)
        (and (<= 0 grade) (<= grade 100))))
    (define valid1
      (lambda (grade)
        (and (<= 0 grade) (>= 100 grade))))
    (define valid2
      (lambda (grade)
        (or (= grade 0)
            (= grade 100)
            (and (> grade 0) (< grade 100)))))
    (define valid3
      (lambda (grade)
        (<= 0 grade 100)))
    (define valid4
      (lambda (grade)
        (and (number? grade) (<= 0 grade 100))))