--- title: Eboard 26 Vectors number: 26 section: eboards held: 2017-10-30 --- CSC 151.01, Class 26: Vectors ============================== _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Notes on Quiz 9 * Questions * Lab * Debrief ### News / Etc. * Welcome to any prospective students here for Discover Grinnell! * Grading still isn't done. I need less paperwork. * Congratulations to Singers for an excellent and moving concert. * Congratulations to Volleyball for making the Midwest Conference Tournament. * Congratulations to Men's and Women's Soccer for making the Midwest Conference Tournament. ### Upcoming Work * [Writeup for class 25](../writeups/writeup25) due Monday at 10:30 p.m. * Exercises 1 and 3. * Hand to: Sam (or under Sam's door) * [Writeup for class 26](../writeups/writeup26) due Wednesday at 10:30 p.m. * Exercise 5. No documentation necessary. (But it would be nice and good practice.) * To: * Subject: CSC 151.01 Writeup 26 (YOUR NAMES) * [Assignment 6](../assignments/assignment06) due TOMORROW. * Read [Binary trees](../readings/binary-trees) for Wednesday's class. * Yes, it's ready! ### Extra credit (Academic/Artistic) * CS Table, Tuesday at noon: TBD * Halloween Organ Thing, Tuesday evening in Herrick Chapel. * Rebirth Brass Band, Wednesday at 7:30 p.m. in Herrick. * CS Extras, Thursday at 4:15 p.m.: "Building your own programming language and other reasons to go to graduate school" ### Extra credit (Peer) ### Extra credit (Misc) ### Other good things ### Notes on Quiz 9 Two common problems: Named let and tail-recursive tally. #### Named let Original: ``` (define factorial (lambda (n) (fact-helper 1 n))) (define fact-helper (lambda (ans-so-far k) (if (zero? k) ans-so-far (fact-helper (* k ans-so-far) (- k 1))))) ``` Ideas of named let * Put the helper inside the primary procedure; others don't need it. * Put the parameters and the initial values together. * (Strange syntax) * The body of the helper should not change, except that you may change the name of the helper. Form ``` (let NAME ([PARAM-1 INITIAL-1] [PARAM-2 INITIAL-2]) BODY) ``` Revised ``` (define factorial (lambda (n) (let kernel ([ans-so-far 1] [k n]) (if (zero? k) ans-so-far (kernel (* k ans-so-far) (- k 1)))))) ``` * (Eventually, you will just write the helpers within the procedure, rather than write them separately and move them within the procedure.) #### Tail-recursive list tally * Idea: We use a helper to keep track of both the tally and what we have left to explore. * When we're done, the tally should be done; no need to backtrack. Chart ``` tally remaining ----- --------- 0 '(5 -2 1 4 3 -7) 1 '(-2 1 4 3 -7) 1 '(1 4 3 -7) 2 '(4 3 -7) 3 '(3 -7) 4 '(-7) 4 '() ``` Code ``` (define tally-positive (lambda (lst) (let kernel ([tally 0] [remaining lst]) (cond [(null? remaining) tally] [(positive? (car remaining)) (kernel (increment tally) (cdr remaining))] [else (kernel tally (cdr remaining))])))) ``` ### Questions Why do you tell us not to use quote to create lists? : If you haven't thought deeply enough about it, you don't get the results you expect. `'(5 (sqrt 4))` : For really simple lists, it's fine. For other things, it can make your brain hurt. Why is it that grabbing the ith element of a vector is faster than grabbing the ith element of a list? On problem 2, can we use reverse? : No Can we use `equal?` : I suppose, but char=? is better. Lab --- The correct path is "/home/rebelsky/Desktop/pg1260.txt" Writeup: Exercise 5. Call it `my-vector-fill!`. Do not use the existing `vector-fill!` Debrief ------- Why did we see inconsistent times to build a list of random elements from a vector? : Four key operations: Find the length. Choose a random number. Get that element from the vector. Add that element to a new list. : The first three should be consistent. It turns out that adding an element to a list is less consistent than you might expect. You won't notice it for a few elements, but you may notice it for a lot. : I expect that the timer is also not perfect. Why are the cpu times all multiples of four? : Perhaps the granularity of the timer is 1/250 of a second. Why doesn't my `my-vector-fill!` work? (What are your common "helpful" comments.) : Remember that `vector-set!` does not return anything. If you want to recurse with the vector, you'll need to do a sequence of statements. : Remember that if you use `cond` or `when` you can have multiple consequents. : Don't create a new vector. : Reindent.