6.001 Staff notes for week of Feb 23, 1998

Administration:

-- usage survey of sections

-- students missing tutorials?

-- Quiz 1 is two weeks from Wednesday!  Grading is on Thursday


Lecture 6: Tuesday, 24 Feb 1998

     Lists and Aggregate Data

Pairs: CONS, CAR, CDR, PAIR?  names.

Common Patterns of Usage:

   Mixing and Matching Interchangeable Parts
    SEQUENCE as a critical conventional idea

   Lists as a convention for using pairs to represent SEQUENCES
    NIL is the name of the empty list
    CDR recursion: CDRing down a list, CONSing up an answer

   Abstracting common patterns
      map, filter, accumulate

   Permutations of a list of numbers

   Conventional Interfaces
     Representations of Standard Interfaces


Recitation: Wednesday, 25 Feb 1998

Reminder: Quiz 1 in two weeks, 5-7pm xor 7-9pm

  I would do lots of examples of lists and procedures for manipulating
them -- e.g. use of map, filter, accumulate
        e.g. reverse

  Be sure to include box-and-pointer diagrams.

  I/O representation of list structure

     reality is box-and-pointer diagrams

     I/O is character based, 
       so we use parens, spaces, and dots to notate trees.

     Printed representation of lists.  You probably should tell
     them about dotted pair notation as brighter students will
     notice that the story for lists is not sufficient.

     Output ambiguity:

         (let ((x (cons 1 2)))
           (cons x x))

       prints out the same as

         (cons (cons 1 2) (cons 1 2))

       but box-and-pointer diagram is different.  
       This does not matter until we have side-effects.

       procedures and other complicated objects


Lecture 7: Thursday, 26 Feb 1998

Starts with an introduction to the Communications Decency Act and
the issue of free speech vs protection of children on the Internet.
PICS as an example of part of the technology to solve the problem.
Then shows the fundamental data structures used by PICS -- which
really ARE Scheme list syntax:

(define test-label
  '(PICS-1.0  "http://www.rsac.org/ratingsv01.html"
	      l gen true comment "RSACi North America Server"
	      by "rodgerb@playboy.com"
	      for "http://www.playboy.com"
	      on "1996.04.04T08:15-0500"
	      r (n 4 s 3 v 0 l 4)))

(define test-service
  '((PICS-version 1.1)
    (rating-system "http://www.rsac.org/ratingsv01.html")
    (rating-service "http://www.rsac.org/")
    (name "The RSAC Ratings Service")
    (description "The Recreational Software Advisory Council rating
service.  Based on the work of Dr. Donald F. Roberts of Stanford
University, who has studied the effects of media on children for
nearly 20 years.")
    (default (label-only true))
    (category  (transmit-as "v")
	       (name "Violence")
	       (label (name "Conflict")
		      (description "Harmless conflict; some damage to objects")
		      (value 0))
	       (label (name "Fighting")
		      (description "Creatures injured or killed; damage to objects; fighting")
		      (value 1))
	       (label (name "Killing")
		      (description "Humans injured or killed with small amount of blood")
		      (value 2))
	       (label (name "Blood and Gore")
		      (description "Humans injured or killed; blood and gore")
		      (value 3))
	       (label (name "Wanton Violence")
		      (description "Wanton and gratuitous violence; torture; rape")
		      (value 4)))
    (category (transmit-as "s")
	      (name "Sex")
	      (label (name "None")
		     (description "Romance; no sex")
		     (value 0))
	      (label (name "Passionate kissing")
		     (description "Passionate kissing")
		     (value 1))
	      (label (name "Clothed sexual touching")
		     (description "Clothed sexual touching")
		     (value 2))
	      (label (name "Non-explicit sexual activity")
		     (description "Non-explicit sexual activity")
		     (value 3))
	      (label (name "Explicit sexual activity; sex crimes")
		     (description "Explicit sexual activity; sex crimes")
		     (value 4)))
    (category (transmit-as "n")
	      (name "Nudity")
	      (label (name "None")
		     (description "No nudity or revealing attire")
		     (value 0))
	      (label (name "Revealing Attire")
		     (description "Revealing attire")
		     (value 1))
	      (label (name "Partial Nudity")
		     (description "Partial nudit")
		     (value 2))
	      (label (name "Frontal Nudity")
		     (description "Non-sexual frontal nudity")
		     (value 3))
	      (label (name "Explicit")
		     (description "Provocative frontal nudity")
		     (value 4)))
    (category  (transmit-as "l")
	       (description "Language")
	       (label (name "Slang")
		      (description "Inoffensive slang; no profanity")
		      (value 0))
	       (label (name "Mild Expletives")
		      (description "Mild expletives")
		      (value 1))
	       (label (name "Expletives")
		      (description "Expletives; non-sexual anatomical references")
		      (value 2))
	       (label (name "Obscene Gestures")
		      (description "Strong, vulgar language; obscene gestures")
		      (value 3))
	       (label (name "Explicit")
		      (description "Crude or explicit sexual references")
		      (value 4)))))

Introduce symbols as the way to allow us to build these data
structures, emphasizing that the only interesting thing about the new
data type is that "two symbols that look the same, are the same" and
introducing the operation EQ? as the fundamental (and fast!) way to
tell if two things are exactly the same.

Lecture ends by writing selectors and a procedure for taking a PICS
label and a PICS service description (shown above) and expanding the
label by filling in the names and descriptions of the categories and
values included in the label.  This will build on the
map/filter/accumulate model from the previous lecture.

If time allows (unlikely) I'll introduce quotation of lists and the
notation '() for the empty list.

Recitation: Friday, 27 Feb 1998

Go over EQ? again.  Stress that all bets are off when neither arg is a
symbol -- the rule is that EQ? will say things are the same only if
there is no way whatsoever to tell them apart.  In particular, don't
use EQ? on numbers (use = instead) or strings (use string=?).

Review cons, list and append (maybe write code for append), and get them
to draw box-and-pointer diagrams for things like (introduce quoted
lists if it didn't happen in lecture).

  (cons 'a '(b))
  (cons '(a) '(b))
  (list 'a 'b)
  (list '(a) '(b))
  (append '(a) '(b))

Be prepared for much confusion concerning pairs and lists.  (Remember
there is always an inward pointing arrow...)  This is probably *not*
the right time to introduce the fact that 'A is the same as (QUOTE A)
-- unless you want to say it in passing and come back to it later when
we do the evaluator.

If you want, do the symbolic derivative example in the text.

   Note that quotation is a hard concept

If time, do more list and especially tree examples, including
box-and-pointer diagrams.

  Do some examples that stress that lists can be trees -- not
  just flat sequences.  You'll have an chance to do more of
  this later, but it wouldn't hurt to do a little of it now.

    reversing a list on all levels
    computing the fringe of a tree

  Another nice thing to do is the Representation of Sets
   example section 2.4.3