mega calculator

#include <stdlib.h>
#include <locale.h>
#include <iostream>
#include <string>
#include <cmath>
#include <iomanip>

using namespace std;

#define PRECISION 5

// todo think of removing unnecessary zeroes in the beginning of the num

bool validNumForDouble(string num) {
    return true;
}

enum NumSign {
    POSITIVE,
    NEGATIVE,
    NOT_CORRECTED
};

NumSign getNumSign(string num) {
    int countMinusInTheBeginning = 0;
    int countPliusInTheBeginning = 0;

    for(int i = 0; i < num.length(); i++) {
        if(num[i] == '-') countMinusInTheBeginning++;
        if(num[i] == '+') countPliusInTheBeginning++;
    }

    if(countMinusInTheBeginning > 1 || countPliusInTheBeginning > 1) return NOT_CORRECTED;

    if (num.length() == 0) {
        return NOT_CORRECTED;
    }

    if (num[0] == '-') {
        return NEGATIVE;
    } else if (num[0] == '+') {
        return POSITIVE;
    } else if (num[0] >= '0' && num[0] <= '9') {
        return POSITIVE;
    } else {
        return NOT_CORRECTED;
    }
}

bool numIsValid(string num, bool *isPositive, bool *isReal) {
    bool hasDigit = false;
    bool metPoints = false;

    NumSign numSign = getNumSign(num);
    if(numSign == NOT_CORRECTED) return false;

    string numWithoutSign = num;
    if(num[0] == '-' || num[0] == '+') numWithoutSign = num.substr(1, num.length());

    if(numWithoutSign[0] < '0' || numWithoutSign[0] > '9') return false;

    (*isPositive) = numSign == POSITIVE;

    for (char c : numWithoutSign) {
        if (c >= '0' && c <= '9') {
            hasDigit = true;
        } else if (c == '.' || c == ',') {
            if (metPoints) return false;
            metPoints = true;
        }
        else return false;
    }

    if (!hasDigit) {
        return false;
    }

    (*isReal) = metPoints;

    return true;
}

bool isPositive(string num) {
    if (num[0] == '-') {
        return false;
    } else if (num[0] == '+') {
        return true;
    } else if (num[0] >= '0' && num[0] <= '9') {
        return true;
    }

    return false;
}

void sum() {
    bool firstIsPositive = false;
    bool secondIsPositive = false;

    bool firstIsReal = false;
    bool secondIsReal = false;

    string firstNum;
    string secondNum;

    while(true) {
        cout << endl << "First num: ";
        cin >> firstNum;
        if(!numIsValid(firstNum, &firstIsPositive, &firstIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    while(true) {
        cout << endl << "Second num: ";
        cin >> secondNum;
        if(!numIsValid(secondNum, &secondIsPositive, &secondIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    cout.precision(PRECISION);
    cout << firstNum << " + " << secondNum << " = " << fixed <<(stold(firstNum) + stold(secondNum));

    cout << endl;
    system("pause");
}

void substract() {
    bool firstIsPositive = false;
    bool secondIsPositive = false;

    bool firstIsReal = false;
    bool secondIsReal = false;

    string firstNum;
    string secondNum;

    while(true) {
        cout << endl << "First num: ";
        cin >> firstNum;
        if(!numIsValid(firstNum, &firstIsPositive, &firstIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    while(true) {
        cout << endl << "Second num: ";
        cin >> secondNum;
        if(!numIsValid(secondNum, &secondIsPositive, &secondIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    cout.precision(PRECISION);
    cout << firstNum << " - " << secondNum << " = " << fixed << (stold(firstNum) - stold(secondNum));

    cout << endl;
    system("pause");
}
void multiply() {
    bool firstIsPositive = false;
    bool secondIsPositive = false;

    bool firstIsReal = false;
    bool secondIsReal = false;

    string firstNum;
    string secondNum;

    while(true) {
        cout << endl << "First num: ";
        cin >> firstNum;
        if(!numIsValid(firstNum, &firstIsPositive, &firstIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    while(true) {
        cout << endl << "Second num: ";
        cin >> secondNum;
        if(!numIsValid(secondNum, &secondIsPositive, &secondIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    cout.precision(PRECISION);
    cout << firstNum << " * " << secondNum << " = " << fixed << (stold(firstNum) * stold(secondNum));

    cout << endl;
    system("pause");
}

void divide() {
    bool firstIsPositive = false;
    bool secondIsPositive = false;

    bool firstIsReal = false;
    bool secondIsReal = false;

    string firstNum;
    string secondNum;

    while(true) {
        cout << endl << "First num: ";
        cin >> firstNum;
        if(!numIsValid(firstNum, &firstIsPositive, &firstIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    while(true) {
        cout << endl << "Second num: ";
        cin >> secondNum;
        if(!numIsValid(secondNum, &secondIsPositive, &secondIsReal)) {
            cout << "Num is not valid";
        } else {
            if(stold(secondNum) == 0.0) {
                cout << endl << "Divider cannot be equal to zero";
                continue;
            }
            break;
        }
    }

    int precision = -1;
    while(true) {
        cout << endl << "Enter precision: ";
        cin >> precision;
        if(precision >= 0) break;
    }

    cout.precision(precision);

    cout << endl << "precision is " << precision;
    cout << endl << firstNum << " / " << secondNum << " = " << fixed << (stold(firstNum) / stold(secondNum));

    cout << endl;
    system("pause");
 }

bool validExtent(string extent) {
    for (char c : extent) {
        if (!isdigit(c)) {
            return false;
        }
    }
    return true;
}

void pow() {
    bool firstIsPositive = false;
    bool secondIsPositive = false;

    bool firstIsReal = false;
    bool secondIsReal = false;

    string firstNum;
    string extent;

    while(true) {
        cout << endl << "Num: ";
        cin >> firstNum;
        if(!numIsValid(firstNum, &firstIsPositive, &firstIsReal)) {
            cout << "Num is not valid";
        } else break;
    }

    while(true) {
        cout << endl << "extent: ";
        cin >> extent;
        if(!validExtent(extent)) {
            cout << endl << "The extent is not valid";
        } else break;
    }

    cout.precision(PRECISION);

    cout << endl << firstNum << " ^ " << extent << " = " << fixed << (pow(stold(firstNum),stold(extent)));

    cout << endl;
    system("pause");
 }

int main() {

    int menuItem = -1;

    bool isExit = false;
    while(!isExit) {
        system("clear");
        cout
            << "\n 0 - Exit"
            << "\n 1 - Sum"
            << "\n 2 - Subsctract"
            << "\n 3 - Miltiply"
            << "\n 4 - Divide"
            << "\n 5 - Pow: ";

        cin >> menuItem;
        if(menuItem < 0 || menuItem > 5) continue;

        switch(menuItem) {
            case 0: isExit = true; break;
            case 1: sum(); break;
            case 2: substract(); break;
            case 3: multiply(); break;
            case 4: divide(); break;
            case 5: pow(); break;
            default: cout << "Menu item " << menuItem << " cannot be found";
        }
    }

    return 0;
}