Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Questions on the reading.
• Quick description of steps.
• Lab.
• Reflection.

Preliminaries

Admin

• Maintain lab partners from Friday.
• Quick observations from prologues.
  • Folks are worried about how to do the checkerboard concisely. (I hope that you're not worried about how to do the checkerboard.)
    • Looking at other problems may help with concision.
  • Many problems are, indeed, relatively close to the problems on the procedures lab. Getting the details right (and making sure that you have the details right) will be a bit hard.
• I'm glad to see that many of you used the Sunday afternoon tutor session. Thanks Alex!
• Quiz 2 returned.
  • Because of difficulties on the quiz, I'm playing with the schedule a bit.
  • I'll probably use the first half of Tuesday's class to talk about issues from the quiz. (I sent you notes yesterday afternoon.)
• Notes on HW 2 distributed.
  • Let me know if you have questions.
• There's a request that I keep things in the top 2/3 of the screen. I'll do my best to do so. If I fail, please let me know.

Upcoming Work

• NO WRITEUP FOR TODAY'S LAB!
• HW 3 due Tuesday night.
• Reading for Tuesday:
  • Design and Color

Campus Events of Interest

• Rosenfield seminar on Scotland, Tuesday noon and Thursday afternoon.
• Rebecca Hamlin book talk Wednesday at 4:15 pm.
• Internship talks Wednesday and Thursday afternoons.

Extra Credit Opportunities

Academic

• Convocation Wednesday: President Kington
• CS Table Friday: Browser Tracking

Peer Support

• Football Games (???)
• Women's Volleyball (???)
  • Congrats on three victories this weekend!
• Men's Tennis (???)
• Women's Tennis (???)
• Anna Christie, Oct. 9-12 (SB plays Marthy)

Questions (administrative, homework, etc)

If we're defining the add-shadow procedure, what should we do for our postconditions?

• I'm happy if you say something like "The details of the shadow are in the documentation for shadow." This assumes that you've documented shadow.
• I would suggest that you add something about the relative appearance of the shadow. (in front, behind, other ..)

Should we document every procedure we write?

• "Take your medicine, it's good for you."
• Please do so for anything we say "document" for.
• In other cases, perhaps do it mentally. But include at least a short sentence.

Questions on the Reading

What is the epsilon in the reading?

What's the difference between a check, a test, and a suite?

What is the optional message?

Discussion of Those Questions

• We have a procedure.
• How do we know that it's correct?
  • Proof
  • Experiment!
• E.g., If we have procedure, (drawing-center-x drawing), that's supposed to find the x coordinate of the center of a drawing.
  • Start with a drawing with left edge 10 and right edge 50. Does the procedure return 30?
  • We want to try a variety of experiments.
    • Different shapes (circles, squares, rectangles, ellipsoids, and combinations thereof)
    • Different positions (on the screen, off to the left, both, ...)
• How do we write our experiments?
• Racket gives us three sets of procedures
  • (check-equal? EXPRESSION EXPECTED)
  • (check-= EXPRESSION EXPECTED EPSILON) - for numbers
    • But Scheme approximates numbers. One way to see is that it has a strange numerator or denominator. If there's a decimal point, we know that it's approximate.
    • Approximations may be different (at, say the 20th digit)
    • When comparing inexact numbers, we want to say "How close is close enough to be equal?" That's episolon.
  • We want to group experiments together and add notes.
  • (test-case DESCRIPTION (check-equal? ....) (check-equal? ....) ...)
    • Checks immediately!
  • If we want to just write down the instructions, but not run them immediately, we use (define NAME (test-suite DESCRIPTION ...))

Lab

Reflection