Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions (Exam/Admin).
• Review of Key Ideas.
• Topic Prep.
• Lab.

Preliminaries

Admin

• Continue lab partners.
• No Sam tomorrow. Prof. Weinman will run class.
• Office hours: Thu/Fri 1:45-3:15; Also walking hours 1:15-1:45.
• I've started a short reference on error messages. Please feel free to pass along some you get.
• Two important recent comments from students.
  • "I realized that it would be much easier to do the test and study for quizzes if I took notes along the way."
    • Evidence (research): Taking notes helps you learn, even if you never read them again.
    • Sam belives that the total time is less if you take notes.
  • "In high school, I did homework at night (or in the early morning). I learned that I'm much better off doing homework in my open slots during the day, rather than waiting until night time."
• Other notes on quizzes
  • "Quiz anxiety"
  • Length

Upcoming Work

• Lab writeup: Exercises 4a, 5b, and 5e http://bit.ly/151-2014F-w15 Due Friday.
• Exam 1 due tonight.
  • Bring printed copies on Wednesday.
• Reading for Wednesday:
  • Making and Manipulating Homogeneous Lists

Extra Credit Opportunities

Academic

• Convocation Wednesday: Tara Zahra on Emigration from Eastern Europe
• CS Extras Thursday at 4:30: Mathematical Image Synthesis Toolkit (MIST)
• CS Table Friday at noon: ???
• Campus Town Hall September 30

Peer Support

• Noteworthy, Sat., Sept. 27, 3:30 p.m. Herrick
• Science poster session, Sat., Sept. 27, morningish, science elbow
• Football Games (Saturday, Sept. 27, at 1pm)
• Men's Tennis (???)
• Women's Tennis (???)
• Swimming (???)
• Anna Christie, Oct. 9-12 (SB plays Marthy)
• Evan's radio show 5pm Friday nights on KDIC
• Donna's radio show Sunday midnight on KDIC

Miscellaneous

• Participate in #GrinWell. http://www.grinnell.edu/forms/sign-grinwell-presidents-wellness-challenge
• Friends of Drake Library needs help setting up for their annual booksale on Thursday, October 2 (conveniently, the same night as Grinnell High School's homecoming parade). You can help a good cause and probably get a few free books, too.
  • Followup: They can also use help on Wednesday, October 1.

Questions on the Exam

There are some cases in the Quincunx problem in which the center seems to have a non-integer coordinate. What should I do in such cases?

Read the email.

Summary: At least two choices. You can refuse to compute. You can use a nearby pixel. Your documentation MUST specify which you've chosen.

Can you show a sample test for numerator-sum-fraction-reduced?

Yes.

    (define nsfr-tests
      (test-suite
       "Tests of numerator-sum-fraction-reduced"
       (test-case
        "1/2 plus 1/2"
        (check-= (numerator-sum-fraction-reduced 1 2 1 2) 1 0))
       (test-case
        "1/3 plus 1/2"
        (check-= (numerator-sum-fraction-reduced 1 3 1 2) 5 0))))a

How many tests is enough for that test suite?

Enough that you feel confident that a procedure that passes your tests is almost certainly correct.

About a dozen, provided they explore different potential challenges. E.g., just asking 1/2+1/3, 1/3+1/4, 1/4+1/5, .... doesn't really challenge the procedure in different ways.

Do we have to use the generalized approach that you used for problem 4, or can we just work with particular values.

I'm fine if you just work with particular values.

How carefully should I write my postconditions for problem 1?

Carefully enough that someone who wants to be lazy and just meet the postconditions will still have to achieve the goals of the procedure.

Please try to use "math" or Scheme rather than English to specify the postconditions.

You may have to refer to the results of the other procedures (but I think we noted that).

Can I define a procedure that takes an image and another value as input?

Yes.

    (define nt
      (lambda (value image)
        COMPUTATION))

    (define uniform-pixel
    (lambda (n image)
    (image-set-pixel! image n n "red")))

Can you show me how to log in to MathLAN from my computer?

Yes, but not today.

Review of Key Ideas

• (image-variant image FUN) and (image-transform! image FUN)
  • One is pure, one has side effects.
• We can combine existing functions into new functions by using compose and o
  • (define rg (o irgb-redder irgb-greener))
  • Works right-to-left
  • Note: We are defining procedures without lambda
• You can use lambda outside of a procedure definition, when you want a procedure but don't want to bother naming it.
  • (image-variant kitten (lambda (color) (irgb (+ 45 (irgb-red color)) 128 0)))

Topic Prep

Preliminary Questions

Can you only compose two procedures if they take the same parameters?

It's a bit more complicated than that. You can only compose two procedures if the output of one can serve as the input to the next. For example, since image-load takes a string and returns an image, and image-show takes an image (and returns an image), we can write (compose image-show image-load), but not the other way around.

Right now, we will primarily compose functions that take the same kind of input and have the same kind of output, such as color transformations.

Can we return multiple value?

After tomorrow.

Followup Questions

Talk to your partner: What, if anything, was confusing about the reading?
(Feel free to post to the question board.)

Followup Questions

Can you explain the following?

    (image-variant kitten (lambda (color) (irgb (irgb-red color) 0 0)))

How can the computer use the lambda expressions?

    (image-variant kitten (lambda (color) (irgb (irgb-red color) 0 0)))

Why does the image-variant work even though you haven't provided an input?

    (image-variant kitten irgb-redder)

Can I use the composition operator with math functions?

Yes

    (define qad (o square square))

Can I use the composition operator to write something that returns f^n(x)?

Yes, eventually.

Lab

Three approaches I've seen to undoing irgb-redder, at least for most values

(define anti-redder (o irgb-darker irgb-darker irgb-greener irgb-bluer))

(define anti-redder (o irgb-complement irgb-redder irgb-complement))

(define anti-redder (lambda (color) (irgb (- (irgb-red color) 16)
                                          (irgb-green color)
                                          (irgb-blue color))))