• New Partners!

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Lessons from yesterday's lab.
• A useful tool for repetition: for-each.
• Contrasting map, for-each, repeat, and recursion.
• Lab.

Preliminaries

Admin

• Review sessions Thursday at 9:15am (Sam), 1:15pm (Sam), and 8pm (Alex).
• After reflecting, I flipped the topics for today's class and Friday's class. Sorry!

Upcoming Work

• Lab writeup! Problems 8, 9, and 10. http://bit.ly/151-2015S-lab34
• Reading for Friday: Geometric Art
• Assignment 7 distributed: Producing Playful Polygons.

Extra Credit Opportunities

Academic

• Scholars Convocation.
• CS Table Friday: Therac 25 (readings coming soon)

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• Contact JMU to host a prospie. You also get a good dinner in quad and the ability to attend an incredible improv show. Sunday April 12th. Most importantly, you convince awesome people that they should come to Grinnell.
• KY's radio show, Mondays, 9-10pm, "At Least We Think We're Funny"
• Baseball game Thursday at whatever time is on the campus calendar.

Miscellaneous

• Tonight's "Moving Forward" discussion.

Other Good Things (no extra credit)

• 7:30 p.m. on April 2: Gender as a Process. EE talks afterwards about using mathy things to understand gender.

Exam 2 returned.

Strongest arguments

• "The main goal of the exams is that we learn. We've learned. Exam grades are likely to be relatively consistent across exams, so not counting this exam is unlikely to have an effect on students who did not get inappropriate help."
• "Many of us worked very hard on this exam and should see rewards for our hard work."
• "You should have a policy that does not discourage people from coming forward."

Questions

Learning outcomes from yesterday's lab

• A few small tools and mechanisms for combining them give great opportunity.
• To use turtles successfully, you need to be able to determine where they are after each step (and, often, restore some characteristic).

A useful tool for repetition: for-each

• for-each is a variant of map.
  • map applies a function to every element of a list.
  • and produces a new list (each element of which is the result of applying the function)
• To make a spiral, we might have a list like '(1 2 3 4 5 6 7 ...)` and for each number, move forward that amount and turn 10 degrees
• If we used map, we'd end up with a useless list of turtles.
• for-each is just like map, except that it returns NOTHING
  • We call the functions for SIDE EFFECTS rather than to get a new value

Contrasting map, for-each, repeat, and recursion

> (map (lambda (s) (display s) (newline))
       (list "Happy" "April" "Sam's" "Day"))
Happy
April
Sam's
Day
'(#<void> #<void> #<void> #<void>)
> (for-each (lambda (s) (display s) (newline))
            (list "Happy" "April" "Sam's" "Day"))
Happy
April
Sam's
Day
> (repeat 4
          (lambda (s) (display s) (newline))
          (list "Happy" "April" "Sam's" "Day"))
(Happy April Sam's Day)
(Happy April Sam's Day)
(Happy April Sam's Day)
(Happy April Sam's Day)

Lab

• You can clear your image with "Select All" and then "Delete"

Some lessons

• Systematic procedures can produce very complex systematic images
• Systematic procedures can produce things that don't seem very systematic at all (e.g., things Erin would call "derpy")
• Serendipity!
• Build something more complex by repeating a simple action. Then build something even more complex by repeating that repetition. And so on and so forth.