Scala Spark DataFrame : dataFrame.select multiple

问题:

val columnName=Seq("col1","col2",....."coln");

Is there a way to do dataframe.select operation to get dataframe containing only the column names specified . I know I can do dataframe.select("col1","col2"...) but the columnNameis generated at runtime. I could do dataframe.select() repeatedly for each column name in a loop.Will it have any performance overheads?. Is there any other simpler way to accomplish this?

回答1:

val columnNames = Seq("col1","col2",....."coln")

// using the string column names:
val result = dataframe.select(columnNames.head, columnNames.tail: _*)

// or, equivalently, using Column objects:
val result = dataframe.select(columnNames.map(c => col(c)): _*)

回答2:

Since dataFrame.select() expects a sequence of columns and we have a sequence of strings, we need to convert our sequence to a List of cols and convert that list to the sequence. columnName.map(name => col(name)): _* gives a sequence of columns from a sequence of strings, and this can be passed as a parameter to select():

  val columnName = Seq("col1", "col2")
  val DFFiltered = DF.select(columnName.map(name => col(name)): _*)