1/25 

NOTE: 2/01 -- fixed the rules for doing an assignment.
              steps 2 and 3 were in the wrong order.

Outline
+ reminder: registration, waiver, demo, P1
+ 1:25 sections way too full -- we will do something...
+ feedback mechanisms
+ building programs
    review					NEW
  + variables and assignment			tracing: memory model, tabular
  + sequencing, semicolon to supress output	comma
  + Quiz
  + (for-)loops					loop template
_______________________________________________________________________________

please ask questions in lecture!  

  i will give bonus points for asking good questions in lecture --
  at the end of lecture, please hand in a piece of paper with your name 
  and short paraphrases of your questions.

  note: if you have an advanced question, then i am interested in 
  answering it, but please post it to the newsgroup instead of
  asking in lecture.

lecture evaluations

  i plan to start having online evaluation form for each lecture.
  you can be anonymous or give your netid, in which case i will 
  give you a bonus point.
-------------------------------------------------------------------------------
principle 1: learn and use strategies to avoid pitfalls
+ ex: start early
+ ex: read all instructions
+ ex: pronounce assignment "=" as "gets", "becomes", or "is assigned", 
      NEVER "equals"

principle 2: repetitious code is an opportunity for improvement
+ ex: do not overparenthesize
+ ex: variables and abstraction help reduce redundancy
+ ex: loops help reduce redundancy

principle 3: indent substructures
+ ex: indent loop bodies
+ ex: indent conditionals
_______________________________________________________________________________

review: ways to build bigger programs out of smaller ones

  + building blocks: assignment, ... <-- seen already
  + sequential execution	     <-- seen already
  + iterative (repeated) execution   <-- seen already (for-loop)
  + conditional execution	     <-- new concept, to be done tue 1/30
_______________________________________________________________________________

review: variables and assignment, semicolon to suppress printing
new: tracing, comma

+ comma, semicolon:

>>  x = 5, y = x + 1, x = 2 * y, z = y, z = 6
>>  x = 5; y = x + 1; x = 2 * y; z = y; z = 6; disp(z)

memory model: trace execution
  + first assignment, declare and initialize variable
  + reference to non-existent variable: ERROR!
  + evaluate lhs <-- rhs
    + evaluate lhs to get location
    + evaluate rhs to get value                 <-- 2/01: swapped these
    + declare variable/location if necessary    <-- two lines into right order
    + put value into location
      + cross out old value, even if the same
      + write new value inside box or nearby

note: for now, easy to get location -- later, will require computation

peek/analogy:
+ the room Olin 155 versus the address "Olin 155"
+ the room is huge, cannot fit on a post-it
+ the address can easily fit on a post-it
+ finding location of "Olin 155" (versus Olin 255) takes computation
  -- need to search for correct room

tabular format: trace execution again
  + mostly ignore declaration
  + one row for each variable
  + one column for each assignment
    + even if value is not changed
    + even if multiple assignments are on the same line
    + only write in value assigned: leave everything else blank
      (easier for you to write and for all of us to read)
  + in a column, to find value for a blank entry, start in that column
    and scan right-to-left until hit a value

+ java note: java uses commas and semicolons differently
  + statements do not produce output by default
  + there are separate printing commands
  + each statement must end with a semicolon
  (pascal note: semicolon in java is statement terminator, not separator)
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Quiz E1

write your name, cuid number, "Quiz E1" at the top.

give pseudocode to perform the following:

   simultaneously x <-- y, y <-- z, z <-- x 

   ex:  if initially   x is 3, y is 5, z is 7,
   then the result has z is 3, x is 5, y is 7
   (but your code should work no matter what values x,y,z initially have)

   hint: suppose you had 3 cups
   + x holds juice
   + y holds milk
   + z holds coffee
   how to do you rearrange the contents of the cups using another cup tmp?

   hint: what if you swap 2 of them?

bonus

   let f(x) stand for x + 1/x.  use a loop in Matlab code to
   + silently compute y = f(3) + f(-7) + f(2) + f(8) + f(2.9) + f(1000000)
   + print the answer y 

   silently means don't display any computated values except the answer y.
_______________________________________________________________________________

solution 1: recall to swap a, b:  tmp <-- a; a <-- b; b <-- tmp;
    a) swap x, y; swap z, y
    b) swap x, z; swap y, x
    c) swap y, z; swap x, z

[2/07] remarks: a *temporary* or *auxiliary* or *helper* variable
($tmp$ in the code above) is necessary.  in pseudocode, you should use
$<--$ instead of $=$ to differentiate the difference from equality,
which is symmetric.

solution 2: tmp <-- x; x <-- y; y <-- z; z <-- tmp;

general loop template:		solution to bonus:

    start			>> y = 0;
    for each value		>> for x = [3 -7 2 8 2.9 1e6]
        process it		>>     y = y + x + 1/x;
				>> end
    finish			>> y

[2/07] remarks: the $for$-loop and accumulator $y$ were required for
credit.

trace 

question: what if wanted f(3) * f(-7) * ...?  

answer: start off y = 1;