How to divide the value of current row with the following one? - scala

Question

In Spark-Sql version 1.6, using DataFrames, is there a way to calculate, for a specific column, the fraction of dividing current row and the next one, for every row?
For example, if I have a table with one column, like so
Age
100
50
20
4
I'd like the following output
Franction
2
2.5
5
The last row is dropped because it has no "next row" to be added to.
Right now I am doing it by ranking the table and joining it with itself, where the rank is equals to rank+1.
Is there a better way to do this?
Can this be done with a Window function?

Answer 1

Window function should do only partial tricks. Other partial trick can be done by defining a udf function
def div = udf((age: Double, lag: Double) => lag/age)
First we need to find the lag using Window function and then pass that lag and age in udf function to find the div
import sqlContext.implicits._
import org.apache.spark.sql.functions._
val dataframe = Seq(
("A",100),
("A",50),
("A",20),
("A",4)
).toDF("person", "Age")
val windowSpec = Window.partitionBy("person").orderBy(col("Age").desc)
val newDF = dataframe.withColumn("lag", lag(dataframe("Age"), 1) over(windowSpec))
And finally cal the udf function
newDF.filter(newDF("lag").isNotNull).withColumn("div", div(newDF("Age"), newDF("lag"))).drop("Age", "lag").show
Final output would be
+------+---+
|person|div|
+------+---+
| A|2.0|
| A|2.5|
| A|5.0|
+------+---+
Edited
As #Jacek has suggested a better solution to use .na.drop instead of .filter(newDF("lag").isNotNull) and use / operator , so we don't even need to call the udf function
newDF.na.drop.withColumn("div", newDF("lag")/newDF("Age")).drop("Age", "lag").show