CS 100, Summer 2001
Wednesday, 6/27
Lecture 3 
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+ Collect Exercise 2
Announcements: (2 mins)
+ Assignment 1 due Thursday, 6/28
+ reading: Savitch 3.2
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Topics:
+ operators
+ expressions
+ statements
+ conditional statements
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Miscellaneous:
- do check out the posted lecture notes - has examples used in class
  (.java and .txt links) 
- fixed-width fonts
- who is reading the newsgroup?
- incomplete surveys
- partners: no (not yet)
- CUid is CUid (not netid)
- submit your Friday notes for fame and recognition
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+ Operators: (Appendix 2 for full list)  
  - Arithmetic: + - / * % (and more)
    + remainder: %
    + x % y generates the remainder of x/y
    + ex) 
      int x1 = 10 % 5;   // -> 0  (10/5 = 2 with 0 as remainder)
      int x2 = 10 % 3;   // -> 1  (10/3 = 3 with 1 as remainder)
  - Comparison: < > <= >= == !=
  - Boolean: && || !
  - Ex) 
     boolean b1 = 3 > 4;     // false
     boolean b2 = 2.5 < 6.5; // true
     boolean b3 = 6 == 3;    // false
     boolean b4 = !false;    // true! 
  - 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
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+ Expressions:
  - 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
ASK YOURSELF:  Can you come up with a different result?
  - use () to force some expressions to evaluate first
  ex) (1 + 1); // () not needed
      2 * (2 - 4); 
      2 - (1 - 3);
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Recall:
characters -> tokens -> statements -> programs
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Statements: 
+ combine tokens to create statements
+ end statements with semicolon
+ going to a new line does NOT mean you have a new statement!
+ Java executes each statement top-down, left-to-right
  (within a statement, operator precedence determines direction of execution)
+ Block statement: put {} around a group of statements (useful for "if")
+ Empty statement: doesn't actually do anything (useful for loops)
  Example:    ; 
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+ Assignment:
  - changes the program's STATE
  - store value in a variable that's been declared
  - the value's type MUST match the variable's type
  - ex) int x;
        x = 1;
ASK YOURSELF: What would be an invalid assignment to x?
  - think x <- 1 because assign operator associates right-to-left
  - can say in English "x gets 1"
+ You should be able to map out the program state as it progresses: 
  Draw boxes to hold the values and update them with every statement.
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+ Swap example (see Swap.java)
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+ Style issues:
  - usually one statement per line
  - use whitespace to separate "paragraphs" of code
  - leave space between operators and operands (values operated upon)
  - use descriptive variable names
  - document your code (comments!)
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Conditional processing:
+ Motivation: sanity checks!
  1. Check type of input (SavitchIn takes care of this for you)
  2. Check valid values (this is your responsibility!)
     - Ex) give negative values to ChangeMaker
+ use if, if-else (and if-else if)
+ syntax: (You MUST use the { } for CS 100 purposes)

  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
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Brief QUIZ (see Quiz.java)
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PICK UP graded Exercise 1