CSC151 2010S, Class 08: Writing Your Own Procedures

Overview:
* Some key drawing concepts
* Components of an algorithm
* What is a procedure?
* Why define your own procedures?
* How to define your own procedures.
* Labs!

Admin:
* Sam is a bit disorganized today: Class prep time was consumed by
  "Dad, I forgot to bring my important notebook to school!"
* There are no additional readings for Monday.  
  * You should, however, review the readings you did for today.
* Quiz 2 distributed.  
  * Due at midnight tonight.  
  * See the quiz policies.
* Assignment 3 distributed. 
  * Due next Wednesday.
* EC for today's CS table.
* EC for tonight's support event for the Booth family.
* Continue to use the same lab partners today.
* Finish Wednesday's lab (except for prob 5) before starting today's lab.  
  * Expect to finish today's lab on Monday.

Key Drawing Concepts
* All of the drawing procedures make NEW drawings,
  and don't affect the original
* scaling also affects the position
  * So it is often best to scale before you shift

Components of an Algorithm
* Repetition / Loops
  * Not yet
* Named Values (sometimes called variables)
  * (define name value)
* Subroutines/Procedures
  * TODAY
* Conditionals
  * Not yet
  * But we do know some ways to get the
    effects of conditional behavior
* Some set of built-in types of values and
  operations
  * drawings in the drawings-as-values model
    e.g., ellipses and rectangles
    * (drawing-hshift)
  * define is a kind of operation
  * numbers
    * (* num1 num2)
    * (sqrt __)
  * colors
    * (context-set-fgcolor! __)
    * (context-set_bgcolor! __)
    * (drawing-recolor DRAWING COLOR)
    * Values look like "red" "orange" "yellow"
  * words (strings)
    * We know that they exist
    * But we haven't done much with them
  * images
    * (image-new __)
    * (image-load __)
    * (image-fill! ___)
* A way to sequence steps
  * Put the instructions in order
    * (define a (+ 2 3))
    * (define b (sqrt a))
    * (define c (+ b b))
  * Via nesting
    (sqrt (+ 2 3))
* Other issues
  * We know that in order to make an operation
    work, we need to put it immediately after
    an open paren

What is a procedure?
* An algorithm
  * A set of instructions
  * That takes some input (parameters)
  * May produce some result
  * May change the underlying value (!)
To open! a jodar
  ; Note: The jodar is the parameter
  ; Note: Probably changes the state of the jar
  if the jodar has a screw top then
    repeatedly turn the top counter-clockwise
      until it or your arm falls off 
  if the jodar has a pop top then
    take advantage of the wonder of leverage
  if nmeither of the previous holds
    take hammer to jodar

Why do we like procedures?
* Encapsulate code so that we can refer to it
  more clearly
  * Clear
  * Concise
  * Parameterized

How do we define procedures?
* Need to be able to give
  NAME
  PARAMETERS - the names we give to the inputs
    to the procedure, so that we can talk about
    them
  INSTRUCTIONS - the steps of the algorithm, 
    that usually work with the parameter
  (define NAME 
    (lambda (PARAMETERS)
      INSTRUCTIONS))

(define square
  (lambda (jadar)
    (* jadar jadar)))
Labs!