C++四则运算计算器

#pragma once
#include"List.h"
using std::string;
//字符串分割
class Arithmetic{
private:
    static List<string> segement_string(const string& expression) {
        List<string> res;
        string temp_num;
        for (int j = 0; j < expression.size(); j++) {
            char temp = expression[j];
            if (temp == '(' || temp == ')' ||
                temp == '+' || temp == '-' ||
                temp == '*' || temp == '/') {//如果是运算或者是括号就检查前面是否有数字，如果有就先压入数字，清空，最后压入当前的符号
                if (!temp_num.empty()) {
                    res.push_back(temp_num);
                    temp_num.clear();
                }
                res.push_back(string(1, temp));
            }
            else {//如果是数字，压入temp_num
                temp_num.push_back(temp);
            }
        }
        if (!temp_num.empty()) {//如果结尾不是符号，就要考虑还有没有数字没有压入res
            res.push_back(temp_num);
            return res;
        }
    }
    //比较运算符的优先级，a是栈外运算符，b是栈外运算符
    static bool isprior(const char& a, const char& b) {
        if (b == '(')
            if (a != ')') return true;
        if (b == '+' || b == '-') {
            if (a == '(' || a == '*' || a == '/') return true;
        }
        if (b == '*' || b == '/') {
            if (a == '(') return true;
        }
        return false;
    }
    //将中缀表达式改为后缀表达式
    static List<string> infix2postfix(const List<string>& infix) {
        List<string> postfix;
        List<string> temp;
        List<string>::const_iterator iter;
        for (iter = infix.begin(); iter != infix.end(); iter++) {
            if (*iter == "(" || *iter == ")" ||//如果是符号的话，在temp为空或优先级大于temp栈顶的优先级时，对temp进行入栈操作，否则出栈
                *iter == "+" || *iter == "-" ||
                *iter == "*" || *iter == "/") {
                while (!(temp.empty() || isprior((*iter)[0], temp.back()[0]))) {
                    if (temp.back() == "(") {//"("是不进入post，并且能让"("出栈的只能是")"，一个")"只能让一个"("出栈，所以需要break
                        temp.pop_back();
                        break;
                    }
                    else {
                        postfix.push_back(temp.back());
                        temp.pop_back();
                    }
                }
                if (*iter != ")")//")"是不进入post的
                    temp.push_back(*iter);
            }
            else//是数值的话直接压入post
                postfix.push_back(*iter);
        }
        while (!temp.empty()) {//最后将栈中的符号都压入post
            postfix.push_back(temp.back());
            temp.pop_back();
        }
        return postfix;
    }
    //将字符串转为double类型的量
    static double string2num(const string& str) {
        double res = 0;
        double isInt = 0;
        for (int i = 0; i < str.size(); i++) {
            unsigned int temp;
            if (str[i] != '.') {
                temp = str[i] - '0';
                if (isInt == 0) {
                    res = res * 10 + temp;
                }
                else {
                    res += temp * isInt;
                    isInt *= 0.1;
                }
            }
            else
                isInt = 0.1;
        }
        return res;
    }
    //后缀表达式的计算
    static double postfixCompute(const List<string>& postfix_expression) {
        double temp1;
        double temp2;
        List<double> temp;
        List<string>::const_iterator iter;
        for (iter = postfix_expression.begin(); iter != postfix_expression.end(); iter++) {
            if (*iter == "+") {
                temp2 = temp.back();
                temp.pop_back();
                temp1 = temp.back();
                temp.pop_back();
                temp.push_back(temp1 + temp2);
            }
            else if (*iter == "-") {
                temp2 = temp.back();
                temp.pop_back();
                temp1 = temp.back();
                temp.pop_back();
                temp.push_back(temp1 - temp2);
            }
            else if (*iter == "*") {
                temp2 = temp.back();
                temp.pop_back();
                temp1 = temp.back();
                temp.pop_back();
                temp.push_back(temp1 * temp2);
            }
            else if (*iter == "/") {
                temp2 = temp.back();
                temp.pop_back();
                temp1 = temp.back();
                temp.pop_back();
                temp.push_back(temp1 / temp2);
            }
            else
                temp.push_back(string2num(*iter));
        }
        return temp.back();
    }
public:
    //计算四则运算
    static double arithmetic(const string& expression) {
        List<string> segments = segement_string(expression);
        List<string> postfix_expression = infix2postfix(segments);
        return postfixCompute(postfix_expression);
    }
};