EBoard 19: Local bindings

Warning This class is being recorded (and transcribed).

Approximate overview

• Quiz (or quizzes)
• Administrivia
• Questions
• Lab

Administrivia

Upcoming Token activities

Academic

• Mentor Session Sunday at 1pm.

Cultural

Peer

• Volleyball vs. Lawrence Saturday at 1pm (Senior Day).
• Play this weekend. Everybody. Friday and Saturday at 7:30 p.m. in Flanagan. Sunday at 2:00 also in Flanagan. Get tickets at the box office starting at noon on Thursday. (Maybe before.) The box office is in Bucksbaum, near the courtyard.

Wellness

Misc

• Fireman’s breakfast, Sunday 6am-noon at the firehouse. Free will donation.
  • I will reimburse you up to $10.

Other good things

Upcoming work

• Sunday night: Next reading due
• Monday morning: Today’s lab due
• Thursday night: MP4

Friday PSA

• Take care of yourselves.
• Consent is essential.

Questions

Administrative

MP4

Do I have to use “the big three” or can I just solve things with recursion?

You can solve things with recursion.

Do I have to use recursion, or can I solve things with the big three?

I’m not sure how you write the most general procedures without some kind of recursion or recursive thinking. (It may not be list recursion, but it’s some kind of recursion.)

Can I use let and let*?

Certainly.

Can you give me an example of something that’s really deeply nested?

Here’s a procedure that will build a nested list of approximately n levels.

    (define deep1
      (lambda (stuff n)
        (if (zero? n)
            (list stuff stuff)
            (deep1-helper stuff (deep1 stuff (- n 1))))))

    (define deep1-helper
      (lambda (stuff more-stuff)
        (list stuff more-stuff)))

Sam will attempt to demo.

> red-narrow
(list 10 20 (color 255 0 0 255))
> (deep1 red-narrow 0)
(list (list 10 20 (color 255 0 0 255)) (list 10 20 (color 255 0 0 255)))
> (deep1 red-narrow 1)
(list
 (list 10 20 (color 255 0 0 255))
 (list (list 10 20 (color 255 0 0 255)) (list 10 20 (color 255 0 0 255))))
> (deep1 red-narrow 2)
(list
 (list 10 20 (color 255 0 0 255))
 (list
  (list 10 20 (color 255 0 0 255))
  (list (list 10 20 (color 255 0 0 255)) (list 10 20 (color 255 0 0 255)))))
> (deep1 red-narrow 3)
(list
 (list 10 20 (color 255 0 0 255))
 (list
  (list 10 20 (color 255 0 0 255))
  (list
   (list 10 20 (color 255 0 0 255))
   (list (list 10 20 (color 255 0 0 255)) (list 10 20 (color 255 0 0 255))))))
> (solid-ellipses (deep1 red-narrow 3))
(list . (list . (list . (list . .))))
> (solid-ellipses (deep1 red-narrow 10))
(list
 .
 (list . (list . (list . (list . (list . (list . (list . (list . (list . (list . .)))))))))))`

Other

Will this recursion ever feel natural?

Prior experience suggests that it will eventually feel natural for most of you.

But it may take until week 10 or beyond.

Lab