我是新来的。我已经把csv文件解析成一个Dataframe。我无法从数据框中选择某些列。下面是我的表的模式：root

|-- _c0: integer (nullable = true)
 |-- id: integer (nullable = true)
 |-- track_name: string (nullable = true)
 |-- size_bytes: long (nullable = true)
 |-- currency: string (nullable = true)
 |-- price: double (nullable = true)
 |-- rating_count_tot: integer (nullable = true)
 |-- rating_count_ver: integer (nullable = true)
 |-- user_rating: double (nullable = true)
 |-- user_rating_ver: double (nullable = true)
 |-- ver: string (nullable = true)
 |-- cont_rating: string (nullable = true)
 |-- prime_genre: string (nullable = true)
 |-- sup_devices.num: integer (nullable = true)
 |-- ipadSc_urls.num: integer (nullable = true)
 |-- lang.num: integer (nullable = true)
 |-- vpp_lic: integer (nullable = true)
 |-- Gigabytes: double (nullable = true)

虽然我可以选择一些列，但当我试图选择列“ipadscu url.num”（Dataframe中的一列）时，我得到了一个错误。