Held: Monday, 8 September 2014

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
• EBoard (Source) (HTML)

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

• Please return your flash drives so that I can supply them to the other section of the class.
  • Note: I've given you virtual machines primarily so that you can do the short end-of-reading problems and finish labs in the comfort of your room.
  • I strongly recommend that you come to 3813/3815 to work on homework assignments and other problems.
• Quiz 1 returned.
  
  • Quizzes have two main purposes: They let you gauge how you well you are grasping some material and they let me see how the class is doing overall.
  • If you had difficulty on the quiz, you may want to chat with me or one of the mentors. (If you also have difficulty on quiz 2, you definitely should talk to me or one of the mentors.)
  • Greatest confusion: (ceiling -9.3) -> -9.0.
  • The return of quiz 1 serves as my attendance for today.
• Apologies for the broken link for part two of today's reading.
  
  • It should have been fixed at about 7pm Sunday night.
  • Please do let me know when you find broken links (and what link you are clicking).
• When you run DrRacket in MathLAN, make sure that you are running 5.3.6 and not 5.2.1.
• There's a lot of lecture/recitation today. It seems necessary.

Upcoming Work

• Lab writeup for the drawings-as-values lab (due Wednesday): Exercise 4, parts a, c, and e.
  See link on schedule or list of labs for link to submit it.
• I'll assign the writeup for the procedures lab tomorrow (due Friday).
• HW 2 is due Tuesday.
  • Clarification: Only one assignment per group.
  • Clarification: No need to submit A.6.
  • Note: You may want to carbon copy your colleagues.
• Readings for Tuesday:
  • How Scheme Evaluates Expressions (Take 2)
  • How Pair Programming Really Works - Distributed in paper form.

Extra Credit Opportunities

• Note: Please send a short (one paragraph) reflection on any extra credit activity you attend. And reflect, don't summarize.
• I'd like to see more of you taking advantage of these opportunities.

Academic

• Awesome show at the Faulconer gallery.
• Convo next Wednesday at noon. "Limiting Armed Drone Proliferation" by Micah Zenko, the Douglas Dillon fellow in the Center for Preventive Action at the Council on Foreign Relations and vice chair of the World Economic Forum Global Agenda Council on Terrorism.
• Any other event in the Rosenfield Drones program.
• CS Extra, Thursday, September 11: Ajuna Kyaruzi '17 on being a SysAdmin

Peer Support

• Football Games (???)
• Women's Volleyball home matches (???)
• Men's Tennis home matches (???)
• Women's Tennis (???)
• Anna Christie, Oct. 9-12 (SB plays Marthy)

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.")