CSC 151.01, Class 34: Iteration

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Reflections on procedures and side effects
• Comparing and contrasting map, for-each, repeat, and recursion
• Lab

News / Etc.

• Continue partners!
• I plan to talk a bit more today than yesterday. Sorry.
• I am working on fixing the brush size problem.
• Prospective!
  • Welcome!
  • Come to Grinnell because we have great opportunities
  • Plus your brother is lonely without seeing you regularly
  • Dairy barn ice cream is better than dhall. But the former gas station (“Candyland”) is even better. Plus they have a board game you played in childhood.

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.

Upcoming Work

• Lab writeup: TBD.
• Homework 7 to be distributed Wednesday.
  • Exam 3 distributed a week later
  • Project distributed a week later
  • Exam 4 distributed two weeks later
• Reading for Wednesday: Geometric Art

Extra credit (Academic/Artistic)

• Today at 4:15 p.m. JRC 101: The 1st Mando lecture. “Reflections By Pulitzer-Prize-winning writer Dale Maharidge & photographer Michael Williamson.”
• Today at noon: CS Table: Gadfly (apps to support democracy)
• BAX Opening Reception (plus Robert Hodierne photographs) Friday 4:30-5:30

Extra credit (Peer)

• Baseball games Saturday at 1pm and 3:30 pm vs. Monmouth (if it ever stops raining)
• Track and Field Saturday at Cornell at 11 am.

Extra credit (Misc)

None right now.

Good things to do

• Women’s Tennis Saturday at 10:00 am

Questions

Reflections on procedures and side effects

• Two different models of procedures.
• Originally: A procedure computes a value. square computes a value by multiplying a number by itself.
  • (square 5) => 25
  • (square 5) => 25
  • (define x 11)
  • (square 5) => 25
• Now, procedures can change the state of the system. You often care more about that change in state than the return value, but sometimes both.
  • (turtle-forward! yertle 10) - yertle has moved forward ten units. We get back a turtle, which looks like #<procedure:...> If the brush is down, we see short line.
  • (turtle-forward! yertle 10) - I get back #<procedure...>, we get back the same turtle, but it’s in a different position. Something different appears on the screen; our state is different.
  • (turtle-x yertle) - [returns a shoot (it is spring, after all)] returns the x position of the turtle.
  • Suppose I call (turtle-x yertle) and get 25. I then do some more work, and call (turtle-x yertle) again. Will I get
    1. It depends. on whether or not I’ve moved the turtle.
• Changing the state can lead to confusing situations.

Comparing and contrasting map, for-each, repeat, and recursion

• map - maps a function onto a list (calling the function on each element of the said list), returns a list of the results of the function calls.
• for-each - calls the function on each element of the list, but returns nothing.
  • (for-each square (list 1 2 3)) -> computes 1 4 9 but doesn’t let you see anything. You should probably use map.
  • (for-each (l-s turtle-action01! yertle) (list 5 1 23 1)) does the action with four different inputs. We don’t really need a list of turtles back, so we don’t use map.
  • Used to repeatedly call a procedure that produces a side effect
• repeat - Repeatedly call a procedure that produces a side effect.
  • (repeat 5 square 10) - pointless
  • repeat always gives the procedure the same parameter; for-each gives it different parameters (potentially)
• recursion
  • Works with everything; most general case. (Tends to be longer.)

Lab

Note: Here’s the fix for the brush size issue

1. Quit DrRacket

2. Open a terminal window

3. Type /home/rebelsky/bin/csc151-update

4. Start DrRacket

5. Try changing the turtle’s brush size!