CSC151.01 2014F, Class 36: Trees ================================ *New partners!* * Grab a card. * Read it, figure out where you go. * Put the card on the plate so that Evan doesn't have to collect it. _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions/Ideas. * Four kinds of repetition. * Lab. * Debrief. (If time.) Preliminaries ------------- ### Admin * Extended/modified office hours continue this week and next. * Survey: Should I switch to online signup for office hours? (Choices are "Please have online signup" and "Please keep physical signup". You may also choose not to vote.) * I've been answering a variety of prereg questions. Feel free to continue asking such questions. * I apologize for missing much of class yesterday. It was an emergency. * I hear from some of the tutors that some of you are letting your partners carry the weight in your groups. Doing so violates self-gov. * Review sessions * Tonight at 8pm with Alex * Tomorrow at 10am with Sam * Tomorrow at 8pm with Ajuna * A great quote from _Made to Stick_: "Trying to solve a problem *beore being taught the solution* leads to better learning, even when errors are made in the attempt." ### Upcoming Work * Lab writeup: Problems 2 and 4a. * Reading for Friday: [Geometric Art](../readings/geometric-art-reading.html) * [Exam 3](../assignments/exam.03.html), distributed today, due next Tuesday. * *Required* prologue due Friday night. * *Required* epilogue due next Wednesday night. * Quiz Friday * Turtles * Iteration * Pairs and pair structures * You may bring a sheet of notes ### Cool Upcoming Events on Campus * Peace studies talk Monday Nov. 10 at 4:15 p.m. in JRC 101 "With All the World’s Violence, Where is Peace?" ### Extra Credit Opportunities #### Academic * CS Extra Thursday: Study-Abroad Opportunities with a CS Component 4:30 p.m., Science 3821. * It's never too early to think about studying abroad! * CS Table Friday: Open Data. 12:10-12:50, Day PDR. #### Peer Support * Karan's radio show 11pm Thursday nights on KDIC * Evan's radio show 5pm Friday nights on KDIC * Donna's radio show Sunday midnight on KDIC * Swimming on Saturday (1pm) against Luther. * The gaming club is hosting a Smash Brothers tournament on Saturday (Loose at 11am ...) (And thats "Loose", not "Lose") * The Go Club is having its first meeting Sunday 1pm, JRC 203 * Aimed for complete beginners. Questions/Recent Ideas/Etc. --------------------------- _When do you use basic recursion and when do you use helper recursion?_ > Husk-and-kernel does one kind of helper recursion, but I think you mean the question more generally. > No general answer (at least not one that I know of). It's mostly which way makes it easier to think about solving the particular problem. > If I find it "natural" to have one or more extra values during your recursion, I start with helper. > I often end up writing whichever feels easier, and then go back and reflect on whether I might improve. > Looking at/remembering similar kinds of problems helps. _How do we write `listp?`_ > A list is either the empty list (i.e., `null `) or a pair whose cdr is a list. (define listp? (lambda (val) (or (null? val) (and (pair? val) (listp? (cdr val)))))) _Is the null list a pair?_ > No, it is the only list that is not a pair. Four Kinds of Repetition ------------------------ What's the relationship between the four kinds of somewhat general repetition? (`for-each`, `map`, `repeat`, recursion)` * Take two minutes to talk to your partner about this question. * Identify similarites between pairs (or groups) * Identify differences between pairs (or groups) * Be prepared to share. * Things you might think about * Input types * Output types * Side effects vs. pure Things we came up with * Both `for-each` and `map` apply a function to every element in a list * map: Goal is to produce a new list * for-each: Goal is to have lots of side effects * repeat does one procedure over and over again * With recursion, you do something over and over again * Repeat is a fixed number of times * recursion uses some "base case test", may be less predictable * for-each and map do the application once to each element, * repeat does it multiple times to the same element * recursion could do anything (once, multiple times, no times, ...) * (repeat 3 proc val) means do it three times * while recursion means doing one single function * Input is a list * map, recursion, for-each (not repeat) * Input is a number (or other non-list) * repeat, recursion (not for-each or map) * Goal is side effects, rather than computing a value * repeat or for-each (or maybe recursion, but not map) * Goal is to produce a list * map - if the result is the same length, and fits the model * recurision - otherwise Lab --- Debrief -------