Held: Friday, 2 October 2015

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 21 (Source 21)

Overview

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

Administrivia

• Continue partners!
• We may have prospective students.
• Friday PSA.

Upcoming Work

• Reading for Monday: Building Images By Iterating Over Positions
• HW 5 due Tuesday
  • Use today's partner.
  • I'll do a demo of parts.
• Today's Lab writeup (due before class Tuesday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 21 (Your Names)
  • Exercise:

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery
• Liz Queathem talk 4pm Friday
• MET Livecast Saturday at noon at Harris Auditorium

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.
• Pun Club, Saturdays, 4pm, Way over Younker.
• Mid-Autumn Festival, Saturday, Oct. 3, 6-8 p.m. in Harris Concert Hall.

Other Good Things

• VBall Saturday at 1pm in Darby
• American Football Saturday at 1pm at the football field
• Women's Unamerican Football Saturday at 11am
• Men's Unamerican Football Saturday at 1pm
• Women's Soccer Sunday at 11am
• Men's Soccer Sunday at 1pm
• GrinnellLink Alumni Reception 4:45 today in JRC 2nd

Notes on Exam 1

• To be returned at the end of class.
• Exam notes distributed via email.
• A few of you did really poorly. It makes no sense for you to go on to take exam 2 until you understand the material on exam 1. I will provide you with a makeup exam. You should come see me for help. We'll negotiate an appropriate due date for exam 2 (probably Thursday after break).
• It's really frustrating to take off points because people didn't do the epilogue. There's a reason I give you the checklist.
• Please don't discuss your grade with other people. (Discuss problems, but not grades.)
• Some good notes from the epilogues.
  • "Start early, start early, start early"
  • Keep scratch paper nearby and work out the details on paper.
  • Move on to the next question if I get stuck.
    • And send Sam email
  • Look back on past readings
  • Ask Sam more questions, and ask Sam questions early on.
  • Be well rested.

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.