EBoard 08: RGB Colors

• Warning This class is being recorded.

Approximate overview

• Administrivia
• A few notes on Quiz 1
• Questions
• Lab

Administrivia

Introductory notes

• Covid and other illnesses remains issues. Consider masking. Clean your keyboards.
• Today’s lab is new this semester. Don’t worry if you don’t get too far in it.
  • I’ll be spending the first few minutes of lab trying to get the autograder working.
• Our mentors graciously provided extra copies of their helpful handouts.
• I read “Grinnell Parents and Families” on Facebook.

Upcoming Token activities

Academic

• Tuesday 12 Sept 2023, Noon, CS Table (readings at ends of the hallway)
• Thursday 14 Sept 2023, 6pm, “Microsoft Campus Visit” (HSSC Kernel/A1231)

Cultural

• Thursday 14 Sept 2023, 8pm, Writers @ Grinnell w/Claire Forrest ‘13

Peer

• Tuesday 12 Sept 2023, 7pm, Volleyball vs. Wartburg

Wellness

Misc

• Friday 15 Sept 2023, Noon, Whale Room, Lunch with Young Alums in Tech

Other Good Things

Upcoming work

• Monday Night: MP2 pre-assessment.
• Tuesday night: Reading responses
• Wednesday morning: Today’s lab writeup due
• Thursday Night: MP2 due.
• Friday: Quiz
• Friday Night: MP2 post-assessment

Quiz notes

(define square
 (lambda (x)
   (* x x)))

(define f
 (lambda (x)
   (square (+ x 1)))

(define g
 (lambda (x)
   (+ (square x) 1)))

(define p
 (lambda (x y)
   (- (f x) (g y))))

(define a 3)

(define b 2)

There should be an “S” (for “Satisfactory”) or “Redo” on your quizzes. There are also somoe comments.

I found myself writing the following things multiple time. Please watch these issues. (None of these, by themselves, were enough to move you from Satisfactory to Redo.)

• We normally evaluate parameters left to right.
• We normally do only one substitution of a variable at a time.
• We normally evaluate only one expression at a time.
• We normally finish evaluating one parameter before moving on to the subexpressions of the next.

More major issues. These usually lead to Redos.

• Please do not skip steps. This comment usually came with people doing something like turning (square 5) into 25 rather than first into (* 5 5), or turning (p -4 5) into something other than (- (f -4) (g 5)).
• Please do not put parentheses around numbers. Parentheses indicate expressions to be evaluated.
• Please include the full context. This comment usually happens when someone goes from (- (f -4) (g 5)) to (square (+ -4 1)) without the surrounding context.

Note that my concern is less that you got “the right answer” at the end than that you can show how we get there. If you’re skipping steps and writing invalid Scheme, you haven’t shown that you’ll be able to handle more complex evaluation.

Here’s the full derivation as I’d hoped to see it. (I don’t plan to go through it, but you can look for it on the eboard.)

    (p (- 8 (* a 4)) (+ (* b b) 1))
--> (p (- 8 (* 3 4)) (+ (* b b) 1))
--> (p (- 8 12) (+ (* b b) 1))
--> (p -4 (+ (* b b) 1))
--> (p -4 (+ (* 2 b) 1))
--> (p -4 (+ (* 2 2) 1))
--> (p -4 (+ 4 1))
--> (p -4 5)
--> (- (f -4) (g 5))
--> (- (square (+ -4 1)) (g 5))
--> (- (square -3) (g 5))
--> (- (* -3 -3) (g 5))
--> (- 9 (g 5))
--> (- 9 (+ (square 5) 1))
--> (- 9 (+ (* 5 5) 1))
--> (- 9 (+ 25 1))
--> (- 9 26)
--> -17

Questions

(rgb ...) isn’t working for me.

Did you update our csc151 library?

Did you remember (require csc151).

What should I see for (rgb ...)?

You should see “color info”. Something like (color 255 0 128 255).

Why can’t I use (color-red "darksalmon") to get the red component of the color "darksalmon"?

color-red expects an RGB (or RGBA) color value, not a string. You need to convert the string (color name) to an RGB color using color-name->rgb.

Will you pass back quizzes?

Yup. As soon as lab starts. (I’ll ask for last names, because I don’t know them.)

Lab

A is closest to the board. A drives first. For today’s lab,

• A: 1,2,5,6
• B: 3,4,7,8

Your image should be in the same folder as your .rkt file.

color-name->rgb turns a color name into RGB.

Once you’ve defined ccc and need three concentric circles, you are much better off calling ccc than rewriting the code.

Good:

(ccc (rgb 255 0 0) (rgb 255 0 255) (rgb 0 0 255))

Bad:

(overlay (circle 10 "solid" (rgb 255 0 0))
         (circle 20 "solid" (rgb 255 0 255))
         (circle 30 "solid" (rgb 0 0 255)))

Debrief

(TPS) What are some important things you took from this lab?