CS100J, Fall 2001
Tues 9/11
Lecture 4
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Announcements:
+ new? See Overview on www.cs.cornell.edu/Courses/cs100j/2001fa/!
+ P1 handed back in Section -- no section #, goes to....
+ Readings: all of Chap 3 for 9/11,9/13,9/18,9/20
+ Future reading: Chap 4.1, Chap 5.1-5.4 (9/25,9/27)
+ office hours set: see Staff-->Staff Listing
+ P2 due 9/18: holidays? need letter from parents and/or rabii
  Try to do early submission on Mon to avoid getting behind, but will accept
  work on 9/20 in lecture
+ group sessions!
+ Macs poll, MATLAB poll/talk
+ New section: Mon 11:15, 12:20, 1:10
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Topics:
+ statements
  - empty
  - expression
  - method
  - declare
  - assignment
  - selection
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Statements:
+ combine tokens to create "sentences"
+ each "sentence" forms an instruction/command
+ "sentences" are called statements
+ end statements with semicolon, which is a punctuation token
+ starting a new line does NOT mean you have a new statement!
  (in MATLAB, a new line DOES start a new statement)
+ Java executes each statement top-down, left-to-right
  (within a statement, operator precedence determines direction of execution)
+ group "related" statements together with a BLOCK STATEMENT with {}
+ see Java in Nutshell pp43-44 for full list
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Statement: Empty
+ doesn't effectively do anything
 ex) ; (Java succeeds and moves to next statement)
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Statement: Expression
+ combine constants with operators
+ precedence: some operators "come before" others
+ associativity: operators "work" left to right or right to left
  ex) 1 + 1
      2 * 2 - 4 // technically means (2*2) - 4 because of precedence
      2 - 1 - 3 // technically means (2-1) - 3 due to associativity
+ use () to force some expressions to evaluate first
  ex) (1 + 1)   // () not needed
      2 * (2 - 4)
      2 - (1 - 3)
+ Java does not actually allow arithmetic expressions as individual statements
  ex) 1+1; // causes a syntax error
  (other languages, like MATLAB, do!)
+ beware of mixing types!
  ex) System.out.println(1/2) yields 0, not 0.5
+ mixing doubles and ints makes doubles
  ex) System.out.println(1.0/2) yields 0.5
+ use CAST to force different value
  syntax: (type) expression
  ex) System.out.println( (int) 9.8) yields 9 (an integer)
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Statement: Declaration:
+ want identifiers to represent other values
+ variables (identifiers) can hold values
+ values can be primitive types 
+ can also be references, but that deals with objects, so we'll skip this
  for now
+ Java is strongly typed, so must tell Java about variables before using
  syntax: variabletype var ;
  ex) char c;
      int value;
+ cannot redeclare a variable once it is declared inside { }
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Statement: Assignment
+ special kind of expression statement
+ store value in a variable that's been declared
+ cannot use variable until it's declared!
+ syntax: declaredvar = value
+ the value's type MUST match the variable's type
  ex) int x;
      x = 1;
+ now, x has the value 1
+ think x <- 1 because assign operator associates right-to-left
+ can say in English "x gets 1"
+ later in code, Java "knows" x means 1
+ cannot redeclare variable unless "elsewhere" in code
  (too hard to explain for now -- so, just don't redeclare!)
+ shortcut:
  o can combine syntax for shortcut:
  o syntax: type var = value
  o ex) int y = 7;
  o called INITIALIZING a variable (declaring and assigning initial value)
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Statement: Method Call
+ something1(something2) has the syntax name(expression)
+ think function --> Java will do whatever something1 instructs
  ex) System.out.println("Hello");
  ex) 
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Statement: Style issues
+ usually one statement per line
+ leave space between operators and operands (values operated upon)
+ use descriptive variable names
+ avoid hard coding values (using specific values that)
  o use named constants
  o variables initialized "early" in code and used instead of the constants
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Conditions:
+ execution of program goes in order of operators, starting from 1st statement
+ some say "top-down" execution
+ some statements can change FLOW OF CONTROL 
  (order of which statements execute)
+ sometimes need to make choices (example: comparison)
+ tree concept
+ use if, if-else (and if-else if)
+ syntax:

  if (c) // if c is true
    s;   // do s
         // otherwise, skip this statement

  if (c) // if c is true
    s1;  // do s1
  else   // otherwise
    s;   // do s2

+ other varieties:

  if (c1)
    s1
  else if (c2)
    s2

  if (c1)
    s1;
  else if (c2)
    s2;
  else
    s3;

+ can have as many else-ifs as you wish
+ indent statements, but entire body counts as SINGLE statement!
  (indenting does not make a new statement, just improves clarity)
+ can do multiple statements under each condition:
  - use statement block
  - { statements }
+ language elements:
  - if, else are keywords
  - conditions? must evaluate to true or false (Boolean)
    o relations: <  (less than)           > (greater than)
                 <= (less than or equal)  > (greater than or equal)
                 == (equal, but do NOT use =)
                 != (not equal)
    o logic:     && (and)  || (or)  ! (not)
    o values:    true  false
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