CSC152 2005F, Class 2: About Object-Oriented Programming

Admin:
* Outlines don't always match what happens.  Deal.
* Please scan the course Web and bring questions tomorrow.
* Homework!  Read "An Intro to Unix" in /Espresso/
* If I type too low, please let me know.

Overview:
* Some background
* OOP, In more detail
* Exercise: Rational numbers

This is a course in computer science
* What is CS?
  * The science of computers
    * Science is ... "the study of" "like trying to answer questions in an experimentation-based method"
    * Trying again ... science is a method of study that follows a set pattern: Observe, Form Hypotheses, Design Experiment, Conduct that Experiment, Analyze Data, go back to the beginning
    * A "Computer" is ... Something that computes
      * Computes: Takes instructions and applies those instructions to data to create a result
    * What kinds of hypotheses can you have?
      * If you apply these instructions to these data on this computer, you'll get the same result as if you apply these instructions to these ata on another computer
  * A tool for science
  * Trying again ... CS is the study of Algorithms.  Computers are involved only as the device that runs those algorithms
    * Three ways to study algorithms:
      * As scientists: This algorithm does what it's supposed to do ; This algorithm runs in the time that we expect
      * As mathematicians: *Prove* that this algorithm does what it's supposed to do ; Form models
      * As engineers: Build and test things
* Clueless people equate computer science with computer programming
  * Programming may be part of CS, but it's not all of CS
  * This class will do some programming and some non-programming

Surveys:
* Programming Languages known:
  * Scheme?
  * Some basic: No advanced users, about 3 who have used it a little
  * C++: A few "a little" users
  * Pascal
* Side note: HTML is not a progtramming language because you can't express *how* to do things in it 

Why are you in this class?
* Test the waters for a possible major in CS.  (After two days of Sam's bad jokes, it's already clearly a bad idea.)
  * CS is fun (at least so far)!
* Lack the vision to think of any major other than a CS major and need to take this course to continue in the major.
* Necessary for "that thing we don't do at Grinnell, but provide a 3-2 plan for those who want to try anyway"
* To understand what to do when a computer does not behave as you want.
  * Normal suggestions: Curse, Defenestrate (learn Latin), Realize that computers are stupid and programmers are more so

On to objects:
* Object is something that collects functionality and data
* A pen is something that
  * Data
    * Has a color
    * Has a name
  * Functionality
    * Permits transfer of ink to certain services
    * Can be thrown across the classroom
* A window on the computer screen
  * Data
    * Position on screen
    * Background color
  * Functionality
    * "Draw this in this part of the screen"
    * "Erase yourself"
    * "Hide yourself"
    * "Go away"
* Why use objects?
  * Original reason (1960's): Want to simulate the world
  * More recent reason: Objects tend to better support large-size
    programs
* Why do objects make it easier to write larger programs?
  * Objects segment the program
  * Objects encapsulate control (and data), so that it's harder for other programmers to screw up what you've done
* Many other characteristics of object-oriented programming are helpful for programming-in-the-large
* Characteristic one: Classes
  * Instead of defining only individual objects, you define commonalities
* Characteristic two: Inheritance
  * You can define new classes of objects in terms of previously-defined classes  
  * When you fix/change something in the original, the change is automatically propagated to the related things
* Characteristic three: Polymorphism
  * If you know how to use something, you know how to use the things defined in terms of it
  * Old objects can use new objects *without even knowing that the new objects have been defined*

[* Non-programming computer scientists like to prove interesting facts about polymorphism and inheritance]

Focus early in this class: Designing classes, using classes, implementing classes

Side note: We talk about functionality differently in object-oriented programming:
* Functional: Do this to this and this
  (showobject "This" window1)
  (+ 2 3)
* OBjects: Hey object, do this to this
  window1.showobject("This")
  2.add(3)

Have a great day!