if-else

6.13 if-else

SYNTAX

if (condition) {
     true_statements ;
}

OR

if (condition) true_statement ;

OR

if (condition) {
     true_statements ;
} else {
     false_statements ;
}

PROGRAM FLOW

  1. First, the boolean expression condition is evaluated.
  2. If condition evaluates to true, the true_statement(s) are executed.
  3. If condition evaluates to false and an else clause exists, the false_statement(s) are executed.
  4. Flow exits the if-else structure.

EXAMPLE

The following example tests to see if a user supplied function f comes within a tolerance of zero and prints an appropriate message. 
	  if( Math.abs(f(x)) <= tolerance) {
	   System.out.println("Zero found at x="+x);
	  }
	  else {
	   System.out.println("Failed to find a zero");
	  }

COMMON ERRORS

Make sure that if you want to test equality in the condition to use the boolean operator == and not the assignment operator =. Consider the following bad piece of code. 
	  double x=0;
          if(x=1) {
            System.out.println("x is 0");
          } else {
            System.out.println("x is not 0. x is "+x);
          }
The output will be
x is not 0. x is 1
since x is assigned to 1 in the condition.
David Maxwell, who is still writing this, would like to hear your comments and suggestions.