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

1. The "if" statement
   1. "if'
   2. Blocks
      1. A block is a statement
      2. Two ways to indent a block
      3. Indenting alone does not form blocks
   3. The empty statement
      1. A semicolon can be an empty statement (bad idea)
      2. A block can be an empty statement
   4. "if-else"
   5. "if-else-if-...-else"
   6. Nested "if" statements
   7. The "dangling-else" problem
2. The conditional (ternary) operator (?:)
   1. An Example
   2. A Better Example
3. Incorrect usage
   1. Negated Condition (with "else" clause)
   2. Empty "if" Clause
   3. Using "if" instead of &&
   4. Avoiding "else"
4. Practice problems

Conditionals

1. The "if" statement
    
   1. "if'
      
      class MyCode {
        public static void main(String[] args) {
          if (1 < 2)
            System.out.println("yes");
          if (5 < 4)
            System.out.println("really?");
        }
      }
       
   2. Blocks
       
      1. A block is a statement
         
         class MyCode {
           public static void main(String[] args) {
             if (1 < 2) {
               System.out.println("yes");
               System.out.println("yep");
               System.out.println("sure!");
             }
             if (5 < 4) {
               System.out.println("really?");
               System.out.println("hardly!");
             }
           }
         }
          
      2. Two ways to indent a block
         
         class MyCode {
           public static void main(String[] args) {
             if (1 < 2) { // open brace on same line as test
               System.out.println("yes");
               System.out.println("yep");
             }
           }
         }
         
         ... or ...
         
         class MyCode {
           public static void main(String[] args) {
             if (1 < 2)
             { // open brace on new line after test
               System.out.println("yes");
               System.out.println("yep");
             }
           }
         }
          
      3. Indenting alone does not form blocks
         
         class MyCode {
           public static void main(String[] args) {
             if (5 < 4)
               System.out.println("really?");
               System.out.println("hardly!");
               System.out.println("surprising!");
           }
         }
          
   3. The empty statement
       
      1. A semicolon can be an empty statement (bad idea)
         
         class MyCode {
           public static void main(String[] args) {
             if (5 < 4);  // ERROR: Extra semicolon!
               System.out.println("really?");
           }
         }
          
      2. A block can be an empty statement
         
         class MyCode {
           public static void main(String[] args) {
             if (5 < 4) {
               // empty block (do nothing)
             }
             System.out.println("really?");
           }
         }
          
   4. "if-else"
      
      class MyCode {
        public static void main(String[] args) {
          if (5 < 4)
            System.out.println("really?");
          else
            System.out.println("not really!");
        }
      }
       
   5. "if-else-if-...-else"
      
      class MyCode {
        public static void main(String[] args) {
          if (5 < 4)
            System.out.println("really?");
          else if (4 < 3)
            System.out.println("curious!");
          else if (3 > 2)
            System.out.println("you bet!");
          else
            System.out.println("why not?");
        }
      }
      
      A Better Example:
      
      class MyCode {
        public static void main(String[] args) {
          int n = -3;
          if (n > 0)
            System.out.println("positive");
          else if (n == 0)
            System.out.println("zero");
          else
            System.out.println("negative");
        }
      }
       
   6. Nested "if" statements

      class MyCode {
        public static void main(String[] args) {
          if (2 < 1) {
            if (3 == 3)
              System.out.println("a");
            else
              System.out.println("b");
          }
          else {
            if (4 == 4)
              System.out.println("c");
            else
              System.out.println("d");
          }
        }
      }

   7. The "dangling-else" problem

      class MyCode {
        public static void main(String[] args) {
          if (2 < 1)
            if (3 == 3)
              System.out.println("a");
          else  // dangling else!
           System.out.println("b");
        }
      }

      Remedy:

      class MyCode {
        public static void main(String[] args) {
          if (2 < 1) {
            if (3 == 3)
              System.out.println("a");
          }
          else
           System.out.println("b");
        }
      }

2. The conditional (ternary) operator (?:)
    
   1. An Example

      class MyCode {
        public static void main(String[] args) {
          System.out.println((1 < 2) ? "yes" : "no");
          int x = ((3 > 4) ? 5 : 6);
          System.out.println(x);
        }
      }

   2. A Better Example

      class MyCode {
        public static void main(String[] args) {
          int p = 5;
          System.out.println("I saw " + p + " " + ((p == 1) ? "person" : "people"));
        }
      }

      Equivalent to:

      class MyCode {
        public static void main(String[] args) {
          int p = 5;
          System.out.print("I saw " + p + " ");
          if (p == 1)
             System.out.println("person");
          else
             System.out.println("people");
        }
      }

3. Incorrect usage
    
   1. Negated Condition (with "else" clause)
       

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     boolean b = true;     if (!b)       System.out.println("no");     else       System.out.println("yes");   } }	class MyCode {   public static void main(String[] args) {     boolean b = true;     if (b)       System.out.println("yes");     else       System.out.println("no");   } }

       

   2. Empty "if" Clause
       

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     boolean b = false;     if (b) {        // do nothing     }     else       System.out.println("no");   } }	class MyCode {   public static void main(String[] args) {     boolean b = false;     if (!b)       System.out.println("no");   } }

       

   3. Using "if" instead of &&
       

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     boolean b1 = true;     boolean b2 = true;     if (b1)       if (b2)         System.out.println("Both!");   } }	class MyCode {   public static void main(String[] args) {     boolean b1 = true;     boolean b2 = true;     if (b1 && b2)       System.out.println("Both!");   } }

       

   4. Avoiding "else"
       

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     boolean b = true;     if (b)       System.out.println("yes");     if (!b)       System.out.println("no");   } }	class MyCode {   public static void main(String[] args) {     boolean b = true;     if (b)       System.out.println("yes");     else       System.out.println("no");   } }

      Another Example:

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     int x = 10;     if (x < 5)       System.out.println("small");     if ((x >= 5) && (x < 10))       System.out.println("medium");     if ((x >= 10) && (x < 15))       System.out.println("large");     if (x >= 15)       System.out.println("extra large");   } }	class MyCode {   public static void main(String[] args) {     int x = 10;     if (x < 5)       System.out.println("small");     else if (x < 10)       System.out.println("medium");     else if (x < 15)       System.out.println("large");     else       System.out.println("extra large");   } }

      Yet Another Example:

      Wrong	Right
      class MyCode {   public static void main(String[] args) {     char c = 'a';     if ((c >= 'A') && (c <= 'Z'))       System.out.println("Uppercase!");     if ((c >= 'a') && (c <= 'z'))       System.out.println("lowercase!");     if ((c < 'A') ||         ((c > 'Z') && (c < 'a')) ||         (c > 'z'))       System.out.println("not a letter!");   } }	class MyCode {   public static void main(String[] args) {     char c = 'a';     if ((c >= 'A') && (c <= 'Z'))       System.out.println("Uppercase!");     else if ((c >= 'a') && (c <= 'z'))       System.out.println("lowercase!");     else       System.out.println("not a letter!");   } }

       

4. Practice problems
   
   Write the following methods:
   1. isEven, isOdd, isPositive, ...
   2. signum (replaces Math.signum)
   3. etc...

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