四則運算表達式C++實現

#include<iostream>
#include<stack>
#include<string>
using namespace std;
/*
*判斷操作數的優先級 
*/
int priority(int state,char a)
{
	int result;
	switch (a){
	case '+':
	case '-':
		result = 1;
		break;
	case '*':
	case '/':
		result = 2;
		break;
	case '(':
		if (state == 0)
			result = 3;
		else
			result = 0;
		break;
	case '#':
		result = 0;
		break;
	default:
		break;
	}
	return result;
}
/*
*計算 
*/
double calculate(char op, double op1, double op2)
{
	double result;
	switch (op){
	case '+':
		result = op1 + op2;
		break;
	case '-':
		result = op1 - op2;
		break;
	case '*':
		result = op1*op2;
		break;
	case '/':
		result = op1 / op2;
		break;
	default:
		break;
	}
	return result;
}
int main()
{
	string s;
	while (cin >> s){  //多次測試 
		stack<char> operation;  //存儲操作符的棧 
		stack<double> operand;  //存儲操作數的棧（浮點類型） 
		operation.push('#');
		string num;
		for (int i = 0; i < s.length(); i++){
			/*
			*將讀取到的string類型的操作數轉換爲double類型並存入操作數棧 
			*/
			if (isdigit(s[i])){
				while (isdigit(s[i]) || s[i] == '.'){
					num.push_back(s[i]);
					i++;
				}
				double a = atof(num.c_str());//string->double
				operand.push(a);
				num.clear();
				i--;
			}
			/*
			*將優先級高的先計算出值，保存 
			*/
			else if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/' || s[i] == '('){
				if (priority(0, s[i])>priority(1, operation.top()))
					operation.push(s[i]);
				else{
					while (priority(0, s[i]) <= priority(1, operation.top())){
						char temp = operation.top();
						operation.pop();
						double op2 = operand.top();
						operand.pop();
						double op1 = operand.top();
						operand.pop();
						operand.push(calculate(temp, op1, op2));
					}
					operation.push(s[i]);
				}
			}
			else if (s[i] == ')'){
				while (operation.top() != '('){
					char temp = operation.top();
					operation.pop();
					double op2 = operand.top();
					operand.pop();
					double op1 = operand.top();
					operand.pop();
					operand.push(calculate(temp, op1, op2));
				}
				operation.pop();
			}
			else{
				cout << "error!" << endl;
				return 0;
			}
		}
		//完成剩餘的計算，此時已經沒有括號（在上面已經計算完成） 
		while (operation.top() != '#'){
			char temp = operation.top();
			operation.pop();
			double op2 = operand.top();
			operand.pop();
			double op1 = operand.top();
			operand.pop();
			operand.push(calculate(temp, op1, op2));
		}
		cout << operand.top() << endl;
	}
	return 0;
}