EBoard 19 (Section 1): Randomness

Approximate overview

• Administrative stuff [~5 min]
• Racket stuff [~10 min]
• Questions [~5 min]
• Lab [~60 min]

Administrivia

Introductory notes

• Thank you to those of you who posted corrections in your reading questions. I enjoy awarding tokens.
• I have sent out your current grade reports.
  • It appears that Gradescope misled me about the readings for class 15; I’ll get the rest of those graded soon.
  • Our graders are trying hard to catch up on grading.
  • If you notice issues on LAs or readings, let me know.
  • You should get another grade report on Sunday or Monday, after I’ve finished grading the SoLA.
• The College required me to post midterm assessments yesterday.
  • Fewer than 7-8 LAs generally earned you an M or an R.
  • I will likely be sending out academic alerts for the same reason.
  • Unfortunately, midterm assessments don’t match well with the mastery grading we use in this course, particularly since the graders have not graded redos and you have not had a chance to do SoLA 2. Take your midterm assessments with a large grain of salt.

Class mask policy

• Please bring your mask to class. Wear it if you wish.
• If your partner is wearing a mask, please put on your mask.
• As a member of a high-risk group, I will be wearing a mask at least through spring break.
• Note: Masks remain good for preventing transmission of cold and flu and other similar diseases.

Reminders

• Please say your name when you ask or answer a question (even if I’ve just called you by name).
• Evening tutors are available 7–10 p.m. Sunday through Thursday as well as 3–5 p.m. on Sunday.
• Mentor sessions are Monday 8–9 p.m. Wednesday, 8–9 p.m., Sunday 4–5 p.m.

Upcoming work

• No reading for Friday.
• Today’s lab due Thursday at 10:30 p.m.
  • As usual, “SAM SAID I CAN STOP HERE”
• SoLA 2 released today at 2:30 and due Thursday the 10th at 10:30 p.m.
• Quiz 7 due Sunday at 4pm: Use higher-order procedures.
• Week seven survey distributed Friday, due Sunday at 10:00 p.m.
• Reading for Monday’s class, also due Sunday at 10:00 p.m.
• MP 5 due next Thursday at 10:30 p.m. To be released Friday.

Upcoming Token-Generating Activities

• THURSDAY 11 a.m., JRC 101: Scholars’ Convocation: Greg Duncan ‘70. America’s Long Struggle to Reduce Child Poverty

Other Upcoming Activities

Sample Quiz 7

Use section and composition to simplify computations.

Consider the following procedures

;;; (vowel? char) -> boolean
;;;   char : char?
;;; Determine if char is a vowel.
(define vowel?
  (let ([vowels (string->list "aeiou")])
    (lambda (ch)
      (integer? (index-of vowels (char-downcase ch))))))

;;; (count-vowels str) -> integer?
;;;   str : string?
;;; Count the number of vowels in str
(define count-vowels
  (lambda (str)
    (tally vowel? (string->list str))))

;;; (select-special-words words) -> list-of string?
;;;   words : list-of string?
;;; Selects all the special words in words using the ALTV criterion.
(define select-special-words
  (lambda (words)
    (filter (o (section > <> 2) count-vowels) words)))

a. What kinds of words does select-special-words select?

b. Explain how (o (section > <> 2) count-vowels) works as a predicate for such words.

c. Rewrite vowel? using section and composition but no lambda.

Bonus hard question

You are unlikely to receive a problem this hard.

Consider the following procedure.

(define silly
  (lambda (lst)
    (map (lambda (x) (sqr (+ 1 x)))
         (filter odd? lst))))

Rewrite the procedure using o and section so that it has no lambdas.

Notes:

• Use o when you want to sequence actions. (Do this to the parameter, then this to the result, then this to the next result, and so on and so forth.)
• Use section when you want to fill in one or more parameters to a procedure, thereby creating a new procedure.
• This is a case in which the lambda-free version is likely much harder to read.

Racket/Lab Stuff

The local bindings self check

Rewrite v2c-ratio so that we don’t duplicate work.

Original

;;; (v2c-ratio str) -> rational?
;;;   str : string
;;; Determine the ratio of vowels to consonants in str
(define v2c-ratio
  (lambda (str)
    (/ (tally vowel? (string->list str))
       (tally consonant? (string->list str)))))

Rewritten (without let)

(define v2c-helper
  (lambda (lst)
    (/ (tally vowel? lst)
       (tally consonant? lst))))

(define v2c-ratio
  (lambda (str)
    (v2c-helper (string->list str))))

Rewritten (with a local helper)

(define v2c-ratio
  (let ([helper
         (lambda (lst)
           (/ (tally vowel? lst)
              (tally consonant? lst)))])
    (lambda (str)
      (helper (string->list str)))))

or

(define v2c-ratio
  (let ([helper
         (lambda (lst) (/ (tally vowel? lst) (tally consonant? lst)))])
    (lambda (str)
      (helper (string->list str)))))

Rewritten, substituting the local helper

Sam will attempt this “live”, perhaps with help.

(define v2c-ratio
  (lambda (str)
    ((lambda (lst) (/ (tally vowel? lst) (tally consonant? lst))) 
     (string->list str))))

No, I don’t expect you to do this.

Termial and natural numbers

> (natural? 0)
#t
> (natural? 1)
#t
> (natural? 4)
#t
> (natural? -1)
#f
> (natural? 1.0)
#f

Moral: If we say that something expects a natural number, we mean “an exact non-negative integer”.

Quiz 6

While I won’t put answers in the eboard, I will go over it in DrRacket.

The bad range procedure

Here’s the “bad” version of range from the lab

(define bad-range
  (lambda (n)
    (if (zero? n)
        null
        (cons n (bad-range (- n 1))))))

As you might have noted, this gives us the values in the wrong order. Although it’s inefficient, we might consider adding a reverse to the mix to get them in the right order.

(define less-bad-range
  (lambda (n)
    (if (zero? n)
        null
        (reverse (cons n (less-bad-range (- n 1)))))))

Will this work? (Ignore the inefficiency.)

Here’s another approach. Since we’re putting the values at the wrong position, just swap the parameters to cons.

(define lesser-bad-range
  (lambda (n)
    (if (zero? n)
        null
        (cons (lesser-bad-range (- n 1)) n))))

Will this work?

Questions

Reading questions

Why do we include (lambda () ...) in the definitions of random functions?

The lambda () makes it a function, which means that we execute the body each time it is called, giving ourselves the opportunity to get different results. If we don’t have the lambda, it’s just a value, and it does not change (at least in one run of the program).

Other issues

I’d like to build a library of procedures I have written that I can then reuse. How do I do so?

At the top of the library (after #lang racket), write (provide (all-defined-out)).

In the file that you want to use the library, write (require (file "my-library.rkt")).

Can I provide only some of my functions?

Yes (provide fun1 fun2 fun3)

Are Sam’s office hours still online?

Until break, yes.

How should we cite required files?

The require suffices.

How should we cite a procedure we copied.

; I copied this procedure from the lab on numeric recursion, which I did with Par Tner

Do we have a mentor session tonight?

No. The exam is live. But Sam will answer DMs.

Lab

Preparation

• Don’t forget to wear your mask if your partner is wearing a mask.
• Have the normal ‘start-of-lab’ discussion.
• Save as random-language.rkt

During Lab

• I saw a group write (average (make-list 10 (roll-a-die))) and get 6. If things are going right, is that likely to happen?
  • Answer: No. The odds are 1/60466176 that you will roll ten 6’s in a row.
• C says “volume is crazy”
• How many times should I win if I roll the dice 3600 times? About 800.

Wrapup