SUNY Geneseo Department of Computer Science

Introduction to Design

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Questions?

Lab deliberately graduated series of exercises

• Expect everyone to do 1
• No-one to do 4

Reading Summary

Sections 2.1 - 2.3, 2.5

Abstraction (concentrating on some essential features while ignoring inessential ones) as a tool for thinking about algorithms

• Object-oriented programming (dividing an algorithm's work between software agents -- objects -- that each solve part of the problem) is a mechanism for using abstraction in modern computer science
• Objects abstract what an object can do (messages it handles) away from how it does those things (methods)

Pre- and postconditions provide an abstract interface to an algorithm: they specify what the algorithm needs in order to work (preconditions) and what it produces in return (postconditions)

Objects can also abstract values/data, as well as actions.

Some comments on this reading summary

• I consciously fit it into the 5 sentences guideline. But short doesn't mean easy -- it took a fair bit of thought (and understanding) to figure out what the 5 most important ideas were and how to state them
• Not just an outline of the text -- lots that was in the text (especially examples) never got mentioned, what did get mentioned was paraphrased to some extent
• I wrote it down -- not required of you, but a good exercise when you need to clearly but concisely lay out the essence of a bunch of complicated ideas
• This summary didn't illustrate the possibility of asking questions in a reading summary, but remember that identifying and asking a small number of key questions is just as legitimate as summarizing like I did above

Mini-Chess

An example of object oriented ideas

Chess, but on a small board and with only 4 pieces per side:

Try a game

What does this have to do with object oriented programming?

• Pieces moving...
  • Classes & subclasses
    • King
    • Queen ... subclassses of ... Piece
    • Knight
  • "Piece" is a class because all pieces respond to "move" -- interface
  • "King", etc are subclasses because they handle "move" differently -- implement
• People were objects
  • They received messages
• "Move to square x and take piece y"
  • Is a message with parameter x
    • (possibly also parameter y)
  • Has precondition that y is opposite color from x
    • Plus preconditions based on movement rules and varying according to class
• Methods were (barely conscious) processes people used to move from place to place
• Color could be a member variable
  • aka "attribute", "property", "instance variable"
  • Or encoded in class

Cell Phones and Abstraction

Two descriptions of how to place a call:

1. Push buttons for the number to call, then push another button to place it
2. Push buttons for the number to call, which generates "interrupts" to the computer in the phone, which stores the number somewhere in its memory; when the "interrupt" for the "dial" button arrives the computer causes the radio transmitter to emit radio waves that vary subtly in their timing....

The first of these is much more useful to someone wanting to make a call, because it is more abstract -- it hides or filters out many details of how a call is "really" placed in order to emphasize just what a person using the phone needs to know.

Next

Introduction to Theory

Read Sections 3.1, 3.2, 3.6

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