--- title: Eboard 26 Vectors number: 26 section: eboards held: 2017-10-30 --- CSC 151.03, Class 26: Vectors ============================== _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Notes on Friday's quiz * Questions * Lab * Debrief ### News / Etc. * Welcome to any prospective students! * Grading still isn't done. I need less paperwork. * 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 6. No documentation necessary. * To: * Subject: CSC 151.03 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: The Google Memo. * Rebirth Brass Band, Wednesday at 7:30 p.m. in Herrick. * CS Extras, Thursday at 4:15 p: "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 colonel ([ans-so-far 1] [k n]) (if (zero? k) ans-so-far (colonel (* 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 What does the ! mean in the procedure name? : We're calling the procedure not for a return value, but because we want to change one of the parameters. : Things without ! generally *don't* change their parameters, which means we call them to get back a new value. A vector example ``` > (define vec (vector "a" "vector" "of" "five" "elements")) > vec '#("a" "vector" "of" "five" "elements") > (vector-set! vec 3 "many") > ; Look! It didn't return anything. vec '#("a" "vector" "of" "many" "elements") ``` A list example ``` > (define phrase (list "a" "list" "of" "five" "elements")) > phrase '("a" "list" "of" "five" "elements") > (append (take phrase 3) (list "many") (drop phrase 4)) '("a" "list" "of" "many" "elements") > phrase '("a" "list" "of" "five" "elements") ``` Lab --- Hints on `vector-fill!` : Remember that `vector-set!` returns nothing. You can't use the return value from `vector-set!` as the parameter to a procedure. : You are changing *the same* vector at each recursive call. There's no need to make it a parameter to the kernel. : You may need to sequence operations. Remember that both `when` and `cond` let you have a sequence of consequents. Writeup: Exercise 6, No documentation required. This is class meeting 26. Debrief ------- What is `gc`? : Garbage collection. The time spent trying to find things you've created but aren't using any more and to free the resources associated with them. Why is the time inconsistent? : You're randomly choosing which elements of a list to take. Those near the front will be quicker than those near the end. What about when I'm using vectors? : You're still building a list. It turns out that the time to allocate space to build one pair is not consistent.