Computer Science 15-100, Fall 2008
Class Notes:  More Conditionals and Loops

1. The switch Statement
2. Branching Statements
   1. break
   2. continue
   3. Labeled break
   4. Labeled continue
3. Infinite Loops
   1. while (true)
   2. for ( ; ; )
4. The do-while Statement
5. The for Statement
   1. Omitting conditions
   2. Extra conditions
   3. Scope
   4. enhanced for

More Conditionals and Loops

1. The switch Statement
   See syntax diagram on p. 232
    
   • Example:

     import java.util.*;
     class MyCode  {
       public static void main(String[] args) {
         Scanner scanner = new Scanner(System.in);
         System.out.print("Enter your choice (0-9): ");
         int choice = scanner.nextInt();
         switch (choice) {
           case 0: System.out.println("Case for choice 0.");
                   break;
           case 1: System.out.println("Case for choice 1.");
                   break;
           // multiple cases:
           case 5:
           case 8: System.out.println("Case for choice 5 or 8.");
                   // no break, fall-through
           case 9: System.out.println("Case for choice 9.");
                   break;
           default:
                   System.out.println("Case for default choice.");
         }
       }
     } 

   • Cases must be compile-time constants
     
     wrong:
     char c = s.charAt(0);
     char d = 'X';
     switch (c) {
        case  d :  System.out.println("X marks the spot!");
                 break;
     }
     
     right:
     char c = s.charAt(0);
     switch (c) {
        case 'X': System.out.println("X marks the spot!");
                  break;
     }
      
   • A mildly compelling example from Sun:
     See http://java.sun.com/docs/books/tutorial/java/nutsandbolts/switch.html:

     class SwitchDemo2 {
         public static void main(String[] args) {
     
             int month = 2;
             int year = 2000;
             int numDays = 0;
     
             switch (month) {
                 case 1:
                 case 3:
                 case 5:
                 case 7:
                 case 8:
                 case 10:
                 case 12:
                     numDays = 31;
                     break;
                 case 4:
                 case 6:
                 case 9:
                 case 11:
                     numDays = 30;
                     break;
                 case 2:
                     if ( ((year % 4 == 0) && !(year % 100 == 0))
                          || (year % 400 == 0) )
                         numDays = 29;
                     else
                         numDays = 28;
                     break;
                 default:
                     System.out.println("Invalid month.");
                     break;
             }
             System.out.println("Number of Days = " + numDays);
         }
     }

2. Branching Statements
    
   1. break

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          System.out.print("Enter a number to test for primality: ");
          int n = scanner.nextInt();
          boolean isPrime = (n > 1);
          for (int k=2; k<n; k++)
            if (n % k == 0) {
              System.out.println(n + " is divisble by " + k);
              isPrime = false;
              break;
            }
          System.out.println("isPrime = " + isPrime);
        }
      }

      Another Example:

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          int count = 0;
          int sum = 0;
          while (count < 3) {
            System.out.print("Enter # " + (count+1) + " of 3 (or 'q' to quit): ");
            String s = scanner.next();
            if (s.equals("q"))
              break;
            sum += Integer.parseInt(s);
            count++;
          }
          System.out.println("sum = " + sum);
        }
      }

   2. continue

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          int count = 0;
          int sum = 0;
          while (count < 3) {
            System.out.print("Enter # " + (count+1) + " of 3 (or 'q' to quit): ");
            if (!scanner.hasNextInt()) {
              String s = scanner.next();
              if (s.equals("q"))
                break;
              else {
                System.out.println("  Not an int: " + s + ".  Please try again.");
                continue;
              }
            }
            sum += scanner.nextInt();
            count++;
          }
          System.out.println("sum = " + sum);
        }
      }

   3. Labeled break

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          System.out.println("This code demonstrates a labeled break.  Note that");
          System.out.println("some people believe you should never use these!");
          System.out.println();
      
          String[] strings = { "abc", "de", "fghij", "klm", "n" };
      
          int i, j=0;
          char key = 'm';
          boolean foundKey = false;
      
          searchForKey:
          for (i=0; i<strings.length; i++)
            for (j=0; j<strings[i].length(); j++)
              if (strings[i].charAt(j) == key) {
                foundKey = true;
                break searchForKey;
              }
      
          if (foundKey == true)
            System.out.println("Found " + key + " at char " + j + " of string " + i);
          else
            System.out.println("Did not find " + key);
        }
      }

   4. Labeled continue

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          System.out.println("This code demonstrates a labeled continue.  Note that");
          System.out.println("some people believe you should never use these!");
          System.out.println();
      
          String string = "We all live on a yellow submarine!";
          String sub = "sub";
      
          int start;
          boolean foundIt = false;
      
          searchForSubstring:
          for (start=0; start<=string.length() - sub.length(); start++) {
            for (int i=0; i<sub.length(); i++) {
              if (sub.charAt(i) != string.charAt(start+i))
                continue searchForSubstring;
            }
            foundIt = true;
            break;
          }
      
          if (foundIt == true)
            System.out.println("Found substring starting at index " + start);
          else
            System.out.println("Did not find the substring");
        }
      }

3. Infinite Loops
    
   1. while (true)

      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          int sum = 0;
          int count = 0;
          while (true) {
            System.out.print("Enter # " + (count+1) + " (or 'q' to quit): ");
            String s = scanner.next();
            if (s.equals("q"))
              break;
            sum += Integer.parseInt(s);
            count++;
          }
          System.out.println("sum = " + sum);
        }
      }

   2. for ( ; ; )

      Style Alert:  Do not use "for ( ; ; )" -- use "while (true)"
      
      import java.util.*;
      class MyCode  {
        public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          int sum = 0;
          int count = 0;
          for (;;) {
            System.out.print("Enter # " + (count+1) + " (or 'q' to quit): ");
            String s = scanner.next();
            if (s.equals("q"))
              break;
            sum += Integer.parseInt(s);
            count++;
          }
          System.out.println("sum = " + sum);
        }
      }

4. The do-while Statement

   import java.util.*;
   class MyCode  {
     public static void main(String[] args) {
       System.out.println("Enter some #'s until their sum exceeds 10.");
       Scanner scanner = new Scanner(System.in);
       int sum = 0;
       do {
         System.out.print("sum = " + sum + ".  Next number: ");
         sum += scanner.nextInt();
       } while (sum <= 10);
       System.out.println("sum = " + sum);
     }
   }

5. The for Statement
    
   1. Omitting conditions
      
      Style Alert:  Usually a bad idea, though -- use a better-suited loop
      
      Ex1:
      int x = 1;
      for ( ; x < 3; x++)
        System.out.println(x);
      
      Ex2:
      int x = 1;
      for ( ; x < 3; ) {
        x++;
        System.out.println(x);
      }
      
      Ex3:
      int x = 1;
      for ( ; ; ) {
        x++;
        System.out.println(x);
      }
       
   2. Extra conditions
      
      Style Alert:  Often a bad idea, though -- put them before the loop or in the body
      int x, y;
      for (x=1,y=5; x < y; x++,y--)
        System.out.println(x);
       
   3. Scope

      Example:
      int x;
      for (x=0; x<2; x++)
        System.out.println(x);
      
      Roughly the same as:
      for (int x=0; x<2; x++)
        System.out.println(x);

      But:  variable's scope is limited to the for loop:
      int x;
      for (x=0; x<2; x++)
        System.out.println(x);
      System.out.println(x);
      
      Cannot be done this way:
      for (int x=0; x<2; x++)
        System.out.println(x);
      System.out.println(x);  // will not compile, x is not in scope here!

      Also:  cannot declare a local variable within the scope of another local variable of the same name
      int x;
      for (int x=0; x<2; x++)  // will not compile -- x is already defined!
        System.out.println(x);
      
      But you can reuse variables in subsequent for loops:
      for (int x=0; x<2; x++)
        System.out.println(x);
      for (int x=0; x<2; x++)
        System.out.println(x);
       

   4. enhanced for
      
      See previous notes on the Enhanced for Loop

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