Held: Monday, 28 September 2015

Back to Outline 17 - Programming the GIMP Tools. On to Outline 19 - Pause for Breath.

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

• New partners!
• I hope you had a great family weekend, whether or not your family was able to visit.
• For the first time in a long time, I was unable to get the CSC 151 quizzes or exams graded over the weekend. A few other tasks with tight deadlines are coming up, so you probably won't get them back until Friday. I apologize.
  • I will get your quizzes back to you tomorrow.

Upcoming Work

• Reading for Tuesday: Anonymous Procedures
• HW 4 due Tuesday night.

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery.
• EE's show in Smith
• CS Table: YouTube Content Identification.

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.
• PALS dinner Thursday night. (Sam will treat if you find it a financial burden.)
• Pun Club, Saturdays, 4pm, Way over Younker.

An Exam Note

What should I do if I got the following as sample output on problem 3?

> (seconds (add-times (time 0 0) (time 0 97)))
37
...
> (seconds (add-times (time 0 0) (time 0 97)))
97

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.