def gtval0(f, c1, c2):
    """For positive values in the first column of the dataframe
    that are greater than the second column, get the value in
    the first column, otherwise return zero. 

    Parameters
    ----------
    f : pandas.DataFrame
        Dataframe containing the two columns ``c1`` and ``c2``.
    c1 : str
        Name of the first column in the dataframe ``f``.
    c2 : str
        Name of the second column in the dataframe ``f``.

    Returns
    -------
    new_val : float
        A positive value or zero.

    """
    if f[c1] > f[c2] and f[c1] > 0:
        new_val = f[c1]
    else:
        new_val = 0
    return new_val
