Figure 4.10  Function comp_tax
/*
 * Computes the tax due based on a tax table.
 * Pre : salary is defined.
 * Post: Returns the tax due for 0.0 <= salary <= 150,000.00;
 *       returns -1.0 if salary is outside the table range.
 */
double
comp_tax(double salary)
{
   double tax;

   if (salary < 0.0)
	tax = -1.0;
   else if (salary < 15000.00)	/* first range	*/
	tax = 0.15 * salary;
   else if (salary < 30000.00)	/* second range	*/
	tax = (salary - 15000.00) * 0.18 + 2250.00;
   else if (salary < 50000.00)	/* third range	*/
	tax = (salary - 30000.00) * 0.22 + 5400.00;
   else if (salary < 80000.00)	/* fourth range	*/
	tax = (salary - 50000.00) * 0.27 + 11000.00;
   else if (salary <= 150000.00)	/* fifth range	*/
	tax = (salary - 80000.00) * 0.33 + 21600.00;
   else
	tax = -1.0;

   return (tax);
}