CSC 151.01, Class 30: Analyzing procedures

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Friday PSA
  • Questions
• Quiz
• Lab
• Debrief?

Preliminaries

News / Etc.

• Warning! Friday the 13th falls on a Friday this month. Please be extra careful.
• Some of you have noted with concern that we did not get enough chance to debrief on vectors or higher-order procedures. If you’d like to ask some questions, I’ll take questions about those issues until 9:00 a.m. or so.

Upcoming work

• Lab writeup for class 30: Exercise 3
• Reading for Monday
  • Project description
• Flash Cards due Wednesday at 9pm.
  • Optional.
  • Grade is percent of eight flashcard assignments you complete (capped at 100%).
• Assignment 7 due next Tuesday at 10:30 p.m.
  • Let me know your partners! (I may check in with you during class.)

Extra credit (Academic/Artistic)

• Any one activity in the student research symposium next week.
• Water Dance performance at CERA on Saturday.
• Art and Science of field work Sunday at CERA. RSVP necessary. You should have received email.
• Medieval Improv concert Saturday at 7:30 p.m. in Sebring-Lewis.

Extra credit (Peer)

• Drag show April 14

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)

• Host one or more prospective students. (I think there’s one visit weekend left.)

Other good things

Friday PSA

• You are thoughtful, intelligent, caring people. I am fortunate to be able to teach you.
• Because you are thoughtful, intelligent, and caring, you have the capability to do advance planning. Reflect what is right for you this weekend and try to stick to your plan.
  • Excess is rarely the right thing.
• If you choose to cohabit, planning can also be good. But remember that no matter what, consent is necessary. Don’t damage another human being.

Questions

Why do we need four cases for number-tree-largest?

• What’s the largest value in the empty tree? There are no values in the empty tree, so it’s a meaningless question.
• Oh! Perhaps my base case is “It’s a leaf.”
• What are the possibilities if it’s not a leaf?
  • Empty left, Nonempty right
  • Nonempty left, Empty right
  • Nonempty left, Nonempty right
• I’m not sure that there’s one common thing for all three cases.
  • Well, we do want maxima, but slightly different computations.

(define number-tree-largest
  (lambda (ntree)
    (cond
      [(leaf? ntree)
       (contents ntree)]
      [(empty? (left ntree))
       (max (contents ntree)
            (number-tree-largest (right ntree)))]
      [(empty? (right ntree))
       (max (contents ntree)
            (number-tree-largest (left ntree)))]
      [else
       (max (contents ntree)
            (number-tree-largest (left ntree))
            (number-tree-largest (right ntree)))])))

Trees seem pointless. Why are you making us do them? Are you just cruel?

• Yes, I am cruel. But, more importantly, trees lead to some more interesting and more complicated patterns of direct recursion.
• Some kinds of trees end up being particularly useful.
  • Binary search trees make searching much faster.
  • If we have 1000 things in a vector, it will take us about 1000 comparisons to discover that something is not in the vector.
  • If we have 1000 things in a balanced binary serach tree, it will take us
    • about 500
    • the depth of the tree (log)
    • um
    • TEN, more or less
• We represent many kinds of data as trees, such as Web pages.

Should we always convert our data to a tree if we want to search it?

• No, there’s a tradeoff involved. If you only want to search once or twice, it’s expensive.

When we take the quiz, can we assume that left, right, contents and others of that ilk are in the library?

• Yes.

Will we always have to copy and paste the tree code?

• No. You can use (require csc151/trees), or at least I think you can.
• We intentionally keep the tree code separately from the primary code.

Quiz

Remember! If you finish early, relish the opportunity to have a few minutes of quiet.

Lab

Writeup: Exercise 3

• Don’t forget to update list-append to count the calls to cons, car, and the rest.
• We’ll talk on Monday about why you get results like 28 calls to cons for a list of length 8.

Debrief