EBoard 12: Conditionals (Section 3)

Warning! You are being recorded and transcribed, provided the technology is working correctly.

We are back to the standard start-of-class procedure.

Approximate optimistic overview

• Administrative stuff [10 min]
• Questions and answers [20 min]
• Lab [30 min] (ends at 3:30)
• Turn in Lab [5 min]
• Quiz [15 min]

Administrative stuff

Introductory notes

• I am out of Otter.ai minutes, so there is no Otter transcription/summary for any classes today.
• In case you hadn’t figured it out yet, you have an hour to complete an LA from the time that you start the LA, even if you close your computer or log out of Gradescope.
• Remember that you can take both/all quizzes. And you can stay until thirty minutes after the end of class to complete them.

Upcoming activities

Scholarly

• Thursday, 20 February 2025, 11:00 a.m.–noon, JRC 101. Scholars’ Convocation: Steven Geofrey. What Information Worlds Reveal about Climate Change Awareness, Advocacy, and Hope
• Tuesday, 25 February 2025, Noon–12:50 p.m., PDR 224C (White Dining Room). CS Table: TBD

Artistic

• Friday, 21 February 2025, 7:00 p.m., The Wall (I think). The Neverland Players
  • Tickets in the Bucksbaum Box Office.
• Saturday, 22 February 2025, 2:00 p.m., The Wall (I think). The Neverland Players
• Saturday, 22 February 2025, 7:00 p.m., The Wall (I think). The Neverland Players
• Sunday, 23 February 2025, 2:00 p.m., The Wall (I think). The Neverland Players

Multicultural

• Friday, 21 February 2025, 4:00–5:00 p.m., HSSC N1170 (Global Living Room). Middle of Everywhere: Beyond the War: Ukraine on the Cultural Front

Peer

Musical, theatric, sporting, and academic events involving this section’s students are welcome.

Wellness

• Friday, 21 February 2025, 9:00 p.m., Noyce Elbow. Nerf at Noyce. Shoot your mentor.
• Tuesday, 25 February 2025, 12:15–12:50 p.m., GCMoA. Yoga in the Museum.
• Tuesday, 25 February 2025, 4:30–6:30 p.m., BRAC P103 (Multipurpose Dance Studio). Wellness Yoga.

Misc

• Sunday, 23 February 2025, 7:30–8:30 p.m., Science 3819. Mentor Session
• Tuesday, 25 February 2025, 7:00–8:00 p.m., Science 3820. Mentor Session

Other good things

These do not earn tokens, but are worth your consideration.

• Thursday night–Sunday night, The Natatorium. Swimming Conference Meet.

Upcoming work

• Wednesday, 19 February 2025
  • Quiz: Documentation
  • Makeup quiz: Tracing
• Thursday, 20 February 2025
  • Submit Wednesday’s lab on Gradescope
  • Readings:
    • List basics
    • Transforming lists
    • Submit reading responses on Gradescope
  • SoLA 1
    • Decomposition
    • Procedures
    • Primitive types
    • Lambda-free anonymous procedures
    • Collaboration
• Friday, 21 February 2025
  • MP4 released
  • Submit post-reflection for SoLA 1 on Gradescope
• Sunday, 23 February 2025
  • Submit first redo for MP2 on Gradescope
  • Submit Friday’s lab on Gradescope
  • Readings: Processing Lists
    • The “big three” list operations
    • Transforming lists
    • Submit reading responses on Gradescope
  • Submit pre-reflection for MP4 on Gradescope

Q&A

Administrative

When will we get grades on today’s quizzes?

Sam hopes to have those done tonight.

What should we submit for redos?

The updated .rkt file.

Any images required by the assignment.

A text (preferably) or PDF file name CHANGES.txt (or CHANGES.pdf) that summarizes what problems you had the first time and what changes you’ve made. It can also be CHANGES.rkt.

I accidentally opened an LA early because I didn’t understand the timing. Can you help me?

Yes, I can reset them if you DM me.

Booleans and conditionals

Why does the equality predicate = not also count as a comparator?

Comparators are supposed to tell you whether one thing comes before another. = does not.

Could you go over the not procedure and its uses?

(not expression) -> Gives the “opposite” result of the expression.

;;; (anything-but-string? val) -> boolean?
;;;   val : any?
;;; Returns false if `val` is a string and true otherwise.
(define anything-but-string?
  (lambda (val)
    (not (string? val))))

I read and reread the truish breakdown and was still confused. What are examples of statements that would result in a truish output? What would produce not false, but then not be true?

(not #f) is always true, #t.

However, there are values that are neither #f nor #t. For example, "hello" or the result of (+ 2 3). We consider those “truish”. If we use them as the test/guard in a conditional, instead of an error, we’ll see them treated as if they were true.

What do we need to understand when making the distinction between keywords and procedures?

You should understand that the order of evaluation is different (and, preferably, what that order is).

Normal (e.g., (+ exp1 exp2 exp3), exp1 is (* 3 4)): Evaluate each expression from left to right and then add them. (In general, evaluate from left to right and then apply the procedure.)

Lambda (i.e., (lambda (params) exp)): When we apply one of these, we substitute the arguments for the corresponding parameters in the expression and then evaluate the updated expression. What about just the lambda? It builds a procedure, but does NOT evaluate the expression until later. (It also doesn’t evaluate the params.)

If (i.e., (if test consequent alternate)): Evaluate the test. If it’s true, evaluate the consequent and return its value. If the test evaluated to false, evaluate the alternate and return its value.

And (i.e., (and exp1 exp2 exp3)): Evaluates the expressions one by one. If any of them are false, immediately returns false and does not evaluate the remaining expressions. If none of them are false (it has evaluated all of them), return the value of the last expression. (and (string? s1) (string? s2) (string-append s1 s2)).

Or (ie.., (or exp1 exp2 exp3)): Evaluates the expressions one at a time. If any of them are truish, returns that value. If all of them are false, returns false.

Lab

A hint for median-of-three

(cond
  [(<= x y z)
   ?]
  [(<= y z x)
   ?
  ...
  ])