• New partners!
  • Grab a card from the stack.
  • Identify the location of the new computer.
  • Drop the card in the jar.
  • When both of you are at the computer, introduce yourselves.
• Grab a pink card, too!

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
  • About Exam 1.
• Documentation, Continued.
• Lab begins.

Preliminaries

Admin

• Review sessions:
  • Wednesday, 8pm (Zachary)
  • Thursday, 10am (Sam)
  • Thursday, 8pm (Erin)
• Mentors and tutors cannot directly answer exam questions. However, they can go through any example from the readings and any problem from the homework.
• If you have questions you'd like me to answer, you can write them on blank index cards and hand them to the mentors. I'll send out answers tomorrow.
• Today's lab is a multi-day lab.

Upcoming Work

• Quiz Friday!
  • Procedures with lambdas
  • Maybe documentation
• No new reading for Friday! Testing Procedures with Rackunit
• Exam 1
  • Prologue this Friday
  • Electronic version next Tuesday
  • Epilogue next Tuesday
  • Printed version next Wednesday
• Lab writeup (due before class Monday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 12 (Your Names)
  • Exercise: Document irgb-add

Extra Credit Opportunities

• I remain a little behind in responding to these. Sorry!

Academic

• No CS Table next week - It's advising week!
• CS Extras today in 3821 at 4:15, Jonathan Wellons '04 from Google
• Convo tomorrow at 11 am in JRC 101: Mike Latham
• Art opening (or any visit to the art show): Friday and beyond
  • Talk, 4pm, Sept. 18
  • Opening Reception, 5:00-6:30 p.m., Sept. 18
• John Gerrard talk, "Corn Bomb", Monday at 8pm

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am, Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker.
• Singers concert on Family weekend! Sept. 26, 2pm., Sebring-Lewis
• Orchestra concert on Family weekend! Sept. 26, 2pm., Sebring-Lewis
• Softball Alumna Game, 9am on Sunday

Questions

Exam 1 Distributed (electronically)

• It has a prologue and an epilogue.
• It has seven questions.
• It has a sanity clause. "There's more to life than CS."
• It has way too many instructions.

Documentation Practice

• Six P's
  • Procedure: The name
  • Parameters: The names of the inputs and their types
  • Purpose: Tells you what the procedure does
  • Produces: The name and type of the output, maybe a little more info about what kind of value we return
  • Preconditions: Give limitations, typically retrictions on the parameters that are required in order for the procedure to work. Those are sometimes covered when we add the data types under parameters.
  • Postconditions: Expectations/conditions of your output. Once again, some get covered in the produces section. Formalizes the results

(max v1 v2)

    ;;; Procedure:
    ;;;   max
    ;;; Parameters:
    ;;;   v1, a real number
    ;;;   v2, a real number
    ;;; Purpose:
    ;;;   Takes the maximum value of v1 and v2, whichever is larger
    ;;; Produces:
    ;;;   result, a real number, the larger of v1 and v2
    ;;; Preconditions:
    ;;;   [No additional]
    ;;; Postconditions:
    ;;;   result = v1 or result = v2
    ;;;   result >= v1
    ;;;   result >= v2
    ;;; Problems:
    ;;;   What happens if the two numbers are equal?  We can use
    ;;;     either of them, since that value will meet the postconditions
    ;;; Process:

• We can express these postconditions conditionally.
• We name the produced value so that we can use it in the postconditions.

(irgb-average color1 color2)

    ;;; Procedure:
    ;;;   irgb-average
    ;;; Parameters:
    ;;;   color1, a color
    ;;;   color2, a color
    ;;; Purpose:
    ;;;   Average the components of color1 and those of color2.
    ;;; Produces:
    ;;;   avecolor, an integer-encoded RGB color
    ;;; Preconditions:
    ;;;   color1 and color2 must be integer-encoded RGB colors
    ;;; Postconditions:
    ;;;   (irgb-red avecolor) = (floor (/ (+ (irgb-red color1) (irgb-red color2)) 2))
    ;;;   (irgb-green avecolor) = (floor (/ (+ (irgb-green color1) (irgb-green color2)) 2))
    ;;;   (irgb-blue avecolor) = (floor (/ (+ (irgb-blue color1) (irgb-blue color2)) 2))
    ;;; Problem: What if the two components sum to a value greater than 255?

When we say "integer-encoded RGB color", what do we mean?

One of those strange numbers given back by (irgb _ _ _)

Why do we do this much work? It seems like more work to write the documentation than it does to write the code?

Forces us to think more carefully about our procedures.

What are the legal inputs? (What form can they take?)

What form does the output take? Useful for two weeks from now when we don't remember.

What might go wrong?

Etc.

Lab