def create_gdp_csv(pair_df, time_df, initial):
    """
    Fills gdp dataframe missing values with previous values. Originally gdp data changes once
    a month, but dataframe must contain entries for every minute.

    Args:
        pair_df: Currency dataframe containing columns for time and gdp
        time_df: Dataframe specifying time range used for lstm training
        initial: Float representing the gdp of the month or quarter before 2018

    Returns:
        Dataframe with gdp column and time column containing data from 2018-2020 for every minute.
    """
    pair_df = time_df.merge(pair_df, how="left", on="Time")
    pair_df.iloc[0, pair_df.columns.get_loc("GDP")] = initial
    pair_df = pair_df.fillna(method="ffill")
    return pair_df