Object Oriented Programming C++ Simple Calculator

This is a beginner C++ OOP program.

#include <iostream>
#include <limits>
#include <math.h>  
#include <cmath> // for log function
using namespace std;

class calculator{


public:
    double add(double a, double b){
        return a+b;
    }

    double subtract(double a, double b){
        return a-b;
    }

    double multiply(double a, double b){
        if(a==0 && b==0){
            cout<<"Error! Invalid operation!"<<endl;
            return std::numeric_limits<double>::quiet_NaN();
        }
        return a*b;
    }

    double divide(double a, double b){
        if(b==0)
        {
            cout<<"Error! Cannot divide by 0!"<<endl;
            return std::numeric_limits<double>::quiet_NaN();
        }
        return a/b;
    }

    double power(double a, double b){
        return pow(a,b);
    }

    double loggy(double a) { //can't be 0 or less
        return log(a);
    }

    double factorial(double a){
        if(a==1 || a==0) 
            return 1;
        else
            return a*factorial(a-1);
    }

};

int main()
{
    calculator k;
    double a,b;
    double rez;
    int f, i=1;

    while(i!=0){

cout << "Choose function you want to use " << endl;
cout << "1 - Addition " << endl;
cout << "2 - Subtraction " << endl;
cout << "3 - Multiplication " << endl;
cout << "4 - Division " << endl;
cout << "5 - a^b"<<endl;
cout << "6 - Compute the log"<<endl;
cout << "7 - Compute factorial"<<endl;
cout << "Input: " << endl;

cin >> f;
cout << endl;

switch(f){

    case 1: //Addition
        cout << "a= ";
        cin >> a;
        cout << "b= ";
        cin >> b;
        cout << a << " + " << b << " = "; 
        cout << k.add(a, b);
        break;          
    case 2: //Subtraction 
        cout << "a= ";
        cin >> a;
        cout << "b= ";
        cin >> b;
        cout << a << " - " << b << " = ";
        cout << k.subtract(a, b);
        break;              
    case 3: //Multiplication
        cout << "a= ";
        cin >> a;
        cout << "b= ";
        cin >> b;
        cout << a << " x " << b << " = ";
        cout << k.multiply(a, b);
        break;      
    case 4: //Division
        cout << "a= ";
        cin >> a;
        cout << "b= ";
        cin >> b;
        cout << a << " / " << b << " = ";
        cout << k.divide(a, b);
        break;
    case 5: //power
        cout << "a= ";
        cin >> a;
        cout << "b= ";
        cin >> b;
        cout<< a<<"^"<<b<<" = ";
        cout<< k.power(a,b);
        break;

    case 6:
        cout << "a= ";
        cin >> a;
        cout<<k.loggy(a);
        break;
    case 7:
        cout<< "a= ";
        cin >> a;
        cout<<a<<"! is: "<<k.factorial(a)<<endl;
        break;
    default:
        cout << "Invalid Input...";
        break;
}
  cout<<endl<<"Do you want to perform another operation? Press 1 for yes or 0 to stop."<<endl;
  cin>>i;

}

    return 0;
}