CSC 151.01, Class 26: Vectors

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• About the exam
• Lab
• Debrief

Preliminaries

News / Etc.

• New partners!
• Review sessions Thursday at 7pm and 9pm.
• Lab writeup from Monday: Note that you should have one pair-o-boxes per call to cons.
  • Please check in with Monday’s partner if you got things marked off.

Upcoming work

• Exam 3 distributed.
  • Prologue due Friday via email
  • Exam due Tuesday via email
  • Epilogue due Wednesday via email
  • Cover sheet due in class
• Lab writeup for class 26: Exercise 4 Due before class Friday.
• Reading for Friday
  • Binary trees
• Flash Cards due today at 5pm.
• Quiz Friday: Local recursive bindings with letrec and named let; randomness; pairs and pair structures.
  • Likely one named let question and one “draw this structure” question.

Extra credit (Academic/Artistic)

• Cello concert tonight.
• Danforth lecture on Thursday at 11 am in JRC 101. Robert G. Bergman. “Irreproducibility in the Scientific Literature or: How Often do Scientists Tell the Truth, the Whole Truth and Nothing but the Truth?”
• Roxane Gay talk Friday noon in Harris Cinema.
• Tabla concert Friday night
• Visit the two exhibits at the Faulconer Gallery. (Are there still two exhibits in the Faulconer gallery?)

Extra credit (Peer)

• Men’s Tennis, April 14, 21, and 22.
• Drag

Extra credit (Recurring peer)

• Listen to KDIC Wednesdays at 6pm - Witty banter with other personalities and/or co-host. Also Indian, Arabic, and Farsi music.
  (Up to two units of extra credit.)
• Peer editing with SS. Talk to SS about the details. Make your English Lit more literate.

Extra credit (Misc)

• Donate on Scarlet and Give Back day tomorrow.
  • Grinnell is a wonderful place; please consider donating.
  • If your funds are too limited to allow a donation, I’ve provided an alternative.
• Host one or more prospective students.
• Discussion on the diversity and inclusion plan tonight at 6pm (I think)

Other good things

• Grinnell Singers in Concert on Sunday at 2pm in Sebring Lewis.
• Passover Seder on Friday, send email to Rabbi Rob to sign up.

Questions

What are the “errors” and “times” things on the grading sheet.

• Everyone gets one point for each error someone finds in the main body of the exam (up to five points), starting 5pm today.
• I ask you to record your times. If you do so, you get 2 points.

What happens if we choose the “wrong” kind of recursion.

• I’ll generally be sympathetic, unless you do something irritatingly inefficient, like calling length again and again and again.

Should we document our named let helpers?

• Yes. A line or so is fine.
• “Find an approximation of the square root of n, given that the root is between lower and upper.”

Can we stop doing flash cards after today

• I suppose. I will grade you on the number of flash cards you turn in divided by 9, with a cap of “100 percent”.

About the exam

Recursion in CSC 151. Wicked neat!

Lab

Can we really use /home/rebelsky/Desktop/pg1260.txt as our source for words?

Sure. It’s Jane Eyre, book 1260 of Project Gutenberg.

What’s the writeup?

Exercise 4, vector-sum.

Debrief

How are vectors implemented so that they provide such fast access?

• That’s a good question.
• Vectors, like pairs, group references, rather than values.
• Every reference is the same size.
• So, to find element i of a vector, we look at the position vector-start + i*reference-size

Could you explain “gc time”?

• That’s also a good question.
• We often build lists (or parts of lists) and then stop referencing them.
• If we keep doing that, we’ll exhaust all possible memory (whiteboard) locations.
• At that point, we’ll need to use the unneeded memory for other purposes.
• “Garbage collection” is DrRacket’s attempt to find the created but unneeded parts of memory and make them available.

For start of next class …

I hear that there are two approaches you saw to vector-sum. What were they?

• Stay tuned!

Why am I getting weird errors when I try to use the result of vector-set!?

• Because vector-set! returns nothing. You’re probably trying to use the result in some future computation.
• You’ll see a slightly different pattern of recursion for procedures in which you modify vectors.
  • Call vector-set! to modify
  • and Recurse
• We now have multiple consequents after a guard. We should therefore use when or cond, depending on the situation.