EBoard 27: Vectors

Warning! You are being recorded (and transcribed).

Approximate overview

• Quiz [15 min]
• Administrative stuff [10 min]
• A note from last class [5 min]
• Questions [10 min]
• Lab [35 min]
• Turn in lab [5 min]

Administrative stuff

• Reminder: Grinnellians have returned from around the world.
  • Consider masking.
• Today is the last day to withdraw from classes. Please chat with me if you are questioning whether or not you will pass the course.
• As you preregister for next semester, please consider taking CSC-161. (You might also consider MAT/CSC-208, but space is limited.)
• MP5 returned.

Token opportunities

Academic/Scholarly

• Tuesday, 2024-04-09, noon, some PDR CS Table (topic TBD).
• Thursday, 2024-04-11, 4pm, HSSC 1231 (the Kernel). CS Poster Session.
• Thursday, 2024-04-11, 7pm, the normal place Mentor Session.

Cultural

• Friday, 2024-04-05, 4pm, Global Living Room in HSSC. Middle of Everywhere.
• Saturday, 2024-04-06, 1–4pm, Cleveland Beach (in front of Cleveland residence hall on South Campus). Holi. Beware, your clothes will get stained.
• Saturday, 2024-04-06, 7:00-8:00pm, Global Living Room. Eid extravaganza.
• Sunday, 2024-04-07, 1:00-4:30pm, Harris. Purim Carnival.
• Sunday, 2024-04-07, 2:00-3:30pm, Sebring-Lewis. Eugene Gaub Recital.
• Thursday, 2024-04-11, 4:15-5:30pm, HSSC S1325 Writers@Grinnell.
• Thursday, 2024-04-11, 8:00-9:30pm, JRC 101 Writers@Grinnell.
• Friday, 2024-04-12, 4:00-5:00pm, HSSC N1170 Middle of Everywhere.

Peer

• Saturday, 2024-04-06, 9am–noon, Field House (?) Men’s Tennis vs. Illinois College.
• Saturday, 2024-04-06, noon-3pm, Pioneer Park Baseball vs. Lawrence.
• Saturday, 2024-04-06, 2:30-5:30pm, Pioneer Park Baseball vs. Lawrence, the sequel.
• Saturday, 2024-04-06, 3pm–6pm, Field House (?) Men’s Tennis vs. Monmouth.
• Sunday, 2024-04-07, 1:00-3:00pm, JRC 225 Fiber Arts Club

Wellness

• Tuesday, 2024-04-09, noon-1pm, BRAC P103. HIIT and Strength Fitness Class.
• Tuesday, 2024-04-09, 12:15–12:50, Bucksbaum 131. Yoga in the Museum.
• Tuesday, 2024-04-09, 4pm, BRAC P103 (Multipurpose Dance Studio): Yoga.
• Friday, 2024-04-12, 3:00–5:00pm, JRC Courtyard Get Nostalgic.

Misc

• Monday, 2024-04-08, 1:00–3:00pm (approx), where appropriate. Watch the eclipse (but don’t stare at the sun).
  • Eclipse glasses being given away at Burling (we think) and by the Physics SEPC (we know).

Other good things (no tokens)

Upcoming work

• Friday, 2024-04-05, 11:00pm, MP6 post-reflection
  • Submit MP6 post-reflection on Gradescope
• Friday, 2024-04-05, 11:00pm, SoLA 3 pre-reflection
  • Submit SoLA 3 pre-reflection on Gradescope
• Sunday, 2024-04-07, 11:00pm, MP4 Redo
  • Submit MP4 redo on Gradescope
• Sunday, 2024-04-07, 11:00pm, No reading.
• Monday, 2024-04-08, 4:00pm, SoLA 3 distributed
• Wednesday, 2024-04-08, 8:30am, Lab writeup
  • Submit today’s lab on Gradescope
  • Preferred: Finish it during class Monday
• Wednesday, 2024-04-10, 11:00pm, SoLA 3 due
• Sunday, 2024-04-14, 11:00pm, MP5 Redo
  • Submit MP5 redo on Gradescope
• Sunday, 2024-04-14, 11:00pm, MP2 Second Redo
  • Submit MP2 redo on Gradescope
• Sunday, 2024-04-14, 11:00pm, MP3 Second Redo
  • Submit MP3 redo on Gradescope
• Sunday, 2024-04-14, 11:00pm, MP1 Third Redo
  • Submit MP1 redo on Gradescope

Friday PSA

• You are awesome, people (or at least whatever I am) care about you. You therefore have a responsibility to take care of yourselves.
• Moderation.
• If you consume alcohol, watch your container.
• Consent is essential (but insufficient).

Notes from last lab

listp?

Here’s what most of you wrote for listp?

(define listp?
  (lambda (thing)
    (cond
      [(null? thing)
       #t]
      [(pair? thing)
       (listp? (cdr thing))]
      [else
       #f])))

(define listp?
  (lambda (thing)
    (cond
      [(null? thing)
       #t]
      [(and (pair? thing) (listp? (cdr thing)))
       #t]
      [else
       #f])))

Statement: thing is a list if either (a) thing is null or (b) thing is a pair whose cdr is a list.

Here’s what a more experienced Schemer might write.

(define listp?
  (lambda (thing?)
    (or (null? thing)
        (and (pair? thing) 
             (listp? (cdr thing))))))

Building arbitrary caaddaadddr procedures

Idea:

• If we treat the middle letters as a string,
• We know how to turn a string into a list of letters. (string->list)
• We could map over that list to get a list of procedures. '(car car cdr cdr car car cdr cdr cdr).
• We can compose all of those together.
• And we end up with aprocedure that does caaddaaddr.

;;; (c_r str) -> procedure?
;;;   str : string? (consisting of only #\a and #\r)
;;; Build the procedure corresponding to c<str>r.
;;;
;;; For example, `(c_r "adddddd")` gives the equivalent of
;;; `caddddddr` (if it were defined).
(define c_r
  (let ([letter->fun (lambda (ch) (if (char=? ch #\a) car cdr))])
    (lambda (str)
      (apply o (map letter->fun (string->list str))))))

(define caddddddr (c_r "adddddd"))

No, you do not need to understand that completely.

Questions

Administrative

Can we get a third redo for MP 1?

Sure.

Can we get second redos for MPs 2 and 3?

Sure.

When will our graders catch up?

Soon. By the end of this weekend.

Vectors

Could you explain what a numeric vector is?

Lists and vectors are normally heterogeneous: They can contain multiple types of elements. E.g, (list "a" 'a 23) or (vector "eh" 'b 23).

Lists and vectors can also be homogeneous: They can contain only one type of element. E.g., (list "a" "b" "c") or (vector 'a 'b 'c) or (vector 1 2 3)`.

When we have a homogeneous list or vector, we might use an adjective like “numeric” to describe it. So a “numeric vector” is a vector that contains only numbers.

We are sometimes too casual with language, so you may find that I use “numeric vector” for “vector of only integers”.

We often use numeric vectors as examples for vector recursion because it’s easiest to see us changing numbers.

We will use the term “palette” for a vector of RGB colors.

Can we look at the last procedure in the reading? vector-set!

Unlike lists, which are fixed, vectors are mutable (but not at Prof. Xavier’s academy), which means that we can change them.

> tuff
'#("tee" "you" "eff" "eff")
> (vector-set! tuff 1 "ewe")
> tuff
'#("tee" "ewe" "eff" "eff")

Lab

Note: (section list-ref list-of-values <>) is a different way to write (cut (list-ref list-of-values <>)).

Note: time reports on how long something takes.

Note: "; SAM SAID TO SKIP EXERCISE 2!"

Note: By number-vector-divide!, we mean number-vector-scale!.

IMPORTANT: We will resume this lab on Monday! Plan to be in the same place with the same partners!