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Q&A How to get conditional running cumulative sum based on current row and previous rows?

Use a collect_list collecting the values from all preceding rows up to the current row into a struct Then filter on that struct based on its value and the current row's threshold Use aggregate ...

posted 4mo ago by congusbongus‭

Answer
#1: Initial revision by user avatar congusbongus‭ · 2024-07-02T01:30:01Z (4 months ago)
- Use a [`collect_list`](https://www.sparkreference.com/reference/collect_list/) collecting the values from all preceding rows up to the current row into a struct
- Then [`filter`](https://spark.apache.org/docs/latest/api/sql/index.html#filter) on that struct based on its value and the current row's threshold
- Use [`aggregate`](https://spark.apache.org/docs/latest/api/sql/index.html#aggregate) to calculate the result based on adding the struct's `acc` field

Note that doing so may reorder the output so I added an order column.

```python
import pyspark.sql.functions as F

df = spark.createDataFrame([(1, 3, 1, 1), (2, 1, 2, 2), (3, 2, 3, 2)], ["order", "acc", "value", "threshold"])
display(
  df
  .withColumn("output", F.expr("""
  aggregate(
    filter(
      collect_list(struct(acc, value)) OVER (ORDER BY order ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW), 
      s -> s.value >= threshold
    ),
    0L, 
    (output, s) -> output + s.acc
  )
  """))
  .orderBy("order")
)
```

| order | acc | value | threshold | output |
|-------|-----|-------|-----------|--------|
| 1     | 3   | 1     | 1         | 3      |
| 2     | 1   | 2     | 2         | 1      |
| 3     | 2   | 3     | 2         | 3      |