Decision Constructs

                    CHAPTER 6: DECISION CONSTRUCTS


   Truth In Perl

     - Expression evaluated in a scalar context

     - A numeric scalar is true if it is not 0, else it is false

     - A string scalar is true if it is not the null string or the 
       string "0", else it is false

     - Ex.

         0                         # False
         14 - 2                    # True
         ""                        # False
         "0"                       # False
         "00"                      # True! (Pathological Case)
         "0.0"                     # True! (Pathological Case)


   If

     - if (test_expr)
       {
         statement(s);     # Executed if test_expr is true
       }

     - The {} are REQUIRED!

     - Syntax is really:  if (test_expr) statement_block

     - The () around the test_expr are also required!

     - A compound statement

     - Ex.

         if ($age < 21)
         {
           print ("Go drink somewhere else!\n");
         }


   If-Else

     - if (test_expr)
       {
         statement(s);     # Executed if test_expr is true
       }
       else
       {
         statement(s);     # Executed if test_expr is false
       }

     - A compound statement

     - Ex.

         if ($age < 21)
         {
           print ("Go drink somewhere else!\n");
         }
         else
         {
           print ("Belly up to the bar!\n");
         }


   If-Elsif-Else

     - if (test_expr_1)
       {
         statement(s);     # Executed if test_expr_1 is true
       }
       elsif (test_expr_2)
       {
         statement(s);     # Executed if test_expr_2 is true
       }
       else
       {
         statement(s);     # Executed if all test expressions
                           #   are false
       }

     - May have as many elsif branches as desired
 
     - Each test expression is evaluated in sequence.  If an expression 
       is true, the corresponding statement block is executed and all 
       remaining test expressions and statement blocks are skipped.  If 
       all test expressions are false, the else statement block is 
       executed.

     - A compound statement

     - Ex.

         if ($age < 21)
         {
           print ("Go drink somewhere else!\n");
         }
         elsif ($age > 55)
         {
           print ("Senior's discount is 50%!!\n");
         }
         else
         {
           print ("Belly up to the bar!\n");
         }
    

   Unless

     - unless (test_expr)
       {
         statement(s);     # Executed if test_expr is false
       }

     - A compound statement

     - Ex.

         unless ($age >= 21)
         {
           print ("Go drink somewhere else!\n");
         }


   Unless-Else

     - unless (test_expr)
       {
         statement(s);     # Executed if test_expr is false
       }
       else
       {
         statement(s);     # Executed if test_expr is true
       }

     - A compound statement

     - Ex.

         unless ($age >= 21)
         {
           print ("Go drink somewhere else!\n");
         }
         else
         {
           print ("Belly up to the bar!\n");
         }


   Special Notes

     - No unlessif

     - No switch or case


   Expression Modifiers

     - Can use if and unless as expression modifiers

     - Similar to the if and unless statement except that only an 
       expression can be modified, NOT a statement block


   If Modifier

     - exec_expr if test_expr;

     - Exec_expr executed if the test_expr is true

     - Equivalent to: if (test_expr)
                      {
                        exec_expr;
                      }

     - The () around the test expression are NOT required here!

     - A simple statement 

     - Ex.

         print ("Go drink somewhere else!\n") if ($age < 21);


   Unless Modifier

     - exec_expr unless test_expr;

     - Exec_expr executed if the test_expr is false

     - Equivalent to: unless (test_expr)
                      {
                        exec_expr;
                      }

     - The () around the test expression are NOT required here!

     - A simple statement 

     - Ex.

         print ("Belly up to the bar!\n") unless ($age < 21);

 
   Short-Circuit AND Operator (&&)

     - expression_1 && expression_2;

     - First expression_1 is evaluated.  If true, Perl then must evaluate 
       expression_2 to find the value of the entire expression.  If 
       expression_1 is false, the entire expression is false, and the 
       evaluation of expression_2 is skipped.

     - Similar to:

         if (expression_1)
         {
           expression_2;
         }

     - Value is value of last expression evaluated

     - A simple statement 

     - Ex.

         # Open "file2" only if the open for "file1" succeeds.

         open (FILE1, "file1") && open (FILE2, "file2");

         # Print a list value if there are still elements remaining.

         ($x = shift (@list)) && print $x;


   Short-Circuit OR Operator (||)

     - expression_1 || expression_2;

     - First expression_1 is evaluated.  If false, Perl then must evaluate 
       expression_2 to find the value of the entire expression.  If 
       expression_1 is true, the entire expression is true, and the 
       evaluation of expression_2 is skipped.

     - Similar to:

         unless (expression_1)
         {
           expression_2;
         }

     - Value is value of last expression evaluated

     - A simple statement 

     - Ex.

         # Open file ".btrc successfully or die!
     
         open (FILE, ".btrc") || die ("Can not open input file\n");


   Three-Operand Conditional (?:)

     - test_expr ? expr_1 : expr_2;

     - If test_expr is true, expr_1 is evaluated, else expr_2 is evaluated 

     - Similar to:

         if (test_expr)
         {
           expr_1;
         }
         else
         {
           expr_2;
         }

     - A simple statement 

     - Ex.

         $access == 1 ? print ("r") : print ("-");