How to get column names with all values null? - scala

Question

I don't have any ideas to get column names when it has null value
For example,
case class A(name: String, id: String, email: String, company: String)
val e1 = A("n1", null, "n1#c1.com", null)
val e2 = A("n2", null, "n2#c1.com", null)
val e3 = A("n3", null, "n3#c1.com", null)
val e4 = A("n4", null, "n4#c2.com", null)
val e5 = A("n5", null, "n5#c2.com", null)
val e6 = A("n6", null, "n6#c2.com", null)
val e7 = A("n7", null, "n7#c3.com", null)
val e8 = A("n8", null, "n8#c3.com", null)
val As = Seq(e1, e2, e3, e4, e5, e6, e7, e8)
val df = sc.parallelize(As).toDF
This code makes dataframe like this :
+----+----+---------+-------+
|name| id| email|company|
+----+----+---------+-------+
| n1|null|n1#c1.com| null|
| n2|null|n2#c1.com| null|
| n3|null|n3#c1.com| null|
| n4|null|n4#c2.com| null|
| n5|null|n5#c2.com| null|
| n6|null|n6#c2.com| null|
| n7|null|n7#c3.com| null|
| n8|null|n8#c3.com| null|
+----+----+---------+-------+
and I want to get column names all of their rows are null : id, company
I don't care the type of output. Array, String, RDD whatever

Answer 1

You can do a simple count on all your columns, then using the indices of the columns that return a count of 0, you subset df.columns:
import org.apache.spark.sql.functions.{count,col}
// Get column indices
val col_inds = df.select(df.columns.map(c => count(col(c)).alias(c)): _*)
.collect()(0)
.toSeq.zipWithIndex
.filter(_._1 == 0).map(_._2)
// Subset column names using the indices
col_inds.map(i => df.columns.apply(i))
//Seq[String] = ArrayBuffer(id, company)

Answer 2

An alternative solution could be as follows (but am afraid the performance might not be satisfactory).
val ids = Seq(
("1", null: String),
("1", null: String),
("10", null: String)
).toDF("id", "all_nulls")
scala> ids.show
+---+---------+
| id|all_nulls|
+---+---------+
| 1| null|
| 1| null|
| 10| null|
+---+---------+
val s = ids.columns.
map { c =>
(c, ids.select(c).dropDuplicates(c).na.drop.count) }. // <-- performance here!
collect { case (c, cnt) if cnt == 0 => c }
scala> s.foreach(println)
all_nulls