CSC151.02 2016S, Class 38: Vectors ================================== _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions. * A bit more on pair structures. * Vectors lab. Preliminaries ------------- ### Admin * Same partners! * I will continue to bring you food (or food-like substances) until I am caught up on grading. * I have not had the opportunity to read much email today. I'm also watching my son play soccer in Pella. If you sent a question, I probably won't get to until 9:00 p.m. ### Reminders * Office hours this week * See http://rebelsky.youcanbook.me. * Ask me about other available times. * Tutor hours * Sunday, 3-5 p.m. * Sunday-Thursday, 7-10 p.m. * Review Sessions * Wednesday at 8pm in CS commons with Sarah (?) * Thursday at 8pm in CS commons with Kumar. * None with Sam this week ### Upcoming Work: * *No reading for tomorrow.* * Lab Writeup: * Send email titled __CSC 151 Lab Writeup 38 (Your Names)__ * Do not include the underscores. * Send to * Due before class on Monday. ### Extra Credit * Send your reports to with subject "CSC 151 Extra Credit". (Do not include the quotation marks.) * Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!" #### Academic / Artistic * Talk Wednesday, 4pm, ARH 302 * Talk Thursday, 4:15 pm, ARH 102, The Stability of Value-added Measures of Teacher Effectiveness Derived under Different Growth Models * Student Research Symposium this week * Symphony Orchestra with Dave Stamey, Herrick. April 16, 7:30-9:00 p.m. * Any of the fora on the report from the Residential Learning Task Force. (Formal announcement forthcoming.) * April 15: Open task force meeting, 1:00-2:30pm in JRC 209 * April 18: Listening session, 8:00-9:00pm TBD * April 22: Open task force meeting, 1:00-2:30pm in JRC 209 * April 26: Listening session, 6:00-7:00pm TBD #### Peer * Lords of the Flies, 7:30 pm April 22nd, 2:00 pm April 23rd, 2:00 9pm April 24th. (Only 50-60 tickets per show) * ISO, Saturday, April 16, 7-9 p.m. Various classmates performing at various times. * Second Annual Contra into Spring Dance, April 22nd, Main Quad, 7:30 p.m. #### Miscellaneous * Host a prospective student this coming weekend. #### Regular Peer * Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio * Post-break ExCo on British Politics Wednesdays at 8:00 in JRC 203. Just show up; you don't need to sign up. * Pun club Saturdays at 4pm in Younker * Electronic Potpourri on KDIC Fridays at Five * Space Odyssey KDIC Fridays at Six * Bollywood, Fridays, 7:30-8:30, Younker * Effective Altruism club, 2:30-3:30 Sundays in JRC 226. #### Misc #### Far in the Future * Adaptation of Rushdie's East West. Early May. ### Questions _What was yesterday's writeup?_ > Exercise 3. _What should `rac` and `rcd` return when given `null` as input?_ > What do `car` and `cdr` return when given `null` as input? _What's wrong with my stencil procedure?_ > Most common answer: You don't have to fit everything into one `let` or `letrec`. You can nest forms of `let`. > Second most common answer: You need to recompute the random column and row at each repetition. Think about your ordering of operations. > Third most common answer: Since there's call to `image-new` at the top of `stencil-quarter-circle`, don't recursively call `stencil-quarter-circle`. Use `repeat` or use a helper. (Sam prefers local helpers, but will cope with non-local helpers in many cases.) _Can you demo `image-set-pixel!`?_ > Sure. Done. _Can you explain the formula for problem 6?_ > The area of a circle is pi*r*r. If the radius is 1, the area of a circle is pi. If it's a quarter circle, the area is pi/4. We're approximating the area of the quarter circle. We can then run the formula backwards to get pi. > See the board for more details. _But how do I get a random number between 0 and 1?_ > `(random)`. _I've written `irgb-closest` using that "doubly recursive method" which we've seen is horribly inefficient. Will you penalize us for that?_ > Yes. Lots! (If I'm in a good mood when grading, and you have nothing else wrong with your solution, you'll be lucky to get a 6/10 on that problem.) _Do you expect us to think at least a little bit about efficiency?_ > Yes. _What about empty substrings?_ > Look at the postconditions. 
> (define str "Hello World")
> (define substr "")
> (define pos 0)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
#t
> (define pos 5)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
#t
> (define pos 1253)
> (equal? (substring str pos (+ pos (string-length substr))) substr)
. . substring: starting index is out of range
  starting index: 1253
  valid range: [0, 11]
  string: "Hello World"
_How much extra credit should we get for identifying the massive typo on how to submit?_ > At least two units. Email it to me, like always. _What is my number?_ > The same one as last time. _I lost it._ > (random 1000000) ; add any leading 0's _Do you really want dead trees?_ > No. Point. _Are we ever getting to the main class material?_ > Maybe. Conveniently, tomorrow is a "pause for breath" and you can do the lab then. _Are you really taking off one point for each minute a student was late to today's class?_ > Considering it. (And yes, that's 50 points for people who missed class, and 30 minutes for ....) A Bit More on Pair Structures ----------------------------- Why are we doing this? (That is, looking at pair structures and vectors.) Why are we studying pair structures, given that we know about list? * To give us a better understanding of `car` and `cdr`. `car` is the thing that the first box points to (aka the contents of the first box), `cdr` is the thing that the second box points to. * Gives you a deeper understanding of how Scheme works. * Scheme does a lot of "reference chasing" (following arrows) * List processing is generally slow, so look for ways to avoid it. * We use linked structures a *lot* in CS; knowing more about linked structures turns out to be useful. * Soon, we will build more complicated linked structures using pairs (or something similar). Pictures help you understand. * Understand the stupid dot at the end of each non-list. Why are we studying vectors? * Vectors provide an interesting alternative to lists because they seem more efficient. * For example, vectors let you change them without creating a new vector. * Vectors let us quickly find any element because we can compute its location. * In contrast, finding an element in a vector can be slow. * Vectors are hard to resize, though. * Is there a `car` for vectors? Not really. There's `vector-ref` * Is there a `cdr` for vectors? No. Vectors Lab -----------