EBoard 09: Boolean values and predicate procedures

• Warning This class is being recorded.

Approximate overview

• Administrivia
• A few notes on Quiz 1
• Debrief on lab.
• Questions
• Lab

Administrivia

Introductory notes

• Today’s lab is about half new.
  • There may be bugs. (Of course, there may also be bugs in old labs.)
  • I’ll need to spend the first bit of lab updating the autograder. (Ran out of time yesterday.)
• Beware! Friday the 13th falls on a Wednesday this month!

Upcoming Token activities

Academic

• Thursday, 14 Sept 2023, 4:15 pm, CS Extras: “????” (Science 3820)
• Thursday, 14 Sept 2023, 6pm, “Microsoft Campus Visit” (HSSC Kernel/A1231)
• Sunday, 17 Sept 2023, 1pm, Mentor Session (Here)

Cultural

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

Peer

• Major Fair, Today 4pm, near the sundial

Wellness

Misc

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

Other Good Things

Upcoming work

• Thursday Night: MP2 due.
• Thursday Night: Reading responses due
• Friday morning: Today’s lab due (but we’ll get it turned in today)
• Friday: Quiz (primitive types)
• Friday Night: MP2 post-assessment (or do that with MP2)

Sample primitive types questions

Express basic computations over primitive values and their associated standard library functions.

As you may know, some children enjoy using a language called “Pig Latin” (or Igpay Atlinlay). The normal rules for Pig Latin are as follows:

• If the word starts with a consonant, move the consonant and any subsequent consonants to the end of the word and add “ay” to the end.
• If the word starts with a vowel, add “yay” to the end of the string.

We’re going to do a simpler version of Pig Latin. For any word, we’ll strip the first letter from the beginning of the word, add the letter to the end, and add “ay”.

> (simplified-pig-latin "pig")
"igpay"
> (simplified-pig-latin "slurp")
"lurpsay" ; It's "uprslay" in traditional Pig Latin
> (simplified-pig-latin "append")
"ppendaay" ; It's "appendyay" in traditional Pig Latin

Write the simplified-pig-latin procedure.

Express basic computations over primitive values and their associated standard library functions.

As you’ve learned by now, the Unicode collating sequence assigns different numbers to the letters of the alphabet than humans do. We think #\a is the first letter of the alphabet; Racket calls it number 97.

Write a procedure, (nth-capital-letter n) that takes an integer between 1 and 26 as a parameter and returns the corresponding capital letter in English. (#\A is 1, #\B is 2, and so on and so forth.)

You may not write a huge conditional; you should write this procedure using the appropriate computation.

You should not use magic numbers; don’t rely on a particular collating sequence. (You may, however, assume that the letters appear in sequence in the collating sequence.)

> (nth-capital-letter 1)
#\A
> (nth-capital-letter 5)
#\E
> (nth-capital-letter 11)
#\K

NOTE: You can bring a sheet of hand-written notes or a stack of flashcards.

Debrief on last class

(TPS) What did you take away from the last lab?

• We practiced pair programming. Like all Grinnellians, we’re awesome, so we did it particularly well. It was a good experience.
• A better understanding of RGB colors, although combining them stlil doesn’t completely make sense.
• We can tweak colors in interesting ways.
• We got practice writing procedures.

Questions

I’ve taken questions from your reading responses, but you should always feel free to add your own.

On Boolean values and conditional expressions

Why does (or #f 0) evaluate to 0?

Although our tracer doesn’t show it (yet), or evaluates its arguments by evaluating each in turn. When it hits the first non-false value, it returns that value. If it runs out of values, it returns false.

Why does (or 11 9) evaluate to 11?

See the prior answer. 11 is “trueish” (that is, not false), so it returns it.

Why does (and #t 5) evaluate to 5?

Although our tracer doesn’t show it (yet), and evaluates its arguments by evaluating each in turn. When it hits the first false, it returns false. If the last value is not false, it returns that value.

On RGB colors

On the forthcoming quiz

I won’t remember all of the procedures. Can I bring a sheet of notes or a stack of flashcards?

You may bring a sheet of hand-written notes or a stack of flashcards.

When can I take the quiz on Friday?

Both quizzes should be available at 8 a.m.

On the mini-project

What file names would you like for the mini-project?

I think they are in the description. Perhaps not.

Why isn’t there a place to submit the mini-project on Gradescope?

Forthcoming.

Where is the grading rubric?

Tonight.

Miscellaneous

What’s this list procedure?

list takes a bunch of expressions as parameters and puts them into a structure we call a “list” (surrounded by parentheses). Right now, all we know how to do with lists is (a) make them (with list) and (b) find out how many values there are with length.

I think we were just trying to show you that undefined values make your life difficult.

Where do you find the garbage (strike that, useful comments) that Sam types every day?

Go to the course Web site.

Go to the schedule.

Click on the subject of a class.

Cross your fingers.

If that doesn’t work, let Sam know.

On administrative stuff

Lab

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

• A: 1,4,5
• B: 2,3,6

Sam is frantically updating Gradescope. (Done.)

Note: for median-of-three, don’t use min and max, just compare x, y, and z to each other.

(<= x y z) is a shorthand for (and (<= x y) (<= y z))

(or (<= x y z) (<= z y x)) indicates that y is the median of x, y, z.

We will continue this lab next class!