if

 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.