CSC151 2009F, Class 52: Objects, Continued

Admin:
* Attendance is EXPECTED next week.  Double penalties for missing class
  (unless excused).
* As you requested, there will be no quiz today.
* Reading for Monday: Files.
* EC for:
  * Latin-American Ensemble Performance on Saturday 7:30 in SL
  * Community Choir, Friday, 7:30, Herrick
  * Dance Troupe Performances this week, lots of times and places
    * 4:15 today in Steiner
    * 5:30 on Saturday somewhere in Noyce
    * Tuesday evening at community meal
    * And more!
  * Friday at 4:15 recitals in SL or Bucksbaum 102 or ...
  * Womens Basketball 5pm today
* There was a troubling alcohol-related incident on campus earlier this week.
* Please behave responsibly before, during, and after Waltz.
* Are there questions on the exam?

Overview:
* Exam stuff.
* Summary of objects in Scheme.
* Questions on objects in Scheme.
* Lab.

Exam Stuff
* Do we have to explain our tests in the unit test problem?
  * No, but it would be nice
* In vector-reverse!, what do you mean when you say "Don't use 'reverse'"?
  * You can't solve it with (list->vector (reverse (vector->list vec)))
  * Of course, that also doesn't change the original vector
* What does string-vector-index-of-largest do?
  * Finds the largest string in a subvector
  * Returns the index of that string

Objects in Scheme
* An object is: A named thing
* Objects have an internal state
* Objects respond to messages
  * And the only way you can access their internal information is
    through those messages
* In Scheme, we implement objects as functions, often with code like
(define object
  (lambda (message)
    (cond
      ((equal? message ':do-this!)
       ...)
      ((equal? message ':do-that!) 
       ...)

* In Scheme, we use let to define the internal state
(define object
  (let ((state (vector whatever)))
    (lambda (message)
      ...)))

* In Scheme, when creating multiple objects, we use an "interesting"
  nesting of lambdas and lets

(define make-object
  (lambda ()
    (let ((state (vector whatever)))
      (lambda (message)
        ...))))

Back to the exam

* How do I swap the elements in a vector at positions x and y
  * This won't work
    (vector-set! vec x (vector-ref vec y))
    (vector-set! vec y (vector-ref vec x))
  * Why not?  Think about it.
  * So, we need to remember the values
    (let ((valx (vector-ref vec x))
          (valy (vector-ref vec y)))
      ...)

* Can you give me a hint as to how to write lookup-in-dictionary using
  binary-search
(define lookup-in-dictionary
  (lambda (word dictionary)
    (let ((result (binary-search _____)))
      ...)))
Questions?

Lab