def pchange2(f, c1, c2):
    """Calculate the percentage change between two variables.

    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_column : pandas.Series (float)
        The array containing the new feature.

    """
    new_column = f[c1] / f[c2] - 1.0
    return new_column
