Overview

• Preliminaries.
  • Admin.
  • Questions.
• Reflecting on procedures and side-effects.
• A useful tool for repetition: for-each.
• Contrasting map, for-each, repeat, and recursion.
• Lab.

Preliminaries

Upcoming Work

• Homework for Wednesday: HW 7: GitHub
• Today's lab writeup: 4c and 4d.
• Reading for tomorrow: Geometric Art through Numeric Recursion
• HW 8 is also posted.

Admin

• If our Web server is not working correctly, you should be able to get to today's lab at the URL
• Continue your partner assignments from Monday. If you missed class on Monday, work with someone else who missed class on Monday.
• CS T shirts soon to be available. Vote if you plan to buy one by sending email to sepc-cs@grinnell.edu. The list of possibilities is at http://imgur.com/a/P0haH#0.
  • You should encourage the SEPC to make a "Curried Lamb-Duh" T-shirt
• Extra credit:
  • Convocation, Wednesday, noon: Elizabeth Kolbert (the Sixth Extinction)
  • Technology in liberal arts education, lots of talks on Thursday
  • No CS extra this Thursday.
  • CS table Friday: The API Performance Contract Robert F. Sproull and Jim Waldo. 2014. The API performance contract. Commun. ACM 57, 3 (March 2014), 45-51. DOI=10.1145/2576792 http://doi.acm.org/10.1145/2576792 http://delivery.acm.org/10.1145/2580000/2576792/p45-sproull.pdf

Questions

Reflecting on procedures and side-effects

• A transition in how we think about procedures (not necessarily positive)
• It used to be, we called procedures to return a result
  • And nothing else
• Now, procedures have "side effects" - they change "the world" (or themselves, or an image)
• When you're doing (+ (square 5) (square 7)), it doesn't matter which order we do the square
• If you do (turtle-forward! t 10) (turtle-turn! t 10)
• Order of operations is much more important!
• We didn't tell you, but map does not necessarily do the transformations in order from left to right
  • (map square (list 1 2 5 7 9))
• map is not a good tool for side-effeting operations, since order matters

What is a procedure with a side-effect?

• One view: The procedure changes something external (e.g., an image, the implicit position of the turtle, ...)
• Another view: We are not assured of getting the same behavior each time we call the procedure on the same parameters

Examples

• Side effects: turtle-forward!, image-select-ellipse!, ...
• No side effects (pure): square, increment, iota, rgb-redder, irgb-new, ...
• Badly named: image-show

A useful tool for repetition: for-each

• When you want to apply the same function to one thing and then another and then another (as in map)
• But you care about the order
• To different values
• and we do this more for the side effects than the resulting list

• USE for-each

  (for-each PROCEDURE LIST-OF-PARAMETERS)

Contrasting map, for-each, repeat, and recursion

• Are we doing the work with side effects or not?
  • With side effects: for-each, repeat, recursion
  • Without: map, recursion
• What does our result look like?
  • A list of transformed values: map
  • Nothing: for-each, repeat
  • Variable: recursion
• Do we something different at each step?
  • No: repeat, (recursion)
  • Yes: for-each, map, recursion

    Lab