Extract a column value and assign it to another column as an array in Spark dataframe

问题

I have a Spark Dataframe with the below columns.

C1 | C2 | C3| C4
1  | 2  | 3 | S1
2  | 3  | 3 | S2
4  | 5  | 3 | S2

I want to generate another column C5 by taking distinct values from column C4 like C5

[S1,S2]
[S1,S2]
[S1,S2]

Can somebody help me how to achieve this in Spark data frame using Scala?

回答1:

You might want to collect the distinct items from column 4 and put them in a List firstly, and then use withColumn to create a new column C5 by creating a udf that always return a constant list:

val uniqueVal = df.select("C4").distinct().map(x => x.getAs[String](0)).collect.toList    
def myfun: String => List[String] = _ => uniqueVal 
def myfun_udf = udf(myfun)

df.withColumn("C5", myfun_udf(col("C4"))).show

+---+---+---+---+--------+
| C1| C2| C3| C4|      C5|
+---+---+---+---+--------+
|  1|  2|  3| S1|[S2, S1]|
|  2|  3|  3| S2|[S2, S1]|
|  4|  5|  3| S2|[S2, S1]|
+---+---+---+---+--------+

来源：https://stackoverflow.com/questions/41294414/extract-a-column-value-and-assign-it-to-another-column-as-an-array-in-spark-data