Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Questions on today's topics.
• Quiz.
• Lab.

Preliminaries

Admin

• New lab partners!
• Welcome parents! (when I checked at 9am, none had signed up, but I'd rather be optimistic)
• Fruit (snacks)
• Office hours: Today, 1:45-3:15; Also walking hours 1:15-1:45. Monday's hours are distorted by prospective student events. I'll try to stay around later in the day than normal.
• The prologues suggest that you are anticipating about 3-4 hours on the HW. That's my goal, too. Let me know where you are after 2 or so hours.
• Let me know by Monday if you want to work alone on HW 5.
• A few of you have said that you cannot sign the second academic honesty statement. I am currently debating what to do next. If you have suggestions, send them anonymously on the questions page.
• It's Friday. I hope you have a chance to restore this weekend.
  Please be thoughtful about your choices this weekend (and not just because there are lots of families on campus).

Upcoming Work

• No lab writeup today (no matter what it says online).
• Reading for Monday:
  • The GNU Image Manipulation Program
  • Programming the GIMP Tools

Extra Credit Opportunities

Academic

• CS Table today: Privacy and Big Data in the Social Sciences
• Campus Town Hall September 30 t 7:30 p.m.
• CS extras next Thursday: Alumni
• CS table next Friday: Serendipity and Computing (with Grinnell's Artist in Residence)
• Grinnell Prize events the following week (particularly events related to the prize winners whose project involves Web tools)

Peer Support

• Science poster session, Sat., Sept. 27, morningish, science elbow
• Football Games (Saturday, Sept. 27, at 1pm)
• Noteworthy, Sat., Sept. 27, 3:30 p.m. Herrick
• 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.
  • Update: They can also use help on Wednesday, October 1.

Questions

Boolean values

What's the difference between a "keyword" and a "procedure"?

Procedures are parameterized collections of instructions. Call with (PROCEDURE inputs)

There are a number of things in Scheme that look like procedures, in that we write (THING values). Scheme interprets these slightly differently.

For a procedure: (* (+ 1 2) (* 3 4)) we always evaluate the parameters before we apply the operation. (* alpha 4) It depends on what we know about alpha. If we know nothing, we get an error. If it's not a number, we get an error.

(define alpha (+ 2 3)). Something that looks like a procedure, but acts diffently. Also not (define (max ...) 5)

We need a way to name things that are like procedures, but which have a different semantics of evaluation. We call them "keywords".

Keywords that you have encountered: define, lambda

New keywords introduced in this reading: and, or, not

and has a different order of evaluation. We look at each parameter one at a time, rather than evaluating them first. (Similar for or.)

Can you explain the "beyond the truth binary" section?

yadda yadda yadda

Why would we do and with non-truth values?

Most frequent reason. I want to do THIS then THIS then THIS, but stop if any of them fail.

Can you be more detailed?

Not right now.