def ROCR(df, n, price='Close'):
    """
    Rate of change ratio: (price/prevPrice)
    Returns: list of floats = jhta.ROCR(df, n, price='Close')
    """
    rocr_list = []
    for i in range(len(df[price])):
        if i + 1 < n:
            rocr = float('NaN')
        else:
            rocr = df[price][i] / df[price][i - n]
        rocr_list.append(rocr)
    return rocr_list
