Converting an Array[Double] Column into a string o

问题:

I hit a snag earlier, trying to do some transformations within Spark Dataframes.

Let's say I have a dataframe of schema :

root
|-- coordinates: array (nullable = true)
|    |-- element: double (containsNull = true)
|-- userid: string (nullable = true)
|-- pubuid: string (nullable = true)

I would like to get rid of the array(double) in coordinates, and instead get a DF with row that look like

"coordinates(0),coordinates(1)", userid, pubuid 
                   or something like 
 coordinates(0), coordinates(1), userid, pubuid . 

With Scala I could do

coordinates.mkString(",")

but in DataFrames coordinates resolves to a java.util.List.

So far I worked around the issue, by reading into an RDD, transforming then building a new DF. But I was wondering if there's a more elegant way to do that with Dataframes.

Thanks for your help.

回答1:

You can use an UDF:

import org.apache.spark.sql.functions.{udf, lit}

val mkString = udf((a: Seq[Double]) => a.mkString(", "))
df.withColumn("coordinates_string", mkString($"coordinates"))

or

val apply = udf((a: Seq[Double], i: Int) => a(i))
df.select(
  $"*", 
  apply($"coordinates", lit(0)).alias("x"),
  apply($"coordinates", lit(1)).alias("y")
)

Edit:

In the recent versions you can also use concat_ws:

import org.apache.spark.sql.functions.concat_ws

df.withColumn(
  "coordinates_string", concat_ws(",", $"coordinates")
)

or simple Column.apply:

df.select($"*", $"coordinates"(0).alias("x"), $"coordinates"(1).alias("y"))