• Continue partners.
• Food? (optional)
• Fun with HW 5.

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
  • Quiz.
• Lab.

Preliminaries

Admin

• Review session Monday at 7:00 p.m.
• One of your classmates passed along the following article which you might find interesting.
  • http://www.wired.com/2015/02/science-one-agrees-color-dress/
• Prospies Monday! Be prepared for special activities.
• Friday PSA
  • In making choices this weekend, please make sure that Nobody Gets Hurt (including you).
  • There are a lot of illnesses going around campus. Rest will help you avoid them better than almost anything. (I'm telling myself the same thing.)

Upcoming Work

• Homework 6. Is optional.
  • Due Tuesday at 10:30 p.m.
  • Replaces lowest HW, provided it is higher than the lowest.
• Exam 2 to be assigned on Tuesday.
• No lab writeup for today.
• Reading for Monday: Recursion Basics
  • Recursion Basics
  • A cool and different topic that we'll be talking about through spring break (and the rest of the semester).

Extra Credit Opportunities

Peer Support (Afternoon Section)

• Indoor Track Today and Saturday. Support Grinnell Pride!
• Belly Dancing 3:30-5:00 p.m. and Wednesday 7:00-8:00 p.m.

Other Good Things

• Neverland Players this weekend.
• Spend thirty minutes going to the thing at Garander tonight at 10:00 p.m.
• Stephen Hawking movie tonight 6pm and 9pm in Harris Cinema.
• Today is last day of Amy Flores show in Smith.

Questions

Quiz!

• When you finish, keep your paper by you. We will be by to pick it up.
• When you finish, start GIMP and DrRacket and look over the lab, but wait for your partner.

Lab

• cons can build things that aren't lists. You only get a list if the second parameter is a list.
• Scheme/Racket show you non-list pair structures by including a period somewhere (traditionally right before the end).
• (cdr (cons a b)) is always b
• (car (cons a b)) is always a
• Lots of pronounciation: cadr, cddr, cadadr

Learning Outcomes

• How to build and disamble lists with cons, car, and cdr.

• We can use a list to represent a group of related information

  • The description of a drawing
  • The description of a class '('CSC 151 2 "Functional Problem Solving" 1)

• If we know that representation, we can build procedures that construct things to match it or that deconstruct things to use them.

HW5, Revisited

• You have written (image-sample image x-coordinates y-coordinates).
• You could also have written (image-sample image shape xs ys).
• It's easy to generate long lists of xs and ys
  • (map (lambda (i) (modulo (* i 11) (image-width image))) (iota 1000))
  • (map (lambda (i) (modulo (* i 13) (image-heightimage))) (iota 1000))
  • Putting it all together, we can get some great pictures.