CSC151.01 2014F, Class 23: Revisiting Lists =========================================== _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions. * Lists, revisited. * Lab. Preliminaries ------------- ### Admin * New partners! * Office hours * Normal (MThF 1:45-3:15; walking 1:15-1:45) * In the Grill 8:00-9:30 Wed and Fri * In my office 8:00-9:30 Thu * Review sessions Wednesday and Thursday at 8pm (led by Evan) ### Upcoming Work * Writeup (due Fri): Problem 4, * [HW 5](../assignments/assignment.05.html) due tonight. * Exam 2 will be distributed tomorrow. * Reading for Wednesday: * [Recursion Basics](../readings/recursion-basics-reading.html) * We will spend at least two days on this topic, so don't worry if it seems hard. ### Cool Things Coming to Campus * Two versions of _Anna Christie_! * Live Streaming of Verdi's _Macbeth_, Saturday at 11:55 a.m. * Pop-up Exhibit, Thursday October 9th, all day in BCA 132 ### Extra Credit Opportunities #### Academic * Grinnell Prize events this week (particularly events related to the prize winners whose project involves Web tools) * Artist Talk, Wednesday October 8th at 4:15pm in BCA 152 Rewriting the Application Programming Interface (API) for Art Making * BCC Faculty Chat, Tuesday, October 14, 7pm #### Peer Support * Football (Saturday at 1pm) * Swimming October 18: Alumni Weekend * 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 * The College's version of _Anna Christie_, Oct. 9-12 (SB plays Marthy) * Men's Tennis (???) * Women's Tennis (???) ### Questions * Note that I tend to respond more quickly to email questions than questions posted to the form. _Could you explain Problem 2 on Assignment 5?_ _Please explain Euclidean distance_ > Requires whiteboard. (Sam will try to transcribe.) Lists, revisited ---------------- * Original model: Homogeneous collections * Make them with `list`, `make-list`, `iota` * Build new lists from old with `map` * Join lists with `append` * `reverse` - obvious * `list-take` and `list-drop` - build a new list by taking the first N elements or dropping the first N elements * New model: Lists are collections that may be heterogenous and that you might decompose * `car` - get the first element * `cdr` - get all but the first element * `cadr` - get the second element * `cons` - given a value and a list, build a new list by shoving a value on the front * `list-ref` - given a list and a number, extract the particular value from the list * We can now work with individual elements, rather than doing the same thing to every value _Is there a way to put an element at the end of a list?_ > No built-in one, but you can probably figure out some strategy. (append lst (list val)) (reverse (cons val (reverse lst))) _How do you get the last element in a list?_ (list-ref lst (+ -1 (length lst))) (car (reverse lst)) Lab --- _How do we refer to the empty list?_ > Any of the following null '() (list) (make-list 0 "anything") > I prefer `null`. _Why do the indices start with 0 rather than 1?_ > Computer scientists like to start counting with 0. > It lets you think of `(list-ref lst i)` as "take the `cdr` i times, then take the `car`" _What would lead you to name three lists `one-two`, `won-too`, and `want-to`?_ > Lack of sleep. > Congratulations, you are the last class that will ever have to deal with these stupid names. _Why did we have that exercise?_ > To reinforce that "if you ssee a period, it's not a list" _What do you think of the following code to find element 5 using `cdr` and `car`?_ ((o car cdr cdr cdr cdr cdr) letters) > Creative. _What do you think of the following for `rectangularize`?_ (define rectangularize (lambda (drawing) (list 'drawing 'rectangle (list-ref drawing 2) (list-ref drawing 3) (list-ref drawing 4) (list-ref drawing 5) (list-ref drawing 6) (list-ref drawing 7)))) > My, that's a lot of work.