CSC151 2007S, Class 08: Writing Your Own Procedures

Admin:
* We'll spend a little bit of time going over the first quiz.
* Assignment 2 returned.  We'll go over it a bit.
* Are there questions on Assignment 3?
  * You may only use one image-creation model for each of the three images
* I still haven't completely caught up on grading, but I'm getting closer.
* Advance notice: Extra credit for attending Thursday's convo.
  * Thusdays at 11 a.m.
  * These days, usually in JRC
  * But maybe in Herrick on Thursday
* There are two readings for tomorrow, 
  one on numeric values 
  one on symbolic values

Overview:
* Why define your own procedures?
* How to define your own procedures.
* Lab!

==Notes on the Quiz==

* Most of the problems were on problem 3
* I don't recommend that you compare grades, as such comparison rarely
  provides a net gain in happiness

===Notes on Problem 3===

(define new-width (/ image-width 4))
* Note that image-width is a procedure.  Dividing
  a procedure by an integer (or by anything) makes no sense.  The
  correct answer is <q>An error would be reported.</q>
* Some of you said that "The image width would shrink to 1/4 of its
  original size".  However, definitions generally don't change the
  underlying thing.  
  * Procedures that do change the underlying value usually end with
    an exclamation point
  * If we wanted to use a procedure to change the width, we might use
    something like image-set-width!.  (No such procedure
    exists.)

==Notes on Assignment 2==

* Good lesson for Sam: Don't ask for complex instructions.
* Be careful of equality in describing circles.  (Yes, this will be
  an issue.)
* Drawing the yellow part of the circle with turtles was hard.

==Why Define Your Own Procedures?==

* YOu've been using procedures as long as you've been using Scheme
  * turtle-forward!
  * turtle-down!
  * image-show
  * +
* Where do they come from?
  * Some are built-in to the language
  * Some come from the DrFu library, which means that a Grinnellian
    wrote them
  * Some come from GIMP/Script-Fu, some wiley hacker wrote them
* Most programmers (eventually) want to write their own procedures
  * Avoids repetitious code
    * Copy and paste is a pain
    * Copy and paste and update is more of a pain
    * Copy and paste and FIX is even more of a pain
  * Clarifies code
    repeat 100 times
     repeat 360 times
       forward 01.cm
       turn 1 degree
     turn 90 degrees
     forward 0.1cm
     turn -90 degrees
    VS
     (turtle-draw-filled-circle! ...)
  * NOTE: THE PREVIOUS CODE DOES NOT DRAW A FILLED CIRCLE!  IT HAS A BUG
  * Encourages us to generalize code
    * Make the preceding code draw a filled circle of an arbitrary radius 

==How to Define Your Own Procedures==

(define NAME-OF-PROC
  (lambda (PARAM_1 ... PARAM_N)
    INSTRUCTIONS))

; THE FOLLOWING CODE HAS A BUG, TOO

(define turtle-draw-filled-circle! 
  (lambda (turtle radius)
    (repeat! 100
     (repeat! 360
      (turtle-forward! turtle (/ (* 2 pi radius) 360))
      (turtle-turn! turtle 1))
     (turtle-turn! turtle 90)
     (turtle-forward! turtle 0.1)
     (turtle-turn! turtle -90)
     (set! radius (- radius 0.1))))

==Lab==