Held: Wednesday, 28 January 2015

Back to Outline 05 - Drawings as Values. On to Outline 07 - Writing Your Own Procedures, Continued.

Summary

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

Related Pages

• Reading: Drawings as Values
• Reading: Writing Your Own Procedures
• Reading: How Scheme Evaluates Expressions (v2)
• Lab: Drawings as Values
• Lab: Writing Your Own Procedures
• Morning EBoard 6 (Source a.m. 6)
• Afternoon EBoard 6 (Source p.m. 6)

Overview

• Sequencing operations in Scheme.
• The drawings-as-values model, revisited.
• Why define your own procedures?
• How to define your own procedures.
• Evaluating expressions in Scheme.

Administrivia

• New partners! These will also be your homework partners.
• Office Hours: Normally: 10:00-11:00 MTWF, 3:15-4:00 M (may change)
• Please follow the suggestions for the title of your email messages; many of the graders have filters running to automatically move work to grade into a particular folder.
• I'm going to try to talk less today; we'll see how well I do.
• Quiz 2 is Friday
  • Evaluation in Scheme (define, order of evaluation, etc.)
  • Drawings as values
  • Not procedures
• Don't forget: Review sessions Thursday at 9:00 a.m. and 1:15 p.m., Mentor session Thursday at 7:00 p.m.
• I'm going to try talking less at the start of class today. You'll need to take responsibility for reading the outline/eboard for announcements.
• We normally expect today's lab to take about 1.5 class periods.

Upcoming Work

• Assignment 3.
  • Due next Tuesday at 10:30 p.m.
• Lab writeup for lab 6 (today), due Friday before class: http://bit.ly/151-2015S-lab06
• No additional readings for Friday.

Extra Credit Opportunities

Academic

• Friday, 12:10 p.m., JRC 224A (Day PDR), CS Table, To be determined.

Peer Support (Morning Section)

• Swimming, Friday, January 30, 6:30 p.m., Osgood Natatorium
• Swimming, Saturday, January 31, noon, Osgood Natatorium
• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m. This week's theme is Transportation/Leaving.

Peer Support (Afternoon Section)

• Meditation group, Saturday, 4 p.m., CRSSJ
• Meditation group, Wednesday, 9:00 p.m., CRSSJ
• Swimming, Friday, January 30, 6:30 p.m., Osgood Natatorium
• Swimming, Saturday, January 31, noon, Osgood Natatorium
• Men's Tennis, Saturday, January 31, Field House

Recommended Events (no EC unless mentioned above)

• #OneGrinnell, Wednesday, February 4 at 4:15 p.m.

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, with 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.

Drawings as Values

• There was some confusion on drawings as values, so we'll spend a little bit of time on these issues before you continue that lab.
• In the MediaScheme/gigls world, drawings are like numbers in that once you create a drawing, you can't change that drawing.

• Compare these two pairs of operations

  (define val 3)
  (sqrt val)
  
  
  (define d drawing-unit-circle)
  (scale-drawing 5 d)
  

• Because of this model, we often describe drawings with a series of nested operations (innermost first).

• We also differentiate "drawings", which are abstract descriptions of an image, from "images", which are the rendering of a drawing on something you can see.
  • You need to use drawing->image to convert the drawing to an image.
  • You need to use image-show to display the image.
• MediaScheme/gigls assigns an integer to each image, corresponding to the "Untitled #" that appears as the image title in Gimp.
• I think that covers the main issues. Let me know if there are other issues.

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

• Continue the lab on drawings as values
• 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.")