The if statement is a conditional statement.  It can be written
  with or without an else clause as follows:

       if ( expression ) statement
       if ( expression ) statement else statement

  In both cases, the expression is evaluated, and if it is not 0, the
  first statement is executed.  If the else clause is included and
  the expression is 0, the statement following else is executed
  instead.  In a series of if-else clauses, the else matches the most
  recent else-less if.