CSC 151.01, Class 25: Preconditions, Revisited

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Quick reading summary
• Lab

News / Etc.

• New partners!
• Quizzes returned; current grade status coming tonight.
  • There was enough confusion on quiz 5 that I will now drop the two lowest quiz grades. (That’s better than not counting it.)
• Note to the Narrator: If you are going to draw cartoons about my exams, make sure that you include one about sending email questions.
• Welcome to any visiting prospective students.
  • Please introduce yourself.
  • Please indicate what other schools you are considering.
    • A lot of places
    • Tulane, MIT, Rice, State School (none of these are like Grinnell)
    • About ten schools
    • East coast schools, like Hamilton
    • Midwest, Seattle, Bay Area
  • You may ask the students in this class a question.
    • Is this class hard?
      • Yes, but when you finally figure things out it’s fun. Plus Sam makes really bad jokes.
    • Did you meet someone new in this class?
      • Yes, whether or not that’s a good thing is questionable
    • How many of you have declared a major?
      • Some
    • What’s something you’ve learned in this class? (Other than CS)
      • I’ve learned there are cool people on the top of my head
      • Don’t spend hours staring at your laptop screen making no progress. Ask for help! (Also don’t hit your head against the wall)
      • A bit about art things, such as colors and color palettes
      • Practice self gov
    • Why Grinnell
      • Get personalized attention
      • Small enough that you’ll meet some people quickly
      • One-on-one individualized attention
      • We bribe you to come, even though Kington wants us to stop
      • Not so distracting, not so expensive
      • Events are free, including the concerts, the plays, the amazing basketball/hockey team.
      • Inarticulate administrators
      • You will have construction on campus for most of your time here. Check out the Facebook page for more details.
      • People are happy (hah!) care about each other (Have you met Prof. Rebelsky?), and like it here (that’s why Kington thinks it’s imperative to increase the retention rate)
      • We’re not Reed.
    • How do you learn in this class?
      • cricket noises

Rotating reminders

• Ask questions via email! I’m always happy to (try to) answer questions via email. There is no need to apologize when sending me questions. If I take too long to answer, send another email (or even text, if it’s a reasonable hour).

Upcoming Work

• Exam 2 due TOMORROW NIGHT at 10:30 p.m.
• Epilogue due Wednesday night
• Lab writeup: Exercise 3
• Reading: Characters and strings

Extra credit (Academic/Artistic)

• CS Table, Tuesday at noon, Generating poetry
  • Readings should be outside Curtsinger’s office
• CS Extras, Thursday at 4:15 pm, The CS curriculum
• Up to two of the events in the Rosenfield Technology and Human Rights symposium

Extra credit (Peer)

• Playboy of the Western World, this weekend (9th-12th).
• Ritalin Test Squad Friday the 10th.
• South Asia Tea Time Friday 4pm in JRC Cultural Suite
• G-Tones concert, March 12 with Opposed to Toy Trains (aka Con Brio), time tbd in Bucksbuam Rotunda
• Grinnell Singers, March 12 at 2pm

Extra credit (Misc)

• Tuesday 11:00 a.m. discussion about controversial speakers

Good things to do

Notes on quiz 5

Part 1

(define v #f)
(define w #t)
(define x 5)
(define y "123")
(define z 0)
(and x y)
(or v y z w)
(< z 4 x 5)
(or (< x z) x)
(not (or (< x z) x))

What does (or exp1 exp2 exp3) do?

Evaluates each expression in turn. If any are trueish it returns the first trueish one (and doesn’t evaluate any remaining expressions). In all of them are #f, we get back #f.

What does (and exp1 exp2 exp3) do?

Evaluates each expression in turn. If any are false, immediately returns false. If all are trueish, returns the value of the last one.

What does (not exp1) do?

Evaluates the expression. If the expression is #f, it returns #t. If the expression is #t or trueish, it returns #f.

(define v #f) (define w #t) (define x 5) (define y "123") (define z 0)
> (and x y) => (and 5 "123") => "123"
> (or v y z w) => (or #f "123" 0 #t) => "123"
> (< z 4 x 5) => (< 0 4 5 5) => #f
> (< 0 4 5 521321321) => #t
> (or (< x z) x) => (or (< 5 0) 5) => (or #f 5) => 5
> (not (or (< x z) x)) => (not (or (< 5 0) 5)) => (not 5) => #f

What is trueish?

Scheme is weird. It has three kinds of truth. There’s #f, which is false. There’s #t, which is true. There’s everything else, which is trueish.

Part 2

Create the list '(20 19 18 17 26 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1).

> '(list 20 19 18 17 26 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1)`.
> (iota 20) ; gives (0 1 2 3 ... 19)
> (reverse (iota 20)) ; (gives 19 18 17 ... 0)
> (map increment (reverse (iota 20))) ; gives what we want
> (reverse (map increment (iota 20))) ; gives what we want
; We need map to do something to each element in the list
> (map - (make-list 20 20) (iota 20)) ; gives the right answer, but builds an extra list
> (map (section - 20 <>) (iota 20))

Questions

What does it look like when DrRacket corrupts our exam files?

When you try to open the file, DrRacket says “Sorry, I can’t read that file.”

Sometimes, Sam can help.

Quick reading review

Discuss with partner and be prepared to report back.

What are preconditions?

Expectations about the input or state of the system that must be met if the procedure is to work correctly.

What normally happens if they are not met?

We can do whatever we want.

How do we verify preconditions so that the client gets an error

message when the preconditions are not met?

Use conditionals and then calling error when they are not met

What is husk-and-kernel programming?

Iowa’s contribution to program design

Lab

Your writeup is Exercise 3.

Writeup

Write up exercise 3 from the lab verifying preconditions.

Send your solution to csc151-01-grader@grinnell.edu.

Title your email CSC 151.01 Writeup for Class 25 (YOUR FULL NAMES).