Held: Monday, 2 November 2015

Back to Outline 33 - Turtle Graphics. On to Outline 35 - Pause for Breath.

Summary

We consider a technique for stepping through the values in a list, and doing some action (e.g., moving a turtle or drawing something with the GIMP tools) for each value in the list.

Related Pages

• Reading: Iteration
• Lab: Iteration
• EBoard 34 (Source 34)

Overview

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

Administrivia

• New partners.
• Quiz 8 returned; new grades distributed Sunday night.
  
  • No, you should not be up as late as I was.
• Exam 2 answer key forthcoming asap. (Still some delays due to a variety of issues.)
• Review sessions this week:
  • Monday, 8pm ()
  • Wednesday, 8pm ()
  • Thursday, 10am (SamR)
  • Thursday, 8pm ()

Upcoming Work

• Reading for Tuesday: Geometric Art.
• HW 6 due Tuesday night.
• Exam 3 to be distributed on Tuesday.
• Today's Lab writeup (due before class Wednesday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 34 (Your Names)
  • Exercises:

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery.
• Learning Deconstructed Workshop Series, Tuesdays at 11 in JRC 209.
• CS Table Tuesday: Why people support and bully online.
• Sexual Misconduct at Grinnell College: Results from the 2013 and 2015 Sexual Conduct Surveys. 1-3 pm, November 8, JRC 101.
• CS Extras Thursday: Bluetooth, Robots, and CS 161.

Peer Support

Regular Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included), Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance.
• Socrates Cafe', Saturdays, Younker, 2pm.
• Pun Club, Saturdays, 4pm, Way over Younker.

Upcoming Peer Support

• November 14, Orchestra Concert, Sebring Lewis, 7:30 p.m.
• Women's Basketball Opening Game, 1pm, Nov. 15, vs. Silver Lake.
• One-act Festival Dec. 5 & 6.

Other Good Stuff to Do

• VOTE on Tuesday

Types of procedures and evaluation order

• This is a short chance to reflect on what we've been doing with some aspects of Scheme.
• The first few procedures we learned, like square or * or even drawing-vscale had a few important properties:
  • They left their arguments unchanged
  • Even if applied to the same argument, it doesn't matter which order we do the computation (other than inside-out)
• With both the gimp tools and turtles procedures, procedures are a bit different.
  • They are likely to change their arguments. For example, if we move a turtle forward, it is now at a new location.
  • Because operations change state, the order in which the operations are applied to the same object does make a difference.

Repetition and Map

• map allows us to repeat operations.
• However, you don't know the order in which the operations are applied. (On some machines, they are even done in parallel. Certainly, when Amazon and Google use their map-reduce technology, they want things done in parallel.)
• So it doesn't make sense to use map with side-effecting operations, unless you're feeling lucky.
• What's the alternative? We can use for-each. It's almost exactly like map, except that
  • Order is guaranteed
  • You're not building a list
• What if you want to build a list and guarantee the order? Some implementation of Scheme include a variant of map called mapcar. And you'll soon learn how to implement that yourself.

Four Mechanisms for Repetition

We've now seen four mechanisms for repetition: map, recursion, repeat, and for-each. When do you use each approach?

• map: Goal is to transform each element in a list, and you want the list of transformed values as a result.
• for-each: Goal is to do a side-effecting-operation for each element in a list, in order, with a focus on accumlating side effects, rather than on computing a result.
• repeat: Goal is to do exactly the same side-effecting operation a fixed number of times., with an emphasis on accumulating side effects, rather than on computing a result.
• Recursion: The most general: Intended to do any kind of repetition, either pure or side-effecting operations, any kind of list.

Another approach to this material

This is the approach I used to use for this material. I've kept it in the eboard for historical reasons (and because I may want to go back).

The Problem: Making Spirals

• We ended the turtles lab trying to make spirals.
• Basically, you need to move forward a little and turn a little
• Three possible strategies:
  • Move the same amount each time and turn the same amount each time
  • Move a little more each time, and turn the same amount each time
  • Move the same amount each time, and turn a different amount each time
• The first strategy procedures a circle, and is possible to do with repeat.
• The other two strategies seem difficult to do without a lot of hand coding.

The for-each Function

• We'd like to use map, but there are two problems.
  • Officially, you don't know the order in which the function is applied when you do a map; it could be first to last, it could be last to first; it could be all at the same time.
  • We get back a list, which we really don't need.
• Scheme provides for-each for situations like this.
• Like map, but for sequences of actions.
  • for-each is guaranteed to work in order.
  • for-each doesn't return anything.

Lab

• Do the lab.
• Be prepared to reflect.