EBoard 27: Pairs and pair structures (Section 3)

New places, new partners, grab whiteboards.

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

Approximate overview

• Administrivia
• Q&A
• Lab
• Quizzes

Administrative stuff

Introductory Notes

• There were some questions about the “hole in the program” messages. Scamper produces that error when we define something as ??.
• There was some confusion about MP redos. The first one is free, even if you turned the MP in late.

Upcoming work

• Due Sunday, 2026-11-08
  • Reading: Vectors
    • This reading needs to be updated for Scamper.
  • Today’s lab: Pairs
• Due Tuesday, 2026-11-10
  • Mini-project 1 second redo
    • Submit redo on Gradescope
    • When submitting a mini-project redo, please include a file called CHANGES.txt that describes what you’ve changed.
• Due Tuesday, 2026-11-17
  • Mini-project 5
    • Not yet tested in Scamper!
    • Not yet posted to Gradescope
• On Friday, 2026-11-13
  • NEW Quiz/LA on pairs (today)
  • NEW Quiz/LA on vectors (Monday)
  • Makeup Quiz/LA on numeric recursion
  • Makeup Quiz/LA on list recursion
  • Makeup Quiz/LA on testing
  • Makeup Quiz/LA on local bindings
  • Makeup Quiz/LA on collaboration
  • Makeup Quiz/LA on lists and list operations
  • Makeup Quiz/LA on conditionals
  • Makeup Quiz/LA on compose, cut, and section
  • Makeup Quiz/LA on tracing
  • Makeup Quiz/LA on decomposition
  • Makeup Quiz/LA on procedures

Upcoming activities

Scholarly

• Monday, 9 October 2026, Noon–1:00 p.m., JRC 209. Hana Hashimoto ‘27 leads a discussion of constitutions.
  • Sign up form in campus memo.
• Tuesday, 10 October 2026, Noon–1:00 p.m., CS Commons (I think). CS Table
• Tuesday, 10 October 2026, 4:15–5:15 p.m., Noyce 2021 (?). Tuesday Extras
• Thursday, 12 October 2026, 11:00 a.m.–Noon, JRC 101. Scholars’ Convocation
• Thursday, 12 October 2026, 4:15–5:15 p.m., Noyce 2021 (?). Thursday Extras

Artistic/Cultural

• Any day. Visit the GCMOA for at least 30 minutes.
• Saturday, 7 November 2026, 7:00–10:00 p.m., Harris Gym. Free Sound presents: Battle of the Bands
• Saturday, 7 November 2026, 7:30–9:30 p.m., Roberts Theatre. Mainstage: Cassandra Listens
• Sunday, 8 November 2026, 2:00–4:00 p.m., Roberts Theatre. Mainstage: Cassandra Listens
• Sunday, 15 November 2026, 4:99 p.m., Sebring-Lewis. Family Folk Machine: Rising Like a Flame

Multicultural

• No middle of everywhere today )-:
• Friday, 13 2026, 4:10–5:00 p.m., HSSC N1170. Middle of Everywhere (Japan)

Peer

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

• Football, 14 November 2026.

Wellness

• Friday, 6 November 2026, 5:15–6:00 p.m., Main Hall Lounge. Yoga with Lisa
• Monday, 9 November 2026, 8:00–9:00 p.m., Prayer Garage in the CRSSJ. Meditation Group
• Tuesday, 10 November 2026, 12:15–12:45 p.m., GCMoA. Yoga in the Museum
• Tuesday, 10 November 2026, 4:30–6:00 p.m., Bear P103. Wellness Yoga
• Tuesday, 10 November 2026, 5:00–8:00 p.m., HSSC North Auditorium. Therapy Dogs
• Tuesday, 10 November 2026, 8:00–10:30 p.m., Harris Cinema. The Muppet Movie

Misc

• Tuesday, 10 November 2026, 7:00–9:00 p.m., Darby. Volleyball vs. Lake Forest, Revisited (Quarterfinals)
• Wednesday, 11 November 2026, 8:00–9:00 p.m., Science 3820. Mentor Session
• Thursday, 12 November 2026, 8:00–9:00 p.m., Science 3820. Mentor Session

Other good things

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

• Saturday, 7 November 2026, 1:00–4:00 p.m., Osgood Natatorium. Swimming and Diving vs. Luther College
• Wednesday, 11 November 2026, 6:00–8:00 p.m., Darby. Men’s Basketball vs. Principia

Friday PSA

• You’re awesome.
• Please keep yourself as such.
• Moderation!
• Decide what is right for you.
• Consent is essential. Also insufficient. Also impossible while impaired.

Q&A

Administrative

Tail recursion

What’s the diff between tail recursion and numeric recursion?

Tail recursion usually involves writing a helper procedure that includes an extra “accumulator” (so-far) that builds up the solution as we go.

Numeric recursion involves a numeric parameter that we “simplify” at every strep. For a numeric recursion, we often subtract 1 until we get to 0, but we might also add 1 until we reach some upper bound, or divide by some number or …

You can use tail recursion to write numeric recursive procedures.

Why do we separate the forms of recursion?

Mostly so that we can talk more specifically about the problems.

Numeric recursion

Pairs

How do we pronounce the various cadr-like things.

“car” - like “automobile”

“cdr” - “could - er”

“cadr” - “cad - er”

“cddr” - “could did er”

“cadar” - “see eh dee eh are”

Lab

• Do the pairs lab.

Notes

• There are not yet test cases. Sorry.

Quizzes

• For the NUMERIC RECURSION quiz, you can assume there are MORE THAN n elements in the list.
• For the NUMERIC RECURSION quiz, there are two names for the procedure. You may use either.
• For the NUMERIC RECURSION quiz, I would recommend against tail recursion. If you use tail recursion, you will probably need reverse somewhere, which complicates things.