表达式解析源代码

对于包含有函数sin、cos、tan、cot、sqrt、asin、acos、atan、log，和运算符+(加)、-(减)、*(乘)、/(除)与^(幂次)的字符串进行解析。可以解析出变量名，由用户赋值然后求表达式的值。

-Parsing the source code for the expression contains a function, sin, cos, tan, cot, sqrt, asin, acos, atan, log, and operators+ (plus),- (minus),* (multiply),/(except) and the ^ (power law) to parse the string. You can parse out the variable name, assignment by the user and then seek an expression value.