Held: Friday, 20 February 2015

Back to Outline 18 - Programming the GIMP Tools. On to Outline 20 - Anonymous Procedures, Revisited.

Summary

We begin to explore Scheme's conditional control structures, particularly if, when, and cond.

Related Pages

• Reading: Conditionals
• Lab: Conditionals
• Morning EBoard 19 (Source a.m. 19)
• Afternoon EBoard 19 (Source p.m. 19)

Overview

• The parts of an algorithm, revisited.
• Choosing between two options with if.
• Optional evaluation with when.
• Making multiple choices with cond.
• Comparing approaches.

Administrivia

• Continue partners!
• Quiz today!
• Review session Monday at 7pm.
• It's Friday. You know what I'd say if I were there.

Upcoming Work

• Homework 5.
  • Due next Tuesday at 10:30 p.m.
• Lab writeup for today: http://bit.ly/151-2015S-lab19
• Reading for Monday
  • Anonymous Procedures

Extra Credit Opportunities

Academic

• Pioneer Diversity Council Diversity Dialogue, 8:30 p.m. 26th, Main Quad "Academic Athletes and Academic Students". Note new date!
• Any two talks from TEDx Grinnell, February 21, 1-5 pm. Live stream at http://tedxgrinnell.com.

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• Host a prospie.

Peer Support (Afternoon Section)

• Spring new year event, Feb. 21 5:30-7:30 in Harris

Other Good Things

The Parts of an Algorithm

Remember those? We did them early on in the semester.

• You need to have some basic operations.
• You need to be able to sequence your operations, explicitly or implicitly.
• It's nice if you can name values.
• You need to be able to write subroutines (aka procedures) that parameterize a piece of code.
• You need to be able to repeat instructions.
• You need to be able to choose between options.

How many of these do we know? Almost all of them.

Choosing Between Options with if

• The first mechanism for choice.
• Form (if TEST CONSEQUENT ALTERNATE)
• Meaning
  • Evalute the TEST.
  • If the TEST is not false ("holds"), evaluate and use the value of the CONSEQUENT.
  • If the TEST is false, evaluate and use the value of the ALTERNATE.
• Note:
  • You only get one consequent
  • You only get one alternate

Optional Evaluation with when

• Useful only when you're sequencing operations.
• Form (when TEST CONSEQUENT_1 CONSEQUENT_2 ... CONSEQUENT_n)
• Meaning
  • Evaluate the TEST.
  • If the consequent is not false ("holds"), evaluate each of the consequents in turn. Use the value of the last consequent.
  • If the consequent is false, do nothing. Return nothing.
• Note:
  • You get as many consequents as you want.

Choosing Between Multiple Options with cond

• Form

  (cond 
    [TEST_1 CONSEQUENT CONSEQUENT ... CONSEQUENT]
    [TEST_2 CONSEQUENT CONSEQUENT ... CONSEQUENT]
    ...
    [TEST_n CONSEQUENT CONSEQUENT ... CONSEQUENT]
    [else ALTERNATE_1 ALTERNATE_2 ALTERNATE_n])
  

• Meaning

  • Evaluate each test in turn.
  • When a test holds (is not false), evaluate all of the consequents in sequence and return the value of the last one.
  • If none of the tests hold, evaluate all of the alternates in sequence and return the value of the last one.

Comparing Approaches

• Returns a value: cond and if.
• Lets you have multiple consequents: when and cond.
• Let's you choose between options cond and if.