int option=0;
    do{
        cout << "Menu: " << endl;
        cout << "\t 1. Functions " << endl << "\t 2. Calculator"<< endl << "\t 3. Quit"<< endl;
        cin >> option;

        switch(option)
        {
            case 1:
            {
                system("cls");
                functions_menu();
                break;
            }
            case 2:
            {
                system("cls");
                functions_calculator();
                break;
            }
            case 3:
            {
                system("cls");
                cout<<endl<<"\t****Option 3 [Quit]"<<endl;
                break;
            }
            default:
            {
                cout<<"\tError, try again!"<<endl;
                system("cls");
                break;
            }
            break;
        }
        break;
    }while(option!=3);