表达式求值

#include
#include
#include
#include
#include
#include

using namespace std;
const int MAXN = 1010;

int priority(char c) // 表达优先级的大小关系
{
if (c == ‘*’ || c == ‘/’) return 2;
if (c == ‘+’ || c == ‘-’) return 1;
return 0;
}

void cal(stack &my_op, stack &my_num) // 将数字栈与操作栈通过引用的形式实现调用与更改
{
double b = my_num.top();
my_num.pop();
double a = my_num.top();
my_num.pop();
switch(my_op.top())
{
case ‘+’:
my_num.push(a + b);
break;
case ‘-’:
my_num.push(a - b);
break;
case ‘*’:
my_num.push(a * b);
break;
case ‘/’:
my_num.push(a / b);
break;
}
my_op.pop();
}

int main()
{

 int t;
 scanf("%d", &t);
 while(t --)
 {
     char s[MAXN];
     scanf("%s", s);
     int len = strlen(s);
     stack <double> my_num;
     stack <char> my_op;

     for(int i = 0; i < len; ++ i)
     {
         if(isdigit(s[i]))      //如果s[i]是数字，不是符号 
         {
             double temp = atof(&s[i]);     //字符串转化为浮点数 
             while(isdigit(s[i]) || s[i] == '.') ++ i;
             my_num.push(temp);
             -- i;
         }
         else if(s[i] == '(')
         {
             my_op.push(s[i]);
         }
         else if(s[i] == ')')
         {
            while(my_op.top() != '(')
                 cal(my_op, my_num);
             my_op.pop();
         }
         else if(my_op.empty())
         {
             my_op.push(s[i]);
         }
         else if(priority(my_op.top()) < priority(s[i]))
         {
             my_op.push(s[i]);
			              }
         else if(priority(my_op.top()) >= priority(s[i]))
        {
             while(!my_op.empty() && priority(my_op.top()) >= priority(s[i])) // 比较的是优先级关系不是ASCⅡ，
                 cal(my_op, my_num);
             my_op.push(s[i]);
			              }
    }
     my_op.pop();
     printf("%.2lf\n", my_num.top());
    my_num.pop();
 }
 return 0;

}