#define _USE_MATH_DEFINES

#include <iostream>
#include <cmath>

using namespace std;

double sphere_volume( double radius ) {
    return M_PI * 4 / 3 * std::pow( radius, 3 );
}

int main() {
    cout << "Welcome User" << endl;

    for( ;;) {
        char option;
        cout << "Do you have diameter or ratio? (d/r/q-exit): ";
        cin >> option;

        if( 'q' == option ) return 0;

        double radius;

        switch( option ) {
        case 'r':
            cout << "What is the radius?: ";
            cin >> radius;
            break;
        case 'd':
            {
                cout << "What is the diameter?: ";
                double diameter;
                cin >> diameter;
                radius = diameter / 2;
            }
            break;
        }

        cout << "The volume is: " << sphere_volume( radius ) << endl;
    }
}