CSC 151.01, Class 16: Preconditions, revisited

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Friday PSA
  • Questions
• An example from Wednesday
• Quiz
• Lab

News / Etc.

• New partners.
• Please make sure to return your computer cards to the jar.
• I was sorry to see so few of you at the Wilson Okello convo. It was one of the better convocations I’ve seen.

Upcoming Work

• Writeup for class 15 due TONIGHT at 10:30 p.m.
  • Exercises 5 and 6
  • To: csc151-01-grader@grinnell.edu
  • Subject: CSC 151.01 Writeup 15 (YOUR NAMES)
• No writeup for today’s class.
• Assignment 5 due Tuesday at 10:30 p.m.
• Reading for Mondays’s class: Recursion basics

Extra credit (Academic/Artistic)

• Any of the many Grinnell Prize events.
• CS Table, Tuesday: Tapia.

Extra credit (Peer)

• Women’s Tennis vs. Cornell, Noon, Sunday (high-school courts)
• Volleyball vs. Knox Tuesday at 7:00 p.m.
• Volleyball vs. Beloit, next Friday at 7:00 p.m.
• Men’s Soccer Wednesday at 4:30 vs. Cornell

Friday PSA

• Think about your choices, in advance.
• Self gov (individually and collectively)
• Consent is absolutely positively necessary. But you should strive for more than consent.

Questions

What is the difference between a Boolean and a predicate?

Booleans are basic values, like integers, reals, and strings. The two Boolean values are #t and #f. The design of Scheme is such that anything that’s not a Boolean is treated as “truish” and equivalent to #t in conditioanls.

Predicates are procedures that return Boolean values.

We put question marks after both predicates and Boolean values to highlight that they are or return true/false.

Can you help with problem 1 on the homework?

Sure. Let’s look at what the problem says.

Step one: Read all the words from the file, file-words

Step two: Convert to a list of lengths. map

Step three: Tally. tally-all

Step four: Scale. map

Step fourb: What do we map with? Something that divides the second element by the number of words.

Step five: Sort

(define explore-lengths
  (lambda (fname)
    (let* ([words (file->words fname)]
           [num-words (length words)]
           [scale-second (lambda (pair) 
                           (list ... (/ (cadr pair) num-words)))]); Divide the second element by num-words
      (plot 
       (discrete-histogram
        (sort (map ... 
                   (tally-all (map FUNCTION 
                                   words)))
              (comparator ...)))))))
  ```

I'm not sure what to put as the postconditions for `explore-lengths`.
  : Yeah, that's a hard one.  You can't be as precise, but say something
    about the number of bars you see and such.

For problem 1 (and others), how do I submit the graphs?
  : Don't.

For problem 1, what should I do about word lengths that aren't included?  I feel like it distorts the results.
  : Ideally, you'd come up with a clever solution.  But it's a rare
    enough case that I do not require you to come up with a solution.

What's the goal in problem 3?
  : To make a collection of "nearby" words, where "nearby" means
    appearing immediately after, or two after, or three after a
    word

Shouldn't I get a length that is a multiple of 3?
  : Usually yes.  But there are some cases in which the word appears
    at the end, so you don't get a multiple of three.

An example from Wednesday's class
---------------------------------

(define zips (read-csv-file “/home/rebelsky/Desktop/us-zip-codes.csv”)) (define zips3 (filter (lambda (entry) (string-equal? (substring (car entry) 0 3) “021”))) zips)

(define lat-lon (lambda (entry) (list (cadr entry) (caddr entry))))

(define lon-lat (lambda (entry) (list (caddr entry) (cadr entry))))

;(plot (points (map lat-lon (filter useful-entry? zips))))

;(plot (points (map lon-lat (filter useful-entry? zips))))

;(plot (points (map lon-lat (filter useful-entry? zips)) ; #:x-min -150 ; #:x-max -50)) ;(plot (list ; (points (map lon-lat (filter useful-entry? zips)) ; #:x-min -150 ; #:x-max 0) ; (points (map lon-lat (filter useful-entry? zips3)) ; #:sym ‘fullcircle6 ; #:fill-color “red”)))

Quiz
----

Double-sided.  Whoo!

Lab
---

Insufficient time.  Whoo!  

Yes, problem 2 is a big `and` statement

(define city? (lambda (thing) (and (list? thing) (= (length thing) 6) (string? (list-ref thing 0)) (real? (list-ref thing 1)) …))) ```

No writeup! Please do take the time to read through all of the problems, though, and to attempt them.

Debrief