Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• A bit more on pair structures.
• Vectors lab.

Preliminaries

Admin

• Same partners!
• I will continue to bring you food (or food-like substances) until I am caught up on grading.
• I have not had the opportunity to read much email today. I'm also watching my son play soccer in Pella. If you sent a question, I probably won't get to until 9:00 p.m.

Reminders

• Office hours this week
  • See http://rebelsky.youcanbook.me.
  • Ask me about other available times.
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Review Sessions
  • Wednesday at 8pm in CS commons with Sarah (?)
  • Thursday at 8pm in CS commons with Kumar.
  • None with Sam this week

Upcoming Work:

• No reading for tomorrow.
• Lab Writeup:
  • Send email titled CSC 151 Lab Writeup 38 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic / Artistic

• Talk Wednesday, 4pm, ARH 302
• Talk Thursday, 4:15 pm, ARH 102, The Stability of Value-added Measures of Teacher Effectiveness Derived under Different Growth Models
• Student Research Symposium this week
• Symphony Orchestra with Dave Stamey, Herrick. April 16, 7:30-9:00 p.m.
• Any of the fora on the report from the Residential Learning Task Force. (Formal announcement forthcoming.)
  • April 15: Open task force meeting, 1:00-2:30pm in JRC 209
  • April 18: Listening session, 8:00-9:00pm TBD
  • April 22: Open task force meeting, 1:00-2:30pm in JRC 209
  • April 26: Listening session, 6:00-7:00pm TBD

Peer

• Lords of the Flies, 7:30 pm April 22nd, 2:00 pm April 23rd, 2:00 9pm April 24th. (Only 50-60 tickets per show)
• ISO, Saturday, April 16, 7-9 p.m. Various classmates performing at various times.
• Second Annual Contra into Spring Dance, April 22nd, Main Quad, 7:30 p.m.

Miscellaneous

• Host a prospective student this coming weekend.

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Post-break ExCo on British Politics Wednesdays at 8:00 in JRC 203.
  Just show up; you don't need to sign up.
• Pun club Saturdays at 4pm in Younker
• Electronic Potpourri on KDIC Fridays at Five
• Space Odyssey KDIC Fridays at Six
• Bollywood, Fridays, 7:30-8:30, Younker
• Effective Altruism club, 2:30-3:30 Sundays in JRC 226.

Misc

Far in the Future

• Adaptation of Rushdie's East West. Early May.

Questions

What was yesterday's writeup?

Exercise 3.

What should rac and rcd return when given null as input?

What do car and cdr return when given null as input?

What's wrong with my stencil procedure?

Most common answer: You don't have to fit everything into one let or letrec. You can nest forms of let.

Second most common answer: You need to recompute the random column and row at each repetition. Think about your ordering of operations.

Third most common answer: Since there's call to image-new at the top of stencil-quarter-circle, don't recursively call stencil-quarter-circle. Use repeat or use a helper. (Sam prefers local helpers, but will cope with non-local helpers in many cases.)

Can you demo image-set-pixel!?

Sure. Done.

Can you explain the formula for problem 6?

The area of a circle is pirr. If the radius is 1, the area of a circle is pi. If it's a quarter circle, the area is pi/4. We're approximating the area of the quarter circle. We can then run the formula backwards to get pi.

See the board for more details.

But how do I get a random number between 0 and 1?

(random).

I've written irgb-closest using that "doubly recursive method" which we've seen is horribly inefficient. Will you penalize us for that?

Yes. Lots! (If I'm in a good mood when grading, and you have nothing else wrong with your solution, you'll be lucky to get a 6/10 on that problem.)

Do you expect us to think at least a little bit about efficiency?

Yes.

What about empty substrings?

Look at the postconditions.

> (define str "Hello World")
> (define substr "")
> (define pos 0)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
#t
> (define pos 5)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
#t
> (define pos 1253)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
. . substring: starting index is out of range
  starting index: 1253
  valid range: [0, 11]
  string: "Hello World"

How much extra credit should we get for identifying the massive typo on how to submit?

At least two units. Email it to me, like always.

What is my number?

The same one as last time.

I lost it.

(random 1000000) ; add any leading 0's

Do you really want dead trees?

No. Point.

Are we ever getting to the main class material?

Maybe. Conveniently, tomorrow is a "pause for breath" and you can do the lab then.

Are you really taking off one point for each minute a student was late to today's class?

Considering it. (And yes, that's 50 points for people who missed class, and 30 minutes for ....)

A Bit More on Pair Structures

Why are we doing this? (That is, looking at pair structures and vectors.)

Why are we studying pair structures, given that we know about list?

• To give us a better understanding of car and cdr. car is the thing that the first box points to (aka the contents of the first box), cdr is the thing that the second box points to.
• Gives you a deeper understanding of how Scheme works.
  • Scheme does a lot of "reference chasing" (following arrows)
  • List processing is generally slow, so look for ways to avoid it.
• We use linked structures a lot in CS; knowing more about linked structures turns out to be useful.
• Soon, we will build more complicated linked structures using pairs (or something similar). Pictures help you understand.
• Understand the stupid dot at the end of each non-list.

Why are we studying vectors?

• Vectors provide an interesting alternative to lists because they seem more efficient.
• For example, vectors let you change them without creating a new vector.
• Vectors let us quickly find any element because we can compute its location.
• In contrast, finding an element in a vector can be slow.
• Vectors are hard to resize, though.
• Is there a car for vectors? Not really. There's vector-ref
• Is there a cdr for vectors? No.

Vectors Lab