• New Partners!

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• The parts of an algorithm, revisited.
• Choosing between two options with if.
• Optional evaluation with when.
• Making multiple choices with cond.
• Comparing approaches.
• Lab.

Preliminaries

Admin

• Epilogue for exam 1: <151-2014F-exam1epi>
• I've been reflecting a bit on the "vocabulary" you need for our quizzes. On one hand, I think you do much better if you memorize what the procedures are and what they do. On the other, I know that such memorization can be quite difficult. Hence, as an experiment on this Friday's quiz, I will allow you to bring one page of notes to the quiz. (8.5x11 or A4, double sided, hand written)
• Office hours (modified due to meetings):
  • Wednesday 8:00-9:45
  • Thursday 8:00-9:45, 2:30-3:15
  • Friday 1:45-3:15 (also Walking hours 1:15-1:45)
• Review sessions 8pm Wednesday and Thursday.
• Let me know by 5pm today if you want to work alone on the next HW.
• I've had a student indicate that they will not be able to start HW 5 until late Sunday afternoon and that they want partners for the HW.
  • Please fill out HW 5 partners form: http://bit.ly/151-2014F-partners5
• To provide you with incentive to take your own notes on each topic we cover: If you send me a summary of what you think are the important takeaways from any reading/lab combination, I will comment on your summary.

Upcoming Work

• HW 4 due tonight.
• HW 5 to be assigned tomorrow.
• Lab writeup: Conditionals, 3c and 4d http://bit.ly/151-2014F-w19 Due Friday!
• Reading for Wednesday
  • Anonymous Procedures

Fun Things with no Extra Credit Value

• GHS Homecoming Parade Thursday at 5:30 pm. (Small town Iowa homecoming is a fascinating experience.)
• Friends of Drake Library Book Sale, Friday night and Saturday. At about 11 am on Saturday, you can get a box of books for a buck.
  (Earlier, it's 50 cents for paperbacks and $1 for hardcovers.)

Extra Credit Opportunities

Academic

• Campus Town Hall TODAY at noon or 7:30 p.m.
• CS extras next Thursday: Alumni
• CS table Friday: Serendipity and Computing (with Grinnell's Artist in Residence)
• Grinnell Prize events the following week (particularly events related to the prize winners whose project involves Web tools)

Peer Support

• Wednesday night's [Concerts]
• Anna Christie, Oct. 9-12 (SB plays Marthy)
• Football (???)
• Men's Tennis (???)
• Women's Tennis (???)
• Swimming (???)
• Evan's radio show 5pm Friday nights on KDIC
• Donna's radio show Sunday midnight on KDIC

Miscellaneous

Questions

What's the relationship between a Boolean and a conditional? Where does the Boolean part come in?

A Boolean is a truth value. True or False.

Scheme takes a slightly different view, False or "not False" "Not false" includes true, but also 45, "hello", "not false", drawing-unit-circle, ...

Procedures that assess some chacteristic of values are called predicatees (return true/false, or ??/false)

The parts of an algorithm, revisited

• We need to be able to write subroutines/procedures
  • Parameters
  • Instructions
  • (define NAME lambda (PARAMETERS) BODY)
• We need to be able to name values ("Variables")
  • (define NAME VALUE)

• We need to be able to sequence operations, implicitly or explicitly

  • Approximate: Nesting. Inner expression gets evaluated first. But if the outer expression has multiple inner expressions, they can be evaluated in any order.

  • Exact: In the body a procedure, if we have one expression, then another, then another, they are evaluated in sequence.

    (define image-quincunx! (lambda (image color) (image-set-pixel! image 0 0 color) (image-set-pixel! image (+ -1 (image-width image)) 0 color)

  • Clever use of lambda can also achieve some control of sequence
• We need to be able to repeat operations
  • Fixed number of times
  • Until a goal is achieved
  • map
  • image-transform! or image-variant
  • We will learn more general approaches next week
• We need a set of basic operations and values
  • Numbers and numeric operations
  • Images and their operations
  • Drawings and their operations
• We need to be able to make choices

if, when, and cond

• See the start of lab.

Comparing approaches

• Do you have an alernative? Use if or cond? If not, use when
• Do you want to do more than one thing if a test holds? If so, use when or cond.
• Do you have multiple tests to do in sequence? If so, use cond (or nested if statements).

Lab

Two ways to write irgb-brighter?

(define irgb-brighter?
  (lambda (color1 color2)
    (if (> (rgb-brightness color1) (rgb-brightness color2))
        #t
        #f)))

vs.

(define irgb-brighter?
  (lambda (color1 color2)
    (> (rgb-brightness color1) (rgb-brightness color2))))

Two ways to write irgb-grey4 with if?

(define irgb-grey4
  (lambda (color)
    (if (< (irgb-brightness color) 0.25)
        grey1
        (if (and (< (irgb-brightness color) 0.5)
                 (>= (irgb-brightness color) 0.25))
             grey2
             ...))))

vs.

(define irgb-grey4
  (lambda (color)
    (if (< (irgb-brightness color) 0.25)
        grey1
        (if (< (irgb-brightness color) 0.5)
            grey2
            ...))))