--- title: Eboard 25 Pairs and pair structures number: 25 section: eboards held: 2018-04-02 link: true --- CSC 151.01, Class 25: Pairs and pair structures ================================================ _Overview_ * Preliminaries * Notes and news * Upcoming work * Extra credit * Questions * Representing lists with pairs and cons cells * Why care about the underlying representation * Printing lists * Lab * Debrief Preliminaries ------------- _Moment of silence._ ### News / Etc. * New partners! * Welcome back from spring break. * We're switching topics/themes post break, from recursion to ways of structuring data. * I've returned exam 2 and provided an answer key. * There was an academic honesty issue on the exam that I needed to report. * *Please don't let the stress of the next exam lead you to make poor choices!* ### Upcoming work * Exam 3 will be distributed on Wednesday. * [Lab writeup for class 25](../writeups/writeup25): Exercises 1 and 3. (Hardcopy lab!) Due before class Wednesday. * Reading for Wednesday * [Vectors](../readings/vectors) * [Flash Cards](../flashcards/flashcards09) due Wednesday. ### Dealing with grief * I will do what I can to support you. * Let me know if you need adjustments in workload to help. ### Extra credit (Academic/Artistic) * CS Table Tuesday: Unknown topic * 4pm Tuesday in DLab: "Updating Close Reading for the 21st Century" * CS Extras Thursday: Unknown topic * Danforth lecture on Thursday at 11 am in JRC 101. Robert G. Bergman. "Irreproducibility in the Scientific Literature or: How Often do Scientists Tell the Truth, the Whole Truth and Nothing but the Truth?" * Visit the two exhibits at the Faulconer Gallery. (Are there still two exhibits in the Faulconer gallery?) * Roxanne Gay talk Friday noon (?). ### Extra credit (Peer) * Men's Tennis, April 14, 21, and 22. ### 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) * Donate on Scarlet and Give Back day. (If donating is a financial burden, I will bring cash on Wednesday to give you to donate.) * Blood drive tomorrow. * Or write to me about why giving blood is important. * Host one or more prospective students. ### Other good things ### Questions _I've been incredibly behind in submitting extra credit writeups, even though they are due within TWO DAYS of the event. Can I still get extra credit?_ > I suppose so. Representing lists with pairs and cons cells -------------------------------------------- * The computer stores data in memory. * We will be looking at an abstraction of how it stores data. * Behind the scenes, everything is just a series of 0's and 1's. There is then some interpretation of the 0's and 1's. * We can't access the underlying representation, so we're going to look at a higher level. * When you call cons, it builds a "cons cell" or "pair" with spaces for two things. * The things are *not* the values you've cons'ed, they are references to elsewhere in memory where the values reside. * When you build a list, you get a sequence of pairs. * They are likely scattered about * But we will generally draw them in a line. * When you build a new list from an old list, they likely share memory. ``` (define lst1 (list 1 2 3)) (define lst2 (cons 'a (cdr lst1))) ``` * Naming * `car` - Contents of address register * `cdr` - Contents of data (decrement?) register * `cons` - Short for "construct" * Even if we change what lst1 is, the shared memory sticks around. `define` builds the rhs and then associates with the left. * Note: Scheme has some interesting policies on when it does and does not build new (copies of) values. * All symbols with the same name are in the same place. (Okay, refer to the same place) * The same string may be in different locations. * eq? lets you compare memory locations. Questions * If I have a list of five symbols, how many pairs do you expect to see? * If I have a list of four numbers, how many pairs do you expect to see? * Observation: The number of cons cells corresponds to the number of elements in the list. * Each cons cell can hold only one value. Why care about the underlying representation -------------------------------------------- Your answers * Thinking about memory can help us write more efficient code. * Helps us understand complex list structures. * Many of us think better visually, can help think about all sorts of issues. My answers * Helps with thinking about costs. If you have to recurse through until the end, that's a lot of work. * Helps us understand the stupid period we get for `(cons 'a 'b)`. Printing lists -------------- When Scheme is printing a value and it enounters a pair, it optimistically says "It's a list" and prints the open paren and the first value. Then it prints the values as it goes. When it hits a null, it prints a right paren. What if it doesn't hit a null? It's already done a lot of work that's hard to erase, so ... it prints a period to say "Whoops" and then the cdr and a right paren. Lab --- * You'll find it useful to draw lists horizontally. * If you have a null as the value, you also draw a slash. `(cons null null)` (or `(list null)`) looks something like the following. +---+---+ | / | / | +---+---+ Debrief ------- Observations: * The list of the empty list is not the same as the empty list. * `(cons null 'a)` is not a list, even though it has a `null` Questions: * Is there a pair that's not a list? Sure `(cons 'a 'a)` is not a list. For the pair to be a list, the second element must be a list. * Is there a list that's not a pair? `null` is a list, but not a pair. Problem 5 A list is either * `null` or * Cons (a pair) of anything and a list Translate to code (define listp? (lambda (val) (or (null? val) (and (pair? val) (listp? (cdr val))))))