表達式的求值(含括號)

#include <cstdio>
#include <iostream>
#include <list>
#include <queue>
#include <cstring>
#include <stack>
#include <algorithm>
using namespace std;



double  calc(double a,char op,double b){
	if(op == '+')
		return a + b;
	if(op == '-')
		return a - b;
	if(op == '*')
		return a * b;
	if(op == '/')
		return a / b;
}

int main(int argc, char const *argv[])
{
	char s[100];
	double x,y;
	int priv[300];
	priv['+'] = priv['-'] = 2;
	priv['*'] = priv['/'] = 1;
	priv['('] = 10;
	while(scanf("%s",s) != EOF){
		stack<double> num;
		stack<char> oper;
		int len = strlen(s);
		char last = 0;
		for(int i = 0;i < len;i++){
			if(isdigit(s[i])){
				num.push(atof(s + i));
				for(;i + 1 < len && (isdigit(s[i + 1]) || s[i + 1] == '.');i++);
			}else if(s[i] == '('){
				oper.push(s[i]);
			}else if(s[i] == ')'){
				while(oper.top() != '('){
					x = num.top();
					num.pop();
					y = num.top();
					num.pop();
					char op = oper.top();
					num.push(calc(x,op,y));
				}
				oper.pop();
			}else if(s[i] == '-' && (last == 0 || last == '(')){
					num.push(0.0);
					oper.push('-');
			}else if(priv[s[i]] > 0){
				while(oper.size() > 0 && priv[s[i]] >= priv[oper.top()]){
					y = num.top();
					num.pop();
					x = num.top();
					num.pop();
					char op = oper.top();
					oper.pop();
					num.push(calc(x,op,y));
				}
				oper.push(s[i]);
			}else
				continue;
			last = s[i];
		}
		while(oper.size() > 0){
			y = num.top();
			num.pop();
			x = num.top();
			num.pop();
			char op = oper.top();
			oper.pop();
			num.push(calc(x,op,y));
		}
		cout << num.top() << endl;
	}
	return 0;	
}