C++简单计算器实现（四则运算加括号）

先说一下实现思路，使用两个栈，一个存放数字，一个存放运算符。这样比较运算符和取出运算的数字都非常方便。再者，在压入栈的过程中，把要压入的运算符和前一个运算符进行比较，如果优先级小于或者等于，就要将前一个运算符进行计算。

ps：支持带括号带负数四则运算。

#include <iostream>#include <string>#include <stack>using namespace std;stack<double> num_stk;stack<char> oper_stk;string experssion;double result;int number_flag;void pop_caculate();int cal_priority(char c);string::size_type get_number_size(string &expr,string::size_type currentPos);bool is_number(char c);void caculate(string expr);int main(){    cout << "input experssion!" << endl;    while (cin >> experssion) {        number_flag = 0;        caculate(experssion);    }}/* * get number length from currentPos of string expr*/string::size_type get_number_size(string &expr,string::size_type currentPos){    string numbers = "0123456789.";    return expr.find_first_not_of(numbers,currentPos) - currentPos;} /* * char c is number?*/bool is_number(char c){    string numbers = "0123456789-";    if (numbers.find(c) == string::npos) {        return false;    }    return true;}/* * the main caculate method*/void caculate(string expr){    string::size_type size = 0;    double num = 0;    for (string::size_type i=0;i<expr.size();) {        if (number_flag == 0 && is_number(expr[i])) {            num = stod(expr.substr(i));            if (expr[i] == '-') {            ++i;            num = -stod(expr.substr(i));            }            size = get_number_size(expr,i);        num_stk.push(num);        i += size;        cout << "num:" << num << endl;            number_flag = 1;        } else {        number_flag = 0;        if (oper_stk.empty()) {                oper_stk.push(expr[i]);        } else if (expr[i] == '(') {            oper_stk.push(expr[i]);        } else if (expr[i] == ')') {                while (oper_stk.top() != '(') {                    pop_caculate();            }            oper_stk.pop();        } else if (cal_priority(expr[i]) <= cal_priority(oper_stk.top())) {                pop_caculate();            oper_stk.push(expr[i]);        } else {                oper_stk.push(expr[i]);        }        ++i;        }    }    while (!oper_stk.empty()) {        pop_caculate();    }    result = num_stk.top();    cout << result << endl;}/* * pop and caculate the two stack to caculate*/void pop_caculate(){    double left=0,right=0,res=0;    char oper = oper_stk.top();    right = num_stk.top();    num_stk.pop();    left = num_stk.top();    num_stk.pop();    oper_stk.pop();    if (oper == '+') res = left + right;    else if (oper == '-') res = left - right;    else if (oper == '*') res = left * right;    else if (oper == '/') res = left / right;    num_stk.push(res);    cout << "caculate:" << left << oper << right << "=" << res << endl;}/*  *  caculate operator priority */int cal_priority(char c){    if (c == '(') return 0;    else if (c == '+') return 1;    else if (c == '-') return 2;    else if (c == '*') return 3;    else if (c == '/') return 4;    return 0;}