Computer Science 15-100 (Sections T & U), Spring 2008
Class Notes:  Ch 4+5:  Classes, Methods, Conditionals, and Loops (2 of 3)

Logistics

1. Schedule
   1. Hw5 due tomorrow
      Tonight's office hours to be announced in today's class
   2. Quiz 4 today
   3. Quiz 5 Tuesday (back to our regular schedule)
      1. Note:  Today's material will be included in quiz 5!
      2. May be on quiz 5:
         1. Chapters 1-3 (of course!) + Sections 4.1, 4.4, 5.1 (boolean expr's), 5.2 (if), 5.3, 5.5 (while), 5.8 (for), 5.9
      3. Will not be on quiz 5 (unless it appears in class notes):
         1. Section 4.2 (instance data), 4.3 (visibility/accessors), 4.5-4.9, 5.4 (switch), 5.6 (iterators), 5.7 (do), 5.10-5.12
   4. Bonus Lecture #1 on Tuesday
      1. Discuss times
2. Reading:
   1. L&L Chapter 4:  Writing Classes
           Section 4.4 today
   2. L&L Chapter 5:  Conditionals and Loops
           Sections 5.1, 5.2., 5.3, 5.5, and 5.8 today

Topic Outline:

Style:  Test Cases + Boundary Cases

Methods, Conditionals, and Loops (Oh my!)

1. Format of a Class with Static Methods:
   
   For now:  A class is a collection of static methods!
   We will add more to this definition very soon.
   Note that the order of the methods does not matter.
   
   class MyCode {
      public static void method1() {
         // ... //
      }
   
      public static void method2() {
         // ... //
      }
   
      // ... //
   
      public static void main(String[] args) {
         // ... //
      }
   }
    
2. Boolean expressions
   a)  Equality and inequality operators (==, !=)
   b)  Relational Operators  (<, <=, >, >=)
   c)  Logical Operators (!, &&, ||)
   d)  Truth Tables and Logical Operators
   e)  Short-Circuit Evaluation

   class MyCode {
     public static boolean t() {
       System.out.print("T");
       return true;
     }
     public static boolean f() {
       System.out.print("F");
       return false;
     }
     public static void main(String[] args){
       System.out.println("AND:");
       System.out.println(t() && t()); // TTtrue
       System.out.println(t() && f()); // TFfalse
       System.out.println(f() && t()); // Ffalse
       System.out.println(f() && f()); // Ffalse
   
       System.out.println("OR:");
       System.out.println(t() || t()); // Ttrue
       System.out.println(t() || f()); // Ttrue
       System.out.println(f() || t()); // FTtrue
       System.out.println(f() || f()); // FFfalse
     }
   }
   
   f)  Using short-circuit evaluation as a "guard"
       // guard against division by zero
       if ((d != 0) && (n/d < 20))
   
       // guard against index out of bounds
       if ((i >= 0) && (i < s.length()) && (s.charAt(i) == 'z')) ...

3. "if" Statements
   
   a)  The "if" statement

      if (1 < 2)
         System.out.println("yes");  // prints "yes"
   
      if (2 < 1)
         System.out.println("yes");  // does not get called!
   

   b)  Note:  A Block { ... } is a Statement!
   See If Statement syntax diagram on p. 218

      if (1 < 2) {
         System.out.println("yes");  // prints "yes"
      }
   
      // the block can include multiple statements:
      if (1 < 2) {
         System.out.println("yep");  // prints "yep"
         System.out.println("yep");  // prints "yep" again!
      }
   
      // the block can even include no statements (an empty block):
      if (1 < 2) {
         // do nothing    <- include this comment in any empty block
      }
   
   c)  Warning:  Indenting Doth Not Make A Block!
      if (2 < 1)
         System.out.println("yes");  // not called
         System.out.println("yep");  // prints "yep"
   
   d)  Warning:  A semicolon is also a Statement (the Empty Statement)
      if (2 < 1);
         System.out.println("yes");  // prints "yes"

         Safety Hint:  Do not use the empty statement.  Ever.
         If you must, use an empty block with a suitable comment like "do nothing" (see above).

   e)  The "if-else" statement

      if (2 < 1)
         System.out.println("yes");  // not called
      else
         System.out.println("no");   // prints "no" 

   f)  The "if-else-if-...-else" construct

      if (2 < 1)
         System.out.println("a");  // not called
      else if (3 < 1)
         System.out.println("b");  // not called 
      else if (4 == 4)
         System.out.println("c");  // prints "c" 
      else
         System.out.println("d");  // not called

   g)  Nested "if" statements

      if (2 < 1) {
         if (3 == 3)
           System.out.println("a");  // not called
         else
           System.out.println("b");  // not called
      }
      else {
         if (4 == 4)
           System.out.println("c");  // prints "c"
         else
           System.out.println("d");  // not called
      }

   h)  The "dangling-else" problem

      if (2 < 1)
         if (3 == 3)
           System.out.println("a");  // not called
      else
        System.out.println("b");  // not called
   

   Remedy:  use a block!

      if (2 < 1) {
         if (3 == 3)
           System.out.println("a");  // not called
      }
      else
        System.out.println("b");  // prints "b"

4. Comparing Data

   a)  Comparing Floats and Doubles

   b)  Comparing Characters

   c)  Comparing Objects (== versus .equals())

   d)  Comparing Strings with compareTo()
    

5. The "while" Statement
   See syntax diagram on p. 235
   
   a)  Simple Example

   class MyCode {
     public static void main(String[] args) {
       java.util.Scanner scanner = new java.util.Scanner(System.in);
       int sum = 0;
       int x;
   
       System.out.print("Enter a # (0 to exit): ");
       x = scanner.nextInt();
   
       while (x != 0) {
           sum += x;
           System.out.println("Running sum = " + sum);
   
           System.out.print("Enter a # (0 to exit): ");
           x = scanner.nextInt();        
       }
      
       System.out.println("Final sum = " + sum);
     }
   }

   b)  Using Infinite Loops and Break Statements to Avoid Duplicate Code
   
   Notice the duplicate code in the example above.  These lines occur twice:  (Do you see why?)
       System.out.print("Enter a # (0 to exit): ");
       x = scanner.nextInt();
   
   We fix this as such:

   class MyCode {
     public static void main(String[] args) {
       java.util.Scanner scanner = new java.util.Scanner(System.in);
       int sum = 0;
       int x;
   
       while (true) {
           System.out.print("Enter a # (0 to exit): ");
           x = scanner.nextInt();
   
           if (x == 0)
             break;
   
           sum += x;
           System.out.println("Running sum = " + sum);
       }
      
       System.out.println("Final sum = " + sum);
     }
   }

   c)  The continue statement

   class MyCode {
     public static void main(String[] args) {
       java.util.Scanner scanner = new java.util.Scanner(System.in);
       int sum = 0;
       int x;
   
       while (true) {
           System.out.print("Enter a # (0 to exit): ");
           x = scanner.nextInt();
   
           if (x == 0)
             break;
   
           else if (x < 0) {
             System.out.println("Ignoring that negative value!");
             continue;
           }
   
           sum += x;
           System.out.println("Running sum = " + sum);
       }
      
       System.out.println("Final sum = " + sum);
     }
   }

6. The "for" Statement
   See syntax diagram on p. 253
   
   Note:  We covered the "for" loop basics already, both in class and on the homework.  We'll cover it in more detail next lecture.  Quiz 5 will only cover "simple" for loops.
    
7. Nested Loops

   class MyCode {
     public static void main(String[] args) {
       int x, y;
       for (x=1; x<=4; x++) {
           for (y=1; y<=4; y++)
               System.out.print("(" + x + "," + y + ") ");
           System.out.println();
       }
     }
   }

   Output:

   (1,1) (1,2) (1,3) (1,4)
   (2,1) (2,2) (2,3) (2,4)
   (3,1) (3,2) (3,3) (3,4)
   (4,1) (4,2) (4,3) (4,4)