#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;}
}