Each operator has a precedence number assigned to it that determines the order in which

　　the operator and its operands are evaluated. The precedence numbers range from 1 （the

　　highest precedence， that is， first evaluated） to 7 （the lowest precedence， that is， last

　　evaluated）。

　　The following rules determine how expressions are evaluated：

　　● The highest precedence operators are evaluated first， then the second highest

　　precedence operators， and so on until the lowest precedence operators are evaluated.

　　● Operators of equal precedence are evaluated from left to right in the expression.

　　● Parentheses （ and ） can be used for grouping operators and operands and for

　　controlling the order in which the expressions are evaluated. For example， the

　　folloin exression ealuates to