Overview

• Preliminaries.
  • Admin.
  • HW5.
• Procedures and side-effects.
• for-each vs. map.
• Lab.

Admin

• Let me know if you have partner preferences for HW5 by 10pm tonight. I'll try to have partners ready by class tomorrow. (Just posted on the Web.)
• You will have a substitute teacher (Mr. Walker) and mentors (Ms. Ryan and maybe Mr. Wheeler) on Wednesday and Friday. Mr. Abuah will provide continuity.
• Work for Wednesday:
  • Finish homework 4. DO NOT DO (drawing-hshift (drawing-vshift (drawing-group (drawing-hshift (drawing-vshift drawing-unit-circle 17)
  • Start looking at homework 5
  • Read Local Bindings with Let
  • Do Lab writeup 9: Turn in 4c, 4d and 5e from the Iteration Lab
    • Try to get it in for Wednesday.
    • Due before class on Friday.
    • Title: "CSC 151.02 Lab Writeup 9: Iteration (NAMES)".
• Don't forget the upcoming mentor sessions
  • Thursday at 7:30pm
  • Sunday at 7pm
• EC Opportunities:
  • CS Extras Thursday @ 4:30: Three Students on Student Sysadmin Work
  • CS Table Friday (Coding the Law)
  • Maybe a scrimmage for Mock Trial. (Details to follow.)
  • AS talk, Monday at noon. Learn about a new algebra. And eat free Pags pizza while being confused.
  • Juggling class Saturday 7-8pm, in Multipurpose Dance Studio in the Bear
  • Super Smash Bros tournament, talk to WP about it.
• Other things you might do (no EC, but possibly a good idea)
  • Poweshiek CARES March Thursday, Oct. 3. Meet at Drake Library at 5 p.m.
  • GHS Homecoming Parade Thursday, Oct. 3. If you've never seen a small-town homecoming parade, it's worth it.

A Quick Overview of HW5

Procedures and side effects

• There's a transition in the way we think about procedures.
• First procdures, things like square, *, drawing-hshift
  • Don't affect their parameters (define x 2) (square x) x (drawing-scale drawing-unit-circle 10)
• Implication: Some order of evaluation doesn't matter (drawing-group (drawing-recolor d1 "red") (drawing-hscale d1 100) (drawing-vscale d1 4) (drawing-shift d1 80))

• New procedures: Change the underlying parameter (turtle-forward! t 100)

  (turtle-forward! t 100) (turtle-turn! t 100) (turtle-up! t (turtle-face! t 100)

• We now need to think much more carefully about the order in which operations are evaluated.

Loops and Repetition

• Repeat a similar operation again and again and again
  • (map proc lst) -> apply a procedure to each element of a list, creating a new list. NO GUARANTEE ABOUT ORDER
  • map doesn't work so well with the new procedures
  • (repeat number action! parameter1 ... parametern) - Do action number times
• map: Same procedure, different inputs (and create a list)
• repeat: Same procedure, same inputs (and no list)
• Need: Same procedure, different input, GUARANTEED ORDER, no list
  • (for-each proc! lst1 lst2 ...) - Apply proc! to first element of each list, then second, then third, ...

for-each vs. map

• Order of evaluation: map order is unknown, for-each, order is left-to-right
• Result: map gives list, for-each gives nothing

Detour: drop vs increment

• (list-drop (iota 6) 1) => (1 2 3 4 5) - cross of the first 1 elements
• (map increment (iota 5)) => (1 2 3 4 5)
• What's the diff?
• map increment requires that we do math for every element of the list

Lab

• Hidden and strange ways to get info about turtles

  (define tortoise (turtle-new ...)) (tortoise ':row) (tortoise ':col) (tortoise ':angle)