def gen_cont(df):
    """
    Generate a data frame only including continuous variables.

    Parameters
    ----------
    df : pandas data frame
        whole data frame.

    Returns
    -------
    df_new: pandas data frame
        new data frame only including continuous variables.

    """
    feat_cont = ['loan_term', 'loan_amount',
                 'property_value','loan_to_value_ratio',
                 'income', 'debt_to_income_ratio',
                 'total_age', 'applicant_age',
                 'co_applicant_age'
                ]
    df_new = df[feat_cont]
    return df_new