switch (a)
{
case 1:
    if(matrix[0][0] == '1')
    matrix[0][0] = player_1;

    else
    {
        cout << "Field is already in use. Try again" << endl;
        input(); //Calling function input to prompt user to key in value again and again
    }
    break;

case 2:
    if(matrix[0][1] == '2')
    matrix[0][1] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input(); //Calling function input to prompt user to key in value again and again
    }
    break;
case 3:
    if(matrix[0][2] == '3')
    matrix[0][2] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input(); //Calling function input to prompt user to key in value again and again
    }
    break;
case 4:
    if(matrix[1][0] == '4')
    matrix[1][0] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input(); //Calling function input to prompt user to key in value again and again
    }
    break;
case 5:
    if(matrix[1][1] == '5')
    matrix[1][1] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input();//Calling function input to prompt user to key in value again and again
    }
    break;
case 6:
    if(matrix[1][2] == '6')
    matrix[1][2] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input();//Calling function input to prompt user to key in value again and again
    }
    break;
case 7:
    if(matrix[2][0] == '7')
    matrix[2][0] = player_1;
    else {

        cout << "Field is already in use.Try again" << endl;
        input();//Calling function input to prompt user to key in value again and again
    }
    break;
case 8:
    if(matrix[2][1] == '8')
    matrix[2][1] = player_1;
    else {

        cout << "Field is already in use. Try again" << endl;
        input();//Calling function input to prompt user to key in value again and again
    }
    break;
case 9:
    if(matrix[2][2] == '9')
    matrix[2][2] = player_1;

    else {

        cout << "Field is already in use. Try again" << endl;
        input();//Calling function input to prompt user to key in value again and again
    }
        break;
default:
    cout << "Number inputted doesn't exist. Please try again" << endl;
    input(); //Calling function input to prompt user to key in value again and again
}