Held: Tuesday, 23 February 2016

Back to Outline 17 - Boolean Values and Predicate Procedures. On to Outline 19 - Naming Local Values.

Summary

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

Related Pages

• Reading: Conditionals
• Lab: Conditionals
• EBoard 18 (Source 18)

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.
• Quizzes returned.
• We'll continue to talk a little bit about testing.

Reminders

• Office hours: MTWF 10-11, Tu 1-2.
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Upcoming Work:

• Reading for Wednesday
  • Naming Values wiht Local Bindings
• Assignment 5 due TONIGHT at 10:30 p.m.
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 18 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic

• Town Hall, Wednesday 7:30 pm in JRC 101
• CS Extras, Thursday: ????

Peer

• Go to Wednesday's SGA meeting at 8:30 p.m in JRC 209 to make noise about the reserve.
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Regular Peer

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Misc

• Christine Tran show in Smith Gallery.

No Extra Credit, But Still Good

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.