def _add_cash_data(prices, interest_rate):
    """Add the historical prices of cash to the ones of the assets.

    Parameters
    ----------
    prices : DataFrame of shape (num_times, num_assets) and float
        The historical prices of the assets.

    interest_rate : float
        The interest rate of cash.

    Returns
    -------
    prices : DataFrame of shape (num_times, num_assets) and float
        The historical prices of the assets with ones of cash.
    """
    cash_list = [1.0]  # init cash price doesn't affect its return and risk
    for i in range(len(prices) - 1):
        cash_list.append(cash_list[-1] * (1.0 + interest_rate / len(prices)))
    prices['CASH'] = cash_list
    return prices