CSC151 2007S, Class 09: Computing with Symbols and Numbers

Admin:
* Are there questions on Assignment 3?
* Advance notice: Extra credit for attending Thursday's convo.
* There is only one readings for tomorrow, 
  Representing Images as Lists of Spots
* The Mac version of DrFu is now available, but experimental.  
  Send comments!
* Upcoming work
  * Another quiz on Friday (and almost every Friday)
  * Exam 1 distributed Friday
* Once again, I'll be running Dr. Davis's class today.
* When should Max's office hours Monday 8-9

Overview:
* Types.
* Kinds of Numbers.
* Key Numeric Operations.
* Lab.

==Types in Scheme (and other languages)==

* Programs deal with different kinds of values
  * Numbers (3.14), Strings ("DrFu", "Red"), turtles, drawings, ...
* Why have different kinds of valeus?
  * Because we think about them differently.
  * Because the computer may represent them differently.
  * Because we want to make sure that operations make sense
    * It doesn't make sense to divide a procedure by 4
    * It doesn't make sense to move the number 4 forward 3 pixels
* Different languages approach types differently
  * Scheme "I'll figure it out from context"
  * Most languages "You have to tell me!"
    * Easier to catch errors
    * But tedious

==Symbols==

* In Scheme and LISP, but not a lot of other languages
* Looks like a string, but without quotation marks
  "red" string
  red   symbol
* Both strings and symbols name things
  * symbols are atomic: YOu think of them as a whole thing
  * strings are compound: You break them up into letters
* We tend to use symbols for efficiency
  * Strings take some extra computational power to compare
  * Symbols don't
* Somewhat boring: Create and compare, but not much else

==Numbers==

* Lots of subtypes