Held: Monday, 24 February 2014

Back to Outline 20 - Naming Local Values. On to Outline 22 - Images as Functions from Position to Colors.

Summary

We revisit procedures in Scheme. We consider a variety of techniques for writing anonymous procedures, procedures without names.

Related Pages

• Reading: Anonymous Procedures
• Lab: Anonymous Procedures
• EBoard (Source) (HTML) (PDF)

Overview

• What is a procedure?
• Describing procedures.
• Anonymous procedures.
• Anonymous procedures through lambda expressions.
• Other kinds of anonymous procedures.

Administrivia

• Reading for Tuesday: Building Images by Iterating Over Positions
• Our computer system is (still) acting up. Some aspects of lab may take longer than they should.
• A few of you are frustrated that your indentation changes when you cut and paste into Google docs. If you use Chromium for accessing Google docs, cut and paste should work.
• I think I only got one request for a partner on the current homework, but we'll do a quick survey.
• Study break tonight at 8pm.
• Writeup: Exercise 5, Parts b-f
  • Subject: CSC 151.02 Writeup 15: Anonymous Procedures (OPTIONAL NAMES)
• Extra credit:
  • Tuesday Town Hall, noon in JRC 101, 7:30 p.m. in Harris.
  • Tuesday wellness event.
  • Thursday CS Extras: New CS Curriculum.

What is a procedure?

• One of the basic building blocks of programming.
• Essentially, an encapsulated algorithm
  • Takes some inputs
  • Generates some outputs

Describing Procedures

Traditionally, we write something like

(define *name-of-procedure*
  (lambda (*parameters*)
    *body*))

When applying a procedure, we effectively substitute the arguments we call the procedure on for the parameters in the body, and then evaluate the body.

Anonymous Procedures

• Procedures without names.
• Useful when we just need a procedure once
  • Saves the need to define it (code is more concise)
  • Saves the need to document it
  • Makes code work on its own
• In Scheme, whereever you write a procedure name, you can also write the corresponding lambda expression.
• Very useful with map
• Examples
• ((lambda (x y) (* (+ x y) (- x y))) 2 3)
• (map (lambda (x) (* 5 (+ 1 x))) (iota 10))

Other ways to build anonymous procedures

• Sectioning
• Composition

Lab

• Do the lab.
• Be prepared to reflect.