How to get the two nearest values in spark scala DataFrame - scala

Question

Hi EveryOne I'm new in Spark scala. I want to find the nearest values by partition using spark scala. My input is something like this:
first row for example: value 1 is between 2 and 7 in the value2 columns
+--------+----------+----------+
|id |value1 |value2 |
+--------+----------+----------+
|1 |3 |1 |
|1 |3 |2 |
|1 |3 |7 |
|2 |4 |2 |
|2 |4 |3 |
|2 |4 |8 |
|3 |5 |3 |
|3 |5 |6 |
|3 |5 |7 |
|3 |5 |8 |
My output should like this:
+--------+----------+----------+
|id |value1 |value2 |
+--------+----------+----------+
|1 |3 |2 |
|1 |3 |7 |
|2 |4 |3 |
|2 |4 |8 |
|3 |5 |3 |
|3 |5 |6 |
Can someone guide me how to resolve this please.

Answer 1

Instead of providing a code answer as you appear to want to learn I've provided you pseudo code and references to allow you to find the answers for yourself.
Group the elements (select id, value1) (aggregate on value2
with collect_list) so you can collect all the value2 into an
array.
select (id, and (add(concat) value1 to the collect_list array)) Sorting the array .
find( array_position ) value1 in the array.
splice the array. retrieving value before and value after
the result of (array_position)
If the array is less than 3 elements do error handling
now the last value in the array and the first value in the array are your 'closest numbers'.

Answer 2

You will need window functions for this.
val window = Window
.partitionBy("id", "value1")
.orderBy(asc("value2"))
val result = df
.withColumn("prev", lag("value2").over(window))
.withColumn("next", lead("value2").over(window))
.withColumn("dist_prev", col("value2").minus(col("prev")))
.withColumn("dist_next", col("next").minus(col("value2")))
.withColumn("min", min(col("dist_prev")).over(window))
.filter(col("dist_prev") === col("min") || col("dist_next") === col("min"))
.drop("prev", "next", "dist_prev", "dist_next", "min")
I haven't tested it, so think about it more as an illustration of the concept than a working ready-to-use example.
Here is what's going on here:
First, create a window that describes your grouping rule: we want the rows grouped by the first two columns, and sorted by the third one within each group.
Next, add prev and next columns to the dataframe that contain the value of value2 column from previous and next row within the group respectively. (prev will be null for the first row in the group, and next will be null for the last row – that is ok).
Add dist_prev and dist_next to contain the distance between value2 and prev and next value respectively. (Note that dist_prev for each row will have the same value as dist_next for the previous row).
Find the minimum value for dist_prev within each group, and add it as min column (note, that the minimum value for dist_next is the same by construction, so we only need one column here).
Filter the rows, selecting those that have the minimum value in either dist_next or dist_prev. This finds the tightest pair unless there are multiple rows with the same distance from each other – this case was not accounted for in your question, so we don't know what kind of behavior you want in this case. This implementation will simply return all of these rows.
Finally, drop all extra columns that were added to the dataframe to return it to its original shape.