def mval(f, c):
    """Get the negative value, otherwise zero.

    Parameters
    ----------
    f : pandas.DataFrame
        Dataframe containing the column ``c``.
    c : str
        Name of the column in the dataframe ``f``.

    Returns
    -------
    new_val : float
        Negative value or zero.

    """
    new_val = -f[c] if f[c] < 0 else 0
    return new_val
