#include <iostream>
#include <stack>
using namespace std;

stack<double> numbers; //存储操作数
stack<char> operations; //存储运算符

/* 设定运算符的优先级，其中以‘#‘作为operations的栈底元素（便于之后操作) */
int priority(char operate)
{
    switch (operate)
    {
    case ‘#‘:case ‘\n‘: return 0;
    case ‘(‘:case ‘)‘: return 1;
    case ‘+‘:case ‘-‘: return 2;
    case ‘*‘:case ‘/‘: return 3;
    }
}

void calculate(char);

int main()
{
    cout << "WELCOME CALCULATOR" << endl << endl
        << "input an algebrais expression to calculate or input ‘q‘ to quit" << endl << endl;
    char command = cin.get();
    while (command != ‘q‘) {
        calculate(command);
        command = cin.get();
    }
    return 0;
}

void calculate(char command)
{
    double num, leftNum, rightNum, result;

    switch (command)
    {
    /* 如果输入是数字，则将该double类型的数据存入栈中 */
    case ‘0‘:case‘1‘:case ‘2‘:case‘3‘:case ‘4‘:case‘5‘:case ‘6‘:case‘7‘:case ‘8‘:case‘9‘:
        cin.putback(command);
        cin >> num;
        numbers.push(num);
        break;

    case ‘(‘:case ‘)‘:case ‘+‘:case ‘-‘:case ‘*‘:case ‘/‘:case ‘\n‘:
        /* 初始化栈底元素为‘#‘*/
        if (operations.empty()) 
            operations.push(‘#‘);

        /* 若现在输入的运算符优先级较高或是输入‘（’，则应该存储现在的操作符，不执行之前的运算符 */
        if (priority(command) > priority(operations.top()) || command == ‘(‘)
            operations.push(command);

        /* 若之前输入的运算符优先级较高，则之前的运算符应该被执行 */
        else {
            while (priority(command) <= priority(operations.top()))
            {
                /* 当运算符完全实现后，露出栈底元素‘#’，输入“\n”则打印结果 */
                if (operations.top() == ‘#‘ && command == ‘\n‘) {
                    result = numbers.top();
                    cout << "the answer is " << result << endl << endl;
                    numbers.pop();
                    operations.pop();
                    break;
                }

                /* 当括号内运算符完全实现后，去除括号,读入下一个字符 */
                else if (operations.top() == ‘(‘&&command == ‘)‘) {
                    operations.pop();
                    cin >> command;
                }

                /* 若非上述两种情况，则完成前一个运算符，即operations.top() */
                else {
                    rightNum = numbers.top();
                    numbers.pop();
                    leftNum = numbers.top();
                    numbers.pop();

                    switch (operations.top())
                    {
                    case ‘+‘:
                        numbers.push(leftNum + rightNum);
                        operations.pop();
                        break;
                    case ‘-‘:
                        numbers.push(leftNum - rightNum);
                        operations.pop();
                        break;
                    case ‘*‘:
                        numbers.push(leftNum * rightNum);
                        operations.pop();
                        break;
                    case ‘/‘:
                        numbers.push(leftNum / rightNum);
                        operations.pop();
                        break;
                    }
                }
            }
            /* 完成前面高优先级的运算后，当前的运算符（除了‘\n’以外）变成最高优先级，所以应存储下来 */
            if(command!=‘\n‘)
                operations.push(command);
        }
        break;
    }
}