--- title: Eboard 33 Project work day 1 number: 33 section: eboards held: 2018-04-20 link: true --- CSC 151.01, Class 33: Project work day 1 ========================================= _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Questions * Making code more efficient * Quiz * Work time Preliminaries ------------- ### News / Etc. * Sit with your project group. * We will miss the crane. * However, last Friday's S&B suggests that worries about crane permissions made the College less willing to explore certain important but controversial changes, so perhaps not. ### Upcoming work * No writeup for today's class. * Reading for Monday: [Searching](../readings/searching) * [Flash Cards](../flashcards/flashcards12) due next Wednesday at 9pm. * Optional. * Grade is percent of eight flashcard assignments you complete (capped at 100%). * [Project Proposal](../assignments/project) due **Monday** at 10:30 p.m. * Due early to give me time to react. ### Extra credit (Academic/Artistic) * Disability panel, Friday, 7pm, JRC 101. * HackGC (or any of the HackGC talks). * Rap concert tonight. (Opening act is a Grinnell alum.) * The Cypher Paradigm: Closing Cypher, Tuesday, 8:30 p.m., ARH 120. * Squire Lecture in Physics, Tuesday at 11:45: STEM Eqwuality and Inclusion, a Female Astronomer's View. Noyce 1023. * CS Table Tuesday: Subject TBD. * Diversity in Filmmaking, Monday, 4:15 p.m., Bucksbaum 152 ### Extra credit (Peer) * Play presentation Friday at 4pm in the Wall Theatre (154). * ISO Cultural Evening Saturday night at 7pm in Harris * Escape Room, 7-9pm Monday, Gates * Thursday, April 26th, Noyce 2022: Mellon Mays Project introductions, 4:15 to 6:30 p.m. * Field Day. * Making a Killing, Friday 7pm, Harris. ### 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.) * Listen to KIDC Thursday at 7pm - Classic Rock. (60's and 70's) * Peer editing with SS. Talk to SS about the details. Make your English Lit more literate. ### Extra credit (Misc) * Any Sexual Assault Awareness Month event. * Host one or more prospective students. (I think this is the last visit weekend.) ### Other good things * Free HIV testing at SHACS. * Dick Young Meet tomorrow. * Water Polo Saturday and Sunday. * Ideation, a play, 7:30 p.m. in Loft Theatre tonight and tomorrow night. and Sunday at 2. ### Questions Making code more efficient -------------------------- We'll consider two examples. ### Example one: Finding the smallest value in a list _What are potential expenses in the following code. How would you reduce those expenses without completely rewriting the code?_ ``` (define list-smallest (lambda (lst) (counter-increment! lsc) (if (= (length lst) 1) (car lst) (if (< (car lst) (list-smallest (cdr lst))) (car lst) (list-smallest (cdr lst)))))) ``` Warning bells! * `length` is expensive. I shouldn't call it on every recursive call. * I also worry about identical expressions: three calls to `(car lst)` and two to `(list-smallest (cdr lst))`. * I worry less about `(car lst)` because `car` is fast. * I worry more about `(list-smallest (cdr lst))` because I know that those are expensive. My worries were appropriate. * `(list-smallest (reverse (iota 4)))` => 15 calls * `(list-smallest (reverse (iota 8)))` => 255 calls * `(list-smallest (reverse (iota 16)))` => 65535 calls * Pattern: Approximately `(- (expt 2 (length lst)) 1)` calls * We gave up after about five minutes on `(list-smallest (reverse (iota 1000)))`. At that point, there had been 4123710043 (4,123,710,043 for those who like commas) calls. Solving the problems. * Use `(null? (cdr lst))` to check for a one-element list. * Use `let` to avoid repeated computations. ``` (define list-smallest (lambda (lst) (counter-increment! lsc) (if (null? (cdr lst)) (car lst) (let ([smallest-of-rest (list-smallest (cdr lst))]) (if (< (car lst) smallest-of-rest) (car lst) smallest-of-rest))))) ``` What happens? * For `(list-smallest (reverse (iota 1000)))`, it was nearly instantaneous and was 1000 calls. Yay! ### Computing a sublist. _Once again, what's wrong with this code?_ ``` ;;; Purpose: ;;; take the sublist starting at position start and ending right ;;; before position finish. (define sublist (lambda (lst start finish) (let kernel ([start start] [finish finish]) (cond [(>= start finish) null] [(>= start (length lst)) null] [else (cons (list-ref lst start) (kernel (+ 1 start) finish))])))) ``` Agh! There's another call to `length`. That makes me nervous. I'll move it outside the kernel. ``` (define sublist (lambda (lst start finish) (let [(len (length lst))] (let kernel ([start start] [finish finish]) (cond [(>= start finish) null] [(>= start len) null] [else (cons (list-ref lst start) (kernel (+ 1 start) finish))])))) ``` Oh. There's still a call to `list-ref`. That makes me nervous. `list-ref` is expensive. Calling it each time will likely add up. (start + start+1 + start+2 + ... is a large sum.) How do we do without the call to `list-ref`? We should probably follow our normal pattern for list recursion, which involves adding the list as a parameter. Here's not-yet-better code. ``` (define sublist (lambda (lst start finish) (let [(len (length lst))] (let kernel ([remaining lst] [start start] [finish finish]) (cond [(>= start finish) null] [(>= start len) null] [else (cons (list-ref lst start) (kernel (+ 1 start) finish))])))) ``` Quiz ---- Work time ---------