Held: Friday, 5 February 2016

Back to Outline 07 - Transforming Images. On to Outline 09 - Writing Your Own Procedures, Continued.

Summary

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

Related Pages

• Reading: Writing Your Own Procedures
• Reading: How Scheme Evaluates Expressions (v2)
• Lab: Writing Your Own Procedures
• EBoard 8 (Source 8)

Overview

• Sequencing operations in Scheme.
• Why define your own procedures?
• How to define your own procedures.
• Evaluating expressions in Scheme.

Administrivia

• The Registrar's Office wants you to know that today is the last day to add or drop a full-semester class.
• Today's lab will continue through Monday's class, although we may have a writeup from early in the lab.
• Friday PSA.

Reminders

• Office hours: MTWF 10-11, Tu 1-2.
  • Sign up at http://rebelsky.youcanbook.me.
  • Also feel free to stop by when my door is open.
  • Or to email me for an appointment.
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with Zacharcy
  • Thursday at 10am in this room with SamR
  • Thursday at 8pm in the CS Commons with Kumar

Upcoming Work:

• No additional readings for Monday.
• Assignment 3 due Tuesday at 10:30 p.m. (Your assignment partner is your partner from today.)
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 8 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Tuesday.

Extra Credit

Academic

• Africans vs African Americans, Noon, Saturday, JRC 101.
• Franz Schubert's Winterreise, Sunday, 2 p.m. Sebring-Lewis
• Next week's Rosenfield Symposium on Campaign Finance Reform.
• Monday, February 8, 4:15 p.m. in Bucksbaum Miriam Langer enjoins you to "Do Your Work in Public" (part of the series "Digital in Public: Intellectual and Technical Skills for Making a Difference")
• Monday, February 8, 7:30-9:00, JRC 101: The Complex Relation Between Alcohol and Sexual Assault
• Noon Tuesday in the White PDR: Bowie, Mickey, and the NFL: Copyright in the 21st century (copies of the reading outside Sam's office)
• 7:30 pm next Tuesday in ARH 131 - Funding for unfunded internships
• Anything in next week's Carnival and Creativity series

Peer

• Track and Field Grinnell Invite February 6. (10 a.m.)
• Swimming February 6 vs. UN and IAState. (1 p.m.)

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio (starting next week)
• Pun club Saturdays at 4pm in Younker (max 2)
• Socrates Cafe' Saturdays (max 2)
• Electronic Potpourri on KDIC Fridays at Five

No Extra Credit, But Still Good

• Fill out the Disability Student Space Survey https://docs.google.com/forms/d/1hIXOjkd1j9goBgoWoDv2EiZu8vhhETOZS3bDTcKypLE/viewform?usp=send_form

Review

• What are the components of algorithms?
• Which of those have we learned so far?
• What things don't make sense?

Sequencing Operations

• We noted that the sequence of operations is an important aspect of an algorithms. (You want a jar open before you put the knife in the jar.)
• Different languages have different approaches to sequencing operations.

• In Scheme, we often write compound expressions, expressions that include subexpressions.

  (sqrt (+ (square x) (square y)))
  

• Which is done first, square, +, or sqrt?

  • Scheme (usually) works from the inside-out, so we do the square operations first, then the addition, then the sqrt.
• Which of the square operations is done first?
  • The Scheme standard says that the interpreter can do them in either order.
• Do we have other ways of sequencing operations?
  • A series of definitions provides one mechanism for doing so.

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 hshift-drawing.
• 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.
  • (We'll see some in Scheme a bit later.*
• Some procedures create new values, without modifying their parameters:
  • sqrt
• 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 (param₁ ... param_n) expression₁ ... expression_m))

• 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 soon.

• When the body of a procedure has multiple expressions (as in the template), only the value of the last expression is returned.
• When you call a procedure, it's like you do a define for each parameter as the corresponding sent value.

• So (square 5) is a lot like

  (define val 5) (* 5 5)

Evaluation in Scheme

• Two main issues:
  • Table that associates names and values.
  • Where we are in the expression we are evaluating.
• General process
  • Evaluate from inside out
  • If you see a name, look it up in the table
  • If you see a value, leave it as is
• Complication from procedures
  • We also look up procedures in the table.
  • When we run a procedure, we temporarily update the table, associating the values of parameters for their names.

Labs

• Begin the lab on procedures
• 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.")