HISTOGRAM.md

Histogram

For a table df like

user	score
Alice	101
Alice	221
Alice	211
Alice	176
Bob	276
Bob	232
Bon	258
Charlie	221

you can compute the histogram for each user

user	≤100	≤200	>200
Alice	0	2	2
Bob	0	0	3
Charlie	0	0	1

as follows:

df.withColumn("≤100", when($"score" <= 100, 1).otherwise(0))
  .withColumn("≤200", when($"score" > 100 && $"score" <= 200, 1).otherwise(0))
  .withColumn(">200", when($"score" > 200, 1).otherwise(0))
  .groupBy($"user")
  .agg(
    sum($"≤100").as("≤100"),
    sum($"≤200").as("≤200"),
    sum($">200").as(">200")
  )
  .orderBy($"user")

Equivalent to that query is:

import uk.co.gresearch.spark._

df.histogram(Seq(100, 200), $"score", $"user").orderBy($"user")

The first argument is a sequence of thresholds, the second argument provides the value column. The subsequent arguments refer to the aggregation columns (groupBy). Only aggregation columns will be in the result DataFrame.

In Java, call:

import uk.co.gresearch.spark.Histogram;

Histogram.of(df, Arrays.asList(100, 200), new Column("score")), new Column("user")).orderBy($"user")

In Python, call:

import gresearch.spark

df.histogram([100, 200], 'user').orderBy('user')