How to get value of previous row in scala apache rdd[row]? - scala

Question

I need to get value from previous or next row while Im iterating through RDD[Row]
(10,1,string1)
(11,1,string2)
(21,1,string3)
(22,1,string4)
I need to sum strings for rows where difference between 1st value is not higher than 3. 2nd value is ID. So the result should be:
(1, string1string2)
(1, string3string4)
I tried use groupBy, reduce, partitioning but still I can't achieve what I want.
I'm trying to make something like this(I know it's not proper way):
rows.groupBy(row => {
row(1)
}).map(rowList => {
rowList.reduce((acc, next) => {
diff = next(0) - acc(0)
if(diff <= 3){
val strings = acc(2) + next(2)
(acc(1), strings)
}else{
//create new group to aggregatre strings
(acc(1), acc(2))
}
})
})
I wonder if my idea is proper to solve this problem.
Looking for help!

Answer 1

I think you can use sqlContext to Solve your problem by using lag function
Create RDD:
val rdd = sc.parallelize(List(
(10, 1, "string1"),
(11, 1, "string2"),
(21, 1, "string3"),
(22, 1, "string4"))
)
Create DataFrame:
val df = rdd.map(rec => (rec._1.toInt, rec._2.toInt, rec._3.toInt)).toDF("a", "b", "c")
Register your Dataframe:
df.registerTempTable("df")
Query the result:
val res = sqlContext.sql("""
SELECT CASE WHEN l < 3 THEN ROW_NUMBER() OVER (ORDER BY b) - 1
ELSE ROW_NUMBER() OVER (ORDER BY b)
END m, b, c
FROM (
SELECT b,
(a - CASE WHEN lag(a, 1) OVER (ORDER BY a) is not null
THEN lag(a, 1) OVER (ORDER BY a)
ELSE 0
END) l, c
FROM df) A
""")
Show the Results:
res.show
I Hope this will Help.