Held: Tuesday, 10 September 2013

Back to Outline 06 - Drawings as Values, Continued. On to Outline 08 - Documenting Programs and Procedures.

Summary

We begin to consider how you can write your own procedures and why you might do so.

Related Pages

• Reading: Procedures
• Lab: Procedures
  • Code for the lab
• EBoard (Source) (PDF)

Overview

• Why define your own procedures?
• How to define your own procedures.
• Lab.

Administrivia

• Reminder: Mentor sessions are Thursdays and Sundays at 7:30 pm.
• Reminder: Wednesday's class is next door in 3815.
• HW3 will be distributed in class tomorrow.
• Reading for Wednesday
  • Documenting Procedures
• EC Opportunities:
  • Humanities Center Speaker Sarah Hendron Wed., Sept. 11, 7:30 p.m., JRC101 Waking the Machines: Art, Design, and Adaptive Technology
  • CS Table Friday (Reflections on Trusting Trust; NYT on NSA)
  • Other?
• How do I work from my own computer? Details and demo in class.

User Defined Procedures

• It's clear that programmers often want to (and need to) define their own procedures.
• By "their own procedures", we mean collections of Scheme commands that are parameterized and referred to be a single name, just like the built-in procedures, such as square and +, or the DrFu procedures, such as drawing-hshift.
• Procedures take inputs (which we call parameters) and may produce a result.
• Some procedures modify their parameters:
  • An "open jar" procedure changes the state of the jar
  • image-fill! adds virtual paint to an image.
• Some procedures create new values, without modifying their parameters:
• User-defined procedures can add clarity to a program.
  • Rather than looking at how code does something, the user of a procedure can focus on what the code does.
  • A reader of the program is much more likely to understand a procedure call than the body of the procedures.
• Programmers can avoid repetitive (and, therefore, error prone) code.
  • Rather than retyping the same code again and again, just changing a few values, a programmer can give a name to the same code.

• How do you define your own procedures? Using the following template:

  (define *your-procedure*
    (lambda (*param<sub>1</sub>* ... *param<sub>n</sub>*)
      *expression<sub>1</sub>*
      ...
      *expression<sub>m</sub>*))
  

• For example,

  (define square (lambda (val) (* val val)))

• You can (and should) document your procedures so that others can understand what they are supposed to do. We'll come back to this issue tomorrow.

• When the body of a procedure has multiple expressions (as in the template), only the value of the last procedure is returned.

Lab on User-Defined Procedures

• Do the lab
• Be prepared to reflect (e.g., to describe the most important or most confusing thing you dealt with today). (And no, you can't say "Sam is the most confusing thing I dealt with today.")