EBoard 18 (Section 1): Numeric Recursion

Approximate overview

• Administrative stuff [~5 min]
• Racket stuff [~0 min]
• Questions [~10 min]
• Lab [~60 min]

Administrivia

Introductory notes

• I’m back. THANK YOU to our amazing class mentors for taking over.
  • There’s a chance I’ve forgotten everyone’s name.
• I lost one of my hearing aids, so may be even less responsive than normal.
• I hope you were safe during the tornado.
• I’d forgotten how busy conferences are. I am even further behind than when I left. (Most of my days ran from 8am to 9pm or later.)
  • I still expect to have quizzes and grade summaries to you Tuesday night.
• Reminder: It’s good practice for the navigator to look at the screen or whiteboard once in a while to see if there are any notes on the exercise.
• I’ve rearranged the schedule slightly. We’ll be doing randomness on Wednesday and more recursion on Friday.

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.

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

• Reading for Wednesday: Randomness (due Tuesday at 10:00 pm)
• Today’s lab due Tuesday at 10:30 p.m.
• Quiz 7 due Sunday at 4pm: Use higher-order procedures
• SoLA 2 due Thursday the 10th at 10:30 p.m.

Upcoming Token-Generating Activities

• TONIGHT: Mentor Session (8pm)
• WEDNESDAY: Mentor Session (8pm)
• 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

Questions

Reading questions

Other issues

Lab

Preparation

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

During Lab

WRITE TESTS! Writing tests helps you

• Check whether your procedure works correctly. (And shouldn’t be much slower than just typing in the expression and reading the result.)
• Think through what the procedure should do.
• Consider potential problems.

Bonus question

• The “rhythm10” of any non-negative number is the number of times that we must divide the number by 10 to reach a number less than or equal to 1.
• E.g., (rhythm10 5) = 1; (rhythm10 512) = 3; (rhythm10 9812.412) = 4
• Write (rhythm10 n) recursively.
  • Note: Simplification will be different than in other numeric recursive procedures.

Wrapup

• Please make sure to do the bonus question before you go on to the extras.
• It’s a “SAM SAID STOP HERE” day.