--- title: Eboard 27 Trees number: 27 section: eboards held: 2018-04-06 link: true --- CSC 151.01, Class 27: Trees ============================ _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Questions * Quiz * A bit more on vectors * A bit about trees * Lab * Debrief (?) Preliminaries ------------- ### News / Etc. * New partners! * Welcome back to our backup mentor, Smarly! * It would have been nice to see students take a creative approach to the burrito challenge. (E.g., "If every person in every class gives $1, we'll all be at 100% and we'll all get burritos.") ### Upcoming work * [Exam 3](../exams/exam03) * Prologue due **TONIGHT** via email * Exam due Tuesday via email * Epilogue due Wednesday via email * Cover sheet due in class * No lab writeup! Due before class Monday. * Reading for Monday * [Higher-order-procedures, revisited](../readings/hop) * [Flash Cards](../flashcards/flashcards10) due Wednesday at 5pm. * Optional. ### Extra credit (Academic/Artistic) * Roxane Gay talk **TODAY** noon in Harris Cinema. * Tabla concert **TONIGHT** * Visit the two exhibits at the Faulconer Gallery. (Are there still two exhibits in the Faulconer gallery?) * Retrospectively watch last night's talk (Marlon James) if you can find it online. ### 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. ### Other good things * Grinnell Singers in Concert on Sunday at 2pm in Sebring Lewis. ### Friday PSA * Please take care of yourselves. I do care about you and I'm fortunate to have you in the class. ### Questions _How do you shorten long and rambly code?_ * Sometimes pull things out and make a separate procedure. * If you have a complex expression, name it as a separate procedure. * Write higher-order procedures, like we did last class. * Practice with other people and critique. Quiz ---- Remember: If you finish early, revel in the chance to sit quietly. Continued debrief on vectors ---------------------------- Vectors have two important characteristics. * Fast access to every element. * Vectors are mutable -> You can replace an element in a vector without building a new vector. (Lists require that you build a new list.) In contrast, lists are dynamic - they can grow and shrink. Some folks also prefer immutable structures. _I hear that there are two approaches you saw to `vector-sum`. What were they?_ * One approach is to use a tail-recursive helper that keeps track of both the position in the vector and the running sum. We introduced a new list sum using this technique. * Another aproach is to use direct recursion, as in the `vector-largest` procedure from the reading. Or in the orignal list-sum. * Let's look at both. ``` (define list-sum-tr (lambda (lst) (let kernel ([remaining lst] [sum-so-far 0]) (if (null? remaining) sum-so-far (kernel (cdr remaining) (+ sum-so-far (car remaining))))))) (define vector-sum-tr (lambda (vec) (let ([len (vector-length vec)]) (let kernel ([pos 0] [sum-so-far 0]) (if (>= pos len) sum-so-far (kernel (+ 1 pos) (+ sum-so-far (vector-ref vec pos)))))))) (define list-sum-direct (lambda (lst) (if (null? lst) 0 (+ (car lst) (list-sum-direct (cdr lst)))))) (define vector-sum-direct (lambda (vec) (let ([len (vector-length vec)]) (let kernel ([pos 0]) (if (>= pos len) 0 (+ (vector-ref vec pos) (kernel (+ pos 1)))))))) ``` Note that we can also compute this right-to-left rather than left-to-right. ``` (define vector-sum-tr-alt (lambda (vec) (let kernel ([pos (- (vector-length vec) 1)] [sum-so-far 0]) (if (negative? pos) sum-so-far (kernel (- 1 pos) (+ (vector-ref vec pos) sum-so-far)))))) (define vector-sum-direct-alt (lambda (vec) (let kernel ([pos (- (vector-length vec) 1)]) (if (>= pos len) 0 (+ (vector-ref vec pos) (kernel (- pos 1))))))) ``` _Why am I getting weird errors when I try to use the result of `vector-set!`?_ * Because `vector-set!` returns nothing. You're probably trying to use the result in some future computation. * You'll see a slightly different pattern of recursion for procedures in which you modify vectors. * Call `vector-set!` to modify * *and* recurse * We now have multiple consequents after a guard. We should therefore use `when` or `cond`, depending on the situation. Example: An incorrect approach to "double all the values" ``` (define vector-double! (lambda (vec) (let ([len (vector-length vec)]) (let kernel ([pos 0] [vec vec]) (if (>= pos len) vec (kernel (+ pos 1) (vector-set! vec pos (* 2 (vector-ref vec pos))))))))) ``` A bit about trees ----------------- _There wasn't time for this._ * Trees provide a third alternative for organizing information. * [Sam may go over other issues on Monday] Lab --- _Not enough time; Sam spent too much time talking about vectors._ _We will continue the lab on Monday. Same partners!_ Debrief ------- Why consider trees? * Can provide an efficient way to organize information. (The finding in trees examples illustrates that.) * Important conceptual idea: We can separate a way of thinking about organizing information (nodes) with the way we actually implement it (with lists or vectors). * This technique is a form of "abstraction" * Abstraction generally involves ignoring ("abstracting away") underlying details. We find it useful as a way to approach both data and procedures.