• Continue partners!
  • We have some illnesses, so we may need to do a bit of swapping.
• Printed copies of tomorrow's reading are available near the crayons

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Pair programming.
• Documentation - A strategy.
• Practice.

Preliminaries

Admin

• EG says "Computer science can be dangerous!"
• Review sessions:
  • Wednesday, 8pm (Zach)
  • Thursday, 10am (Sam)
  • Thursday, 8pm (Erin)
• Mentors and tutors cannot provide direct assistance on exam questions.
  However, they can go through any example from the readings and any problem from the homework.
• If you have questions you'd like me to answer, you can write them on blank index cards and hand them to the mentors. I'll send out answers tomorrow.
  • I cleverly left yesterday's question at home. I'll try to answer it tonight.
• Our graders are even slower than I am. Sorry!
• Control-I is useful in the definitions pane - it reindents code based on the parenthesization.
  
  • It makes your code easier to read
  • It tells you about possible parenthesization error

Upcoming Work

• Read for Tuesday: Testing Procedures with Rackunit
• Assignment 3 due Tonight @ 10:30 p.m.
  • Your partner is the person who you worked with last Wednesday and Friday.
• Exam 1 to be distributed Tomorrow.
• Lab writeup (due before class Friday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 11 (Your Names)
  • Exercise: NONE!

Extra Credit Opportunities

• I'm a little behind in responding to these. Sorry!

Academic

• Tuesday, noon, CS Table, JRC 224C. Hacking Cars
• CS Extras, Wednesday, Jonathan Wellons '04 from Google, 4:15 3821
• Convo Thursday at 11 am in JRC 101: Mike Latham
• Art opening (or any visit to the art show): Friday and beyond

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 1:30 and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker.
• Singers concert on Family weekend! Sept. 26, 2pm., probably JRC 101
• Orchestra concert on Family weekend! Sept. 26, 2pm., probably Sebring-Lewis
• Softball Alumna Game, 9am on Sunday

Questions

Even though I've completely blown the *two days* policy for extra credit, will you have sympathy on me because my sister burned her finger?

Yes

What do you want me to write in the extra credit thing?

A reflection on the event. A few sentences.

Who do we send the extra credit responses to?

rebelsky@grinnell.edu

Doesn't putting your email address on a Web page lead to spam?

Yes.

Can we use your irgb-average on problem 4 from yesterday's lab?

Certainly! The goal was to annotate, not to have to write from scratch.

I think my procedures are right for HW3, but I'm having trouble converting to lambda.

Sam's hypothesized example

    (define color1 (irgb 200 100 40))
    (define color2 (irgb 100 200 50))
    (define average (irgb (* 1/2 (+ (irgb-red color1) (irgb-red color2)))
                          (* 1/2 (+ (irgb-green color1) (irgb-green color2)))
                          (* 1/2 (+ (irgb-blue color1) (irgb-blue color2))))

General model

    (define input1 ...)
    (define input2 ...)
    (define computedvalue EXPRESSION) 

    (define compute-value
      (lambda (input1 input2)
        EXPRESSION))

Back to example (define my-irgb-average (lambda (color1 color2) (irgb (* 1/2 (+ (irgb-red color1) (irgb-red color2))) (* 1/2 (+ (irgb-green color1) (irgb-green color2))) (* 1/2 (+ (irgb-blue color1) (irgb-blue color2))))))

What's the relationship between this color1 and color2 and the ones we defined earlier?

There is none. Scheme will just use those names for whatever you provide as input to your procedure.

Should we have parameters that have the same names as variables (or vv)?

It works, but it's probably not a good idea because it will confuse someone (maybe you, maybe me, maybe the grader)

You should choose good names for your parameters and variables, even though it is possible to pick really bad ones. Think of how hard it is to read

(define irgb-munge
  (lambda (- whatever)
    (irgb (+ whatever (irgb-red -))
          (+ whatever (irgb-green -))
          (irgb-blue -))))

Can I use a procedure as an input to another procedure with lambda?

Yes! As long as you use the parameter in the place we expect a procedure to be.

    (define apply-to-blue
      (lambda (transformation)
        (transformation (irgb 0 0 255))))

Can we define using lambda?

Not right now.

Pair programming

You have a classmate (or mentor) (or teacher) who has not read the article. Your goal is to summarize the central points.

• Two people sit at the same computer and work together.
• They have two roles
  • Driver: One is writing the code
  • Navigator: One is looking over his/her/zir shoulder and making comments (checking correctness, suggesting other approaches, contribute ideas, etc.) Also looks up information!
• Why do this?
  • Different knowledge combines well! Think differently, compare perspectives. Two heads are smarter than one.
  • More eyes catch more errors!
  • Pair pressure - it's better than peer pressure - you don't want to let your partner down; hold each other to higher standards
  • Get stuck less!
  • Explain to anything helps you understand a problem better; explaining to a human being who can respond helps more!

Documentation

You have a classmate who has not read the reading. Your goal is to summarize the central points.

• We document to clarify code for people who don't want to read code
• We document to force ourselves to think more carefully about the code; what inputs are reasonable, what outputs are expected, what do we know about those output?
• Six P's
  • Procedure: The name
  • Parameters: The names of the inputs and their types
  • Purpose: Tells you what the procedure does
  • Produces: The name and type of the output, maybe a little more info about what kind of value we return

Documentation Practice

(max v1 v2)

    ;;; Procedure:
    ;;;   max
    ;;; Parameters:
    ;;;   v1, a real number
    ;;;   v2, a real number
    ;;; Purpose:
    ;;;   Takes the maximum value of v1 and v2, whichever is larger
    ;;; Produces:
    ;;;    result, a number, the larger of v1 and v2
    ;;; Preconditions:
    ;;; Postconditions:

    ;;; Process:

To be continued ...

(max v1 ... vn)

(irgb-average c1 c2)