Does Spark supports melt and dcast [duplicate]

Question

We use melt and dcast to convert data from wide->long and long->wide format. Refer http://seananderson.ca/2013/10/19/reshape.html for more details.

Either scala or SparkR is fine.

I've gone through this blog and scala functions and R API. I don't see functions which does similar job.

Is there any equivalent function in Spark? If not, is there any other way to do it in Spark?

Answer 1

Reshaping Data with Pivot in Spark gives support for reshaping with pivot. I understood melt is roughly the reverse of pivot also called unpivot. I'm relatively new to Spark. With my knowledge i tried to implement melt operation.

    def melt(df: DataFrame, columns: List[String]): DataFrame ={

    val restOfTheColumns =  df.columns.filterNot(columns.contains(_))
    val baseDF = df.select(columns.head, columns.tail: _*)
    val newStructure =StructType(baseDF.schema.fields ++ List(StructField("variable", StringType, true), StructField("value", StringType, true)))
    var newdf  = sqlContext.createDataFrame(sqlContext.sparkContext.emptyRDD[Row], newStructure)

    for(variableCol <- restOfTheColumns){
      val colValues = df.select(variableCol).map(r=> r(0).toString)
      val colRdd=baseDF.rdd.zip(colValues).map(tuple => Row.fromSeq(tuple._1.toSeq.:+(variableCol).:+(tuple._2.toString)))
      var colDF =sqlContext.createDataFrame(colRdd, newStructure)
      newdf =newdf.unionAll(colDF)
    }
    newdf
  }

It does the work. But i am not very sure about the efficiency.

+-----+---+---+----------+------+
| name|sex|age|    street|weight|
+-----+---+---+----------+------+
|Alice|  f| 34| somewhere|    70|
|  Bob|  m| 63|   nowhere|   -70|
|Alice|  f|612|nextstreet|    23|
|  Bob|  m|612|      moon|     8|
+-----+---+---+----------+------+

Can be used as

melt(df, List("name", "sex"))

The result is as below:

+-----+---+--------+----------+
| name|sex|variable|     value|
+-----+---+--------+----------+
|Alice|  f|     age|        34|
|  Bob|  m|     age|        63|
|Alice|  f|     age|       612|
|  Bob|  m|     age|       612|
|Alice|  f|  street| somewhere|
|  Bob|  m|  street|   nowhere|
|Alice|  f|  street|nextstreet|
|  Bob|  m|  street|      moon|
|Alice|  f|  weight|        70|
|  Bob|  m|  weight|       -70|
|Alice|  f|  weight|        23|
|  Bob|  m|  weight|         8|
+-----+---+--------+----------+

I hope it is useful and appreciate your comments if there is room for improvements.