CSC151.02 2003F, Class 7: Lists

Admin:
* Sorry for the heat
* Are there questions on homework 1?
* Did anyone go to the Festival for the Arts events?
* Wednesday at 4:15
  * Professor Jepsen on Polygonal thingys
  * Choed Khannabha on Computer-Assisted Room Draw
* Be prepared to complete the lab on your own.
* Read "Numeric Values in Scheme" for tomorrow.
* If you're registered in Grinnell, don't forget to vote tomorrow

Topics:
* Symbolic values
* Lists
* Lab: Symbols and Lists

LISP's original design: A language for exploring thought
* Theory was that thought is based on symbolic processing of groups of symbols
* LISP/Scheme make it easy to generate symbols and put them in groups

At least two ways of viewing a word or letter:
* As the name of something else
	(define dozen 12)
	(define ta "Reese")
* As the abstractions that we combine
	'cheese

We want to join those symbols into lists
* The empty list: null
  * Why start with an empty list?  You have to start somewhere.  Computers and computer scientists like to start as simply as possible.
* Add something to the front of a list
	(cons NEWTHING OLDLIST) => gives new list with NEWTHIGN at the front
  E.g., if former-tas were (cassie vivek rachel)
	(cons 'reese former-tas) => (reese cassie vivek rachel)

Scheme uses the same syntax for different things and you're expected to figure out the meaning from context.
* YOu can also build lists with list
	(list 'a 'b 'c) => (a b c)
* You can get the first element of a list with car
* You can get all but the first element of a list with cdr
* To get the second element of a list (say students), you take the car of the cdr
	(car (cdr students))
* Because taking the nth element of a list is a common operation, it is also built-in (but ends up defined in terms of car and cdr
	(list-ref LIST NUMBER)
	Takes the appropriate element of the list
	(list-ref (list 'a 'b 'c) 2) => c

How do you build up a list?  Do you name it at every stage?  Yes

(define initial-list null)
(define a (cons 'a null))
(define ba (cons 'b a))
(define aba (cons 'a ba))

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REFLECTION

* Groups work faster than individuals
* cons builds a new list; it does not modify the existing list
* (cons alpha beta gamma) is not the same as (cons 'alpha 'beta 'gamma)
* To get a copy of the previous command to edit, use Esc-P
* The dots you get in the middle of lists are Scheme's subtle way of saying "You screwed up"
* In order to create the list (d (e)) you need to get over a mental barrier.
  * This is a two element list
  * The first element is the symbol d
  * The second element is the list of the symbol e
  * TO make it (list 'd (list 'e))
  * How would you create it with cons?