EBoard 11 (Section 3): Regular Expressions

Approximate overview

• Administrative stuff [~10 min]
• Racket stuff [~10 min]
• Questions [~5 min]
• Lab [~55 min]

Administrivia

Introductory notes

• Please don’t discuss the SoLA problems until Monday’s class. Some people have extensions.
• In spite of a late night last night, I’m behind.

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 Wednesday, 8–9 p.m., Sunday 4–5 p.m., Monday 8–9 p.m.

Upcoming work

• Quiz 4 Today: Style
• Mini-Project 1 redo due Sunday the 20th at 10:30 p.m. (sooner is better)
• Readings for Monday due Sunday at 10:00 p.m.
  • Recursion is hard. It’s okay not to understand it.
• Lab writeup from today due Sunday at 10:30 p.m.
  • Our goal is to finish it in class.
• MP3 due next Thursday at 10:30 p.m.
  • Yup, we’ll discuss it.
• Quiz 5 next Friday: Regular expressions
• Mini-Project 2 redo due Sunday the 27th at 10:30 p.m. (sooner is better)

Friday PSA

• Be kind to yourself. Be kind to others.
• Moderation, even with post-Valentine’s chocolate.
• Consent is essential. (It is necessary, it may not be sufficient.)
• Pre-pandemic norms
  • 30% of Grinnellians report no sexual partners over the past year
  • 30% of Grinnellians report one sexual partner
  • 40% report more
  • The average is still only a bit above 2
  • At least 30% of Grinnellians will not drink this weekend
  • Less than 20% of Grinnellians will engage in illegal drugs
  • Less than 20% will drink to excess

Upcoming Token-Generating Activities

• TODAY: AI Coffee Chat, 5:20-6:00 p.m., Prof. Eliott’s lab.
  “How does it feel to be a university student?”
• TODAY/WEEKEND: Fill out the week four survey, available at https://bit.ly/csc-151-2020Sp-week04. (Only counts as a token if 50% or more of the section fills it out.)
• TODAY/WEEKEND: Conference swimming. Natatorium.
• WEEKEND: Any self-care activity this weekend.
• SUNDAY/MONDAY: Mentor sessions Sunday and Monday.
• THURSDAY, 11am, JRC 101, “Physics Is More Than Problem-solving: Building Inclusivity and Belonging by Practicing Professionalism”. Prof Marty Baylor of Carleton College.
• ANYTIME: Visit the current exhibit in the Grinnell Art Museum. (At least 15 min.)

Other Upcoming Activities

Sample Quiz Question

Write and structure program code in ways that makes it easier to understand.

Although it is early in the semester, you have already started to learn some bas ic approaches to making your code more readable. Here are a few.

• Put the first argument to a procedure on the same line as the procedure.
• Put all of the arguments to a procedure on the same line, or all on separate lines.
  • Similarly, when writing an if statement, put the test, consequent, and alternate all on the same line or all on separate lines.
• Use Ctrl-i to reindent the text.
• When defining a procedure, place the lambda and the parameters on a separate line from the define, and start the body on a separate line from the from the lambda and the parameters.
• Follow “The Zen of Booleans”: Avoid explicitly using #t and #f in expressions if you can write them more clearly with and or or.
• Avoid repeated code.
• Avoid unnecessary code.
• Choose descriptive names for variables and procedures.

a. Using those principles, as well as others you have learned, clean up the following code.

(define rab (lambda
(int 
str)
(if (if (equal? #t (integer? (string->number (substring int str))))
#t #f) (string->number (substring int str))
(if (if (equal? (integer? 
(string->number (substring int 0 str))) #f) #f
#t
) (string->number (substring int 0 
str)) #f))))

b. In English, describe what rab does.

Here are some sample calls to help get you started.

> (rab "876xy12" 0)
#f
> (rab "876xy12" 1)
8
> (rab "876xy12" 2)
87
> (rab "876xy12" 3)
876
> (rab "876xy12" 4)
#f
> (rab "876xy12" 5)
12
> (rab "876xy12" 6)
2
> (rab "876xy12" 7)
#f
> (rab "876xy12" 8)
. . substring: starting index is out of range
  starting index: 8
    valid range: [0, 7]
      string: "876xy12"

Racket/Lab Stuff

Tallying a’s

(define count-of-as-1
  (tally-value book-characters #\a))
(define count-of-as-2
  (tally (section char=? <> #\a) book-characters))
(define count-of-as-3
  (length (filter (section char=? <> #\a) book-characters)))

Grabbing lines 100 to 120

What is happening with this?

(define grab-elements-100-to-120
  (o (section take <> 20) (section drop <> 100)))

• It’s confusing, at least at first.
• grab-elements-100-to-120 is a procedure.
  • We can tell because it combines two procedures with o.
• This drops the first 100 elements and then takes the first twenty remaining elements.
  • It takes a list as input.
  • So while the problem we did on the lab involved lines from a file, this will work with any list.
• Where does the list come from?
  • It’s implicit in the sections and the compose.
• Note that compose will only work with one-parameter procedures.
• If we had a file, we’d want to turn it into a list of lines.
  • (file->chars filename)
  • (file->words filename)
  • (file->lines filename)

Questions

From the readings (not discussing)

From the readings (discussing)

What should the J-r-d code look like?

; Brute force
(define j-r-d/1
  (rex-any-of (rex-string "Jerod")
              (rex-string "Jared")
              (rex-string "Jered")
              (rex-string "Jarod")))

; A bit more elegant
(define j-r-d/2
  (rex-concat (rex-string "J")
              (rex-char-set "ea")
              (rex-string "r")
              (rex-char-set "eo")
              (rex-string "d")))

Other questions

Why can’t I use test-equal? or test-true?

You need to use (require rackunit)

Lab

Preliminaries

Make sure you have the latest version of the lab. Exercise 1 should look like

#| B |#

; +--------------------------------------------------------+---------
; | Exercise 1: Splitting strings with regular expressions |
; +--------------------------------------------------------+

During Lab

• Don’t forget Ctrl-I.
• Look up at the screen every once in a while.

Exercise 1

• In exercise 1, you are defining values, not procedures. So, for example, (define split-at-space (string-split phishy " ")).
• (rex-split-string rex str)
  • rex must be created by the rex procedures (e.g., rex-string)
• If you want to do “a sequence of this pattern”, use rex-repeat.
• Order matters. See code below.

; v1
(rex-any-of (rex-repeat (rex-string " "))
            (rex-repeat (rex-string "\n"))
            (rex-repeat (rex-string "\t")))

; v2
(rex-repeat (rex-any-of (rex-string " ")
                        (rex-string "\n"))
                        (rex-string "\t"))

Exercise 2

• (rex-char-range #\a #\z) gives all lowercase letters.
• (rex-char-range #\A #\Z) gives all uppercase letters.

Exercise 3

• Your goal is to filter strings using the regular expression.
• If you look at the example, you’ll get an idea of how to do so.

Exercise 4

• The words.txt file is linked from the lab page.

Wrapup

• Today is a “SAM SAID I COULD STOP HERE” day.
• But you should read through the rest and think on it.
• Please don’t upload the words.txt file.