(1) switch后面括弧内的“表达式”， ANSI标准允许它为任何类型。

(2) 当表达式的值与某一个case后面的常量表达式的值相等时，就执行此case后面的语句，若所有的case中的常量表达式的值都没有与表达式的值匹配的，就执行default后面的语句。

(3) 每一个case的常量表达式的值必须互不相同，否则就会出现互相矛盾的现象(对表达式的同一个值，有两种或多种执行方案-(1) switch in brackets after the " expression" , ANSI standard allows for any type. (2) When the value of the expression of a case with a constant expression after the equal value, for the purpose of this case behind the statement that if all the constants in the case of the value of expression and the expression did not match the value of on the implementation of the statement after default. (3) Every case constant expression of value must be different from each other, otherwise there will be contradictory phenomenon (for the expression of the same value, there are two or more of the implementation of the program