EBoard 10: Cut and Compose and more

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

Approximate overview

• Administrative stuff [20 min]
• Lab [55 min]
• Turn in lab [5 min]

Administrative stuff

• As we discovered last Wednesday, when you write something like (define kitten (image-load "kitten.jpg")) in your definitions pane, the autograder will crash. That’s because it’s going to try to load “kitten.jpg”.
  • Option 1: Don’t load it in your definitions pane; just do it in your interactions pane.
  • Option 2: Upload any loaded files along with your .rkt file. In this case, you need to make sure that you just use the short name for the file, and not a full path.
• When you miss class, you are responsible for making up lab. You should
  plan to spend at least 60 minutes on the lab.
  • Even when you don’t finish the lab (even when you’re in class), you should review the rest of the lab to make sure you understand the questions.
  • Don’t forget to email me when you miss class.
• One of my hearing aids broke, so I may be even more unsure of what you say than normal.
• I’ve commented on your SoLA 1 pre-reflections.
• Since you’d been asking about quizzes on Gradescope, I’ll do my best to demonstrate how you look at a quiz.

Token activities

Academic/Scholarly

• Tuesday, 2024-02-13, noon–1:00pm, Some PDR. CS Table: Functional programming.
• Thursday, 2024-02-15, 11:00–noon, JRC 101. Scholars’ Convocation: Gaile Pohlhaus on “An Epistemology of the Oppressed: Resisting and Flourishing under Epistemic Oppression”.

Cultural

• Thursday, 2024-02-15, 7:00–9:00pm, Sebring-Lewis. Jazz Concert w/Carol Welsman.
• Friday, 2024-02-16, 4:00–5:00pm, HSSC N1170 (Global Living Room). Middle of Everywhere.
• Sunday, 2024-02-18, 7:00–9:00pm, Harris Cinema. The Moth Storytelling Slam.

Peer

• Friday through Sunday, 2024-02-16 through 2024-02-18. Osgood Pool. Midwest Swimming and Diving Conference Championships.
  • 30 minutes counts.
  • Up to two separate sessions.
• Sunday, 2024-02-18, Simpson College Baseball Diamond. Baseball.

Wellness

• Monday, 2024-02-12, 4:00–5:00pm, HSSC Atrium. Therapy dogs.
• Tuesday, 2024-02-13, noon-1pm, BRAC P103. HIIT and Strength Fitness Class.
• Tuesday, 2024-02-13, 12:15–12:50, Bucksbaum 131. Yoga in the Museum.
• Tuesday, 2024-02-13, 4pm, BRAC P103 (Multipurpose Dance Studio): Yoga.

Misc

• Monday, 2024-02-12, 4:30–5:30pm, JRC 101. Talking about disability policy with Daniel Van Sant, Director of disability policy at the Harkin Institute.

Other good things (no tokens)

• Saturday, 2024-02-17, 1:00–3:00pm, Darby. Men’s Basketball vs. Monmouth.
• Saturday, 2024-02-17, 2:00–5:00pm, Field House. Women’s Tennis vs. Ottwawa.
• Saturday, 2024-02-17, 3:00–5:00pm, Darby. Women’s Basketball vs. Monmouth.

Upcoming work

• Today, 2024-02-12, 4:00pm, SoLA 1 released
  • Decomposition (Quiz week 1.)
  • Procedural abstraction.
  • Primitive types. (Quiz week 3.)
  • Collaboration. (Friday’s topic.)
  • Lambda-free anonymous procedures (aka cut and compose). (Today’s topic.)
  • No question on tracing. That will only be a paper quiz.
• Tuesday, 2024-02-13, 11:00pm: Submit reading responses.
  • Boolean values and predicate procedures
  • Conditional evaluation in Scheme
  • Submit on Gradescope
• Wednesday, 2024-02-14, 8:30am: Submit today’s lab writeup.
  • Submit on Gradescope
  • Preferred: Submit before the end of class today.
• Wednesday, 2024-02-14, 11:00pm, SoLA 1 due
• Friday, 2024-02-16, 8:30am (or as early as 8:00am): Quizzes.
  • New: Conditionals
  • Old: Tracing
  • You may bring one page of hand-written notes.
• Friday, 2024-02-16, 11:00pm: SoLA 1 post-reflection
  • Try to do it earlier.
• Friday, 2024-02-16, 11:00pm: MP3 pre-reflection

Notes on the SoLA

• You don’t have to memorize procedures. This is a semi-open-book exam.
• Don’t worry if you don’t get everything on this SoLA. Historical evidence suggests that you will find most of the questions easier on the next SoLA.

Questions

Administrative

Why can’t I see how I’m doing with tokens?

Because Sam hasn’t tallied them yet. Stay tuned.

Readings (Pair Programming)

Readings (Cut and Compose)

Can you further explain how the computer understands/executes the cut procedure?

First of all, cut isn’t strictly a procedure. It’s a “keyword” (at least as I describe it). Instead of evaluating its parameter, which is the norm for procedures, it transforms its parameter into a procedure.

Basically, it creates a parameter for each diamond/hole at the top level of the expression and then makes a procedure out of it.

Can cut or section be used such that one parameter is used twice?

Neither cut nor section lets you duplicate a parameter. Hence, they have somewhat limited use. They are still useful in a large range of situations.

For check one, I tried to make this work:

(define sub3 (sub1 (sub1 (cut (sub1 <>)))))

But it gave me an error saying the second sub1 wasn’t receiving a number for an input but (cut (sub1 <>)) does output a number. Why doesn’t that code work?

Can you really put cut in the middle of an expression?

cut doesn’t work when you have a nested expression.

Maybe we need to rearrange.

How define works: We evaluate the expression and then associate it with the identifier.

We’re going to try to evaluate (sub1 (sub1 (cut (sub1 <>)))).

If we were to going to evaluate (sub1 (* 2 3)), we’d multiply the 2 by the 3, giving us (sub1 6). And we’d then evaluate the (sub1 6), giving us 5.

Racket will do the same with this expression above.

“I have a sub1. What’s the parameter? Oh, it’s another sub1. What’s the parameter? Oh, it’s a cut. Let’s see … we build another procedure.”

(sub1 (sub1 #<procedure>))

When we try to apply sub1 to a procedure, we get the error message.

In general, Racket is aggressive about evaluating expressions/procedure calls, only a few things (like lambda and cut) delay that evaluation.

“It will make more sense as you get used to it.”

This is just (sub1 (sub1 sub1))

What’s the difference between (cut (rgb-darker <>)) and rgb-darker?

They work the same.

The cut is longer to write.

I discovered that I can’t put a diamond in a nested expression, such as (cut (+ 2 (* 3 <>))). Why not?

Option 1: The person who implemented cut wasn’t clever enough to do that.

Option 2: The person who implemented cut was too lazy to do that.

Option 3: The person who implemented cut thought it wasn’t necessary and wanted students to explore other approaches.

We can break that up. The goal here is “multiply by 3 and then add 2”.

We can write “multiply by three” as (cut (* 3 <>)).

We can write “add two” as (cut (+ 2 <>)).

When we want to do one thing and then another we use composition.

So we could write (o (cut (+ 2 <>)) (cut (* 3 <>))).

Can we write (+ 2 (cut (* 3 <>)))?

No. Because (cut (* 3 <>)) is a procedure. So you’re writing (+ 2 #<procedure>). It doesn’t make sense to DrRacket to add to a procedure.

Computers are dumb. You may know what you’re saying. It can’t figure it out.

I couldn’t find any colors for which (o rgb-darker rgb-lighter) gave a different color. Are there any?

Yes.

(rgb 255 255 255) ; white

(rgb 255 0 0) ; red

> (rgb->string (rgb-lighter (rgb 255 0 0)))
"255/16/16"
> (rgb->string (rgb-darker (rgb-lighter (rgb 255 0 0))))
"239/0/0"
> (rgb->string (rgb-darker (rgb-lighter (rgb 242 243 244))))
"239/239/239"

For very bright components, lighter and darker are not complete inverses.

Could you go over rgb-purpler and rgb-much-darker?

Sure.

(define rgb-purpler (o rgb-redder rgb-bluer)).

Can you show how to load an image in DrRacket? I’m getting errors.

Sure.

Your Racket file needs to be in the same directory as your image.

Can we put a longer path in image-load?

Yes. But not for something you’re submitting on Gradescope.

Misc

Can you ever finish the introductory material promptly?

Yes, but not if I try to answer all the questions.

Last semester’s class said that it’s more important to answer all the questions.

Lab

Oh, cool. I can copy colors and images into the definitions pane!

You can, but please don’t. It makes the file unreadable on Gradescope.

I didn’t finish the lab and it seems really important for the SoLA.

It is. You should try it on your own. DM me on Teams when you have questions.

Why doesn’t greyscale just average the three components?

Because the color theory I looked up suggests that we perceive green as contributing more than blue or red.

The formula I use is (+ (* .3 red-component) (* .11 blue-component) (* .59 green-component))