CSC 151.01, Class 22: Revisiting Lists

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Lab
• Debrief

News / Etc.

• Continue partners!
• While our lab grader seems to be on top of things, our homework grader seems to be having trouble keeping up. I’m looking to hire an additional homework grader. I apologize for the difficulties. I hope things will be resolved soon.
• Strike policy reminder. I respect the student initiative on this issue. But you should have something to strike against. I will still teach class. I will not penalize you for missing class if you send me a one-paragraph reflection on some aspect of the strike.
• New policy forthcoming.

Rotating reminders

• Visit me in my office! I’m always happy to see students during my office hours. I prefer that you book me at https://rebelsky.youcanbook.me, but you can also just show up during office hours and hope that I’m not busy.
• Note that my office hours have been filling as of late. Talk to me individually if you can’t find a booking time.

Upcoming Work

• Assignment 5 due TONIGHT at 10:30 p.m.
• Lab writeup: TBD
• Reading: Recursion basics
• Exam 2 arrives tomorrow.

Extra credit (Academic/Artistic)

• CS Table, Tuesday at noon: Facial Recognition “algorithms”
• W. Kamau Bell, Tuesday at 6pm, Harris.
• Prazak Quartet, Tuesday, 28 February 2017, 7:30 p.m., Herrick.
• CS Extras, Thursday at 4:15 pm, Sam’s research and PM’s research

Extra credit (Peer)

Extra credit (Misc)

• Non-violent protest training session TODAY at 4:00 in JRC 101.
• Preventing sexual assault, Tuesday at 11am, JRC 101
• Host a prospective student THIS COMING WEEKEND. (March 5, April 8, April 23)
• Write the eboard equivalent for class Tuesday, Wednesday, or Friday next week. (Done. They will be Amazingly Terse)

Good things to do

• Bagel party in Math commons, Thursday, 7-9 pm. Lox status unclear.

Questions

Can you use map on heterogeneous lists?

Yes, provided the procedure can be applied to all elements of the list.

(map square (list 1 2 "three")) => boom

(map list (list 1 2 "three")) => '((1) (2) ("three"))

What happens if I am working with multiple heterogeneous lists?

As long as the procedure accepts the corresponding values, it’s fine.

(map + (list 1 2 3) (list 3 4 "five")) => boom

(map list (list 1 2 3) (list 3 4 "five")) => '((1 3) (2 4) (3 "five"))

Oh no! How will we get help next Tuesday?

Ask questions before Tuesday.

Why can’t we use map with define?

define is not a procedure.

Lab

How do you pronounce these words?

car is pronounced how it looks, unless you are Wicked Neat, in which case you pronounce it “cah”.

cdr is pronounced “could - err”. Of course, could is a strange word. I really need IPA. (International Phoentic Alphabet, not the beer.)

cadr is pronounced “cad - err”, with the “cad” you would use for a man who behaves inappropriately.

cddr is pronounced “could did err”.

cdddr is pronounced “could did id err”.

What is element 5 of the list '("a" "b" "c" "d" "e" "f" "g")?

We start numbering at 0. So it’s “f”.

What does it mean when we see a period right before the last element

of something that looks like a list, as in '(a . b) or

'(a b c d e f . g).

It means it’s not quite a list, because the rightmost cons did not take a list as its second parameter.

For now, you should take it as a sign that you’ve done something wrong.

Why can we take car and cdr of things that are not lists?

Because the John McCarthy (designer of Lisp) and his graduate student (implementer of Lisp) allowed it as such in how they created car and cdr.

(car (cons X Y)) => X

(cdr (cons X Y)) => Y

Debrief

See above.

Writeup

Write up exercise 5 from the lab on lists. Include 6P-style documentation.

``` ;;; Procedure: ;;; rectangularize ;;; Parameters: ;;; drawing, a drawing ;;; Purpose: ;;; Convert to an ellipse ;;; Produces: ;;; result, a drawing ;;; Preconditions: ;;; … ;;; Postconditions: ;;; (drawing-width result) = (drawing-width drawing) ;;; … (define rectangularize (lambda (drawing) …))

Send your solution to csc151-01-grader@grinnell.edu.

Title your email CSC 151.01 Writeup for Class 22 (YOUR FULL NAMES).