C++ ПРОСТЕНЬКИЙ МАТЕМАТИЧЕСКИЙ ПАРСЕР

#include <iostream>
#include <fstream>
#include <stack>
#include <cmath>

using namespace std;

int priority(char op) {
    if (op == '+' || op == '-') return 1;
    if (op == '*' || op == '/') return 2;
    if (op == '^') return 3;
    return 0;
}

bool is_operator(char c) {
    return (c == '+' || c == '-' || c == '*' || c == '/' || c == '^');
}

double apply_operator(char op, double b, double a) {
    switch (op) {
        case '+': return a + b;
        case '-': return a - b;
        case '*': return a * b;
        case '/': return a / b;
        case '^': return pow(a, b);
        default: return 0;
    }
}

double calculate(string expression) {
    stack values;
    stack operators;

    for (int i = 0; i < expression.length(); i++) {
        if (expression[i] == ' ') continue;

        if (expression[i] == '(') {
            operators.push(expression[i]);
        } else if (expression[i] == ')') {
            while (operators.top() != '(') {
                char op = operators.top();
                operators.pop();

                double a = values.top();
                values.pop();

                double b = values.top();
                values.pop();

                values.push(apply_operator(op, b, a));
            }

            operators.pop();
        } else if (isdigit(expression[i])) {
            double value = 0;
            while (i < expression.length() && (isdigit(expression[i]) || expression[i] == '.')) {
                value = (value * 10) + (expression[i] - '0');
                i++;
            }
            i--;

            if (!operators.empty() && operators.top() == '-') {
                operators.pop();
                value = -value;
            }

            values.push(value);
        } else if (is_operator(expression[i])) {
            while (!operators.empty() && priority(operators.top()) >= priority(expression[i])) {
                char op = operators.top();
                operators.pop();

                double a = values.top();
                values.pop();

                double b = values.top();
                values.pop();

                values.push(apply_operator(op, b, a));
            }

            operators.push(expression[i]);
        } else {
            cout << "Invalid character in expression: " << expression[i] << endl;
            return 0;
        }
    }

    while (!operators.empty()) {
        char op = operators.top();
        operators.pop();

        double a = values.top();
        values.pop();

        double b = values.top();
        values.pop();

        values.push(apply_operator(op, b, a));
    }

    return values.top();
}

int main() {
    ifstream config_file("config.txt");
    if (!config_file.is_open()) {
        cout << "Error opening config file." << endl;
        return 0;
    }

    string expression;
    getline(config_file, expression);

    config_file.close();

    double result = calculate(expression);
    cout << "Result: " << result << endl;

    return 0;
}

Пример файла конфигурации "config.txt":

2 + 3 * (4 - 1)
-5 * (3 + 2)
10 / 2 - 3 * (7 - 1) / 2