根据 spark 中前一行的同一列的值计算值 [英] Calculate value based on value from same column of the previous row in spark

问题描述

我有一个问题，我必须使用公式计算一列，该公式使用前一行中完成的计算的值.

I have an issue where I have to calculate a column using a formula that uses the value from the calculation done in the previous row.

我无法使用 withColumn API 弄明白.

I am unable to figure it out using withColumn API.

我需要计算一个新列，使用公式:

I need to calculate a new column, using the formula:

MovingRate = MonthlyRate + (0.7 * MovingRatePrevious)

... 其中 MovingRatePrevious 是前一行的 MovingRate.

... where the MovingRatePrevious is the MovingRate of the prior row.

对于第 1 个月，我有该值，因此我不需要重新计算该值，但我需要该值才能计算后续行.我需要按类型分区.

For month 1, I have the value so I do not need to re-calculate that but I need that value to be able to calculate the subsequent rows. I need to partition by Type.

这是我的原始数据集:

MovingRate 列中的期望结果:

Desired results in MovingRate column:

推荐答案

尽管它可能与 Widow 函数有关(请参阅 @Leo C 的回答)，但我敢打赌，每个 Type 聚合一次的性能更高使用 groupBy.然后，分解 UDF 的结果以取回所有行:

Altough its possible to do with Widow Functions (See @Leo C's answer), I bet its more performant to aggregate once per Type using a groupBy. Then, explode the results of the UDF to get all rows back:

val df = Seq(
  (1, "blue", 0.4, Some(0.33)),
  (2, "blue", 0.3, None),
  (3, "blue", 0.7, None),
  (4, "blue", 0.9, None)
)
.toDF("Month", "Type", "MonthlyRate", "MovingRate")

// this udf produces an Seq of Tuple3 (Month, MonthlyRate, MovingRate)
val calcMovingRate = udf((startRate:Double,rates:Seq[Row]) => rates.tail
  .scanLeft((rates.head.getInt(0),startRate,startRate))((acc,curr) => (curr.getInt(0),curr.getDouble(1),acc._3+0.7*curr.getDouble(1)))
)

df
  .groupBy($"Type")
  .agg(
    first($"MovingRate",ignoreNulls=true).as("startRate"),
    collect_list(struct($"Month",$"MonthlyRate")).as("rates")
  )
  .select($"Type",explode(calcMovingRate($"startRate",$"rates")).as("movingRates"))
  .select($"Type",$"movingRates._1".as("Month"),$"movingRates._2".as("MonthlyRate"),$"movingRates._3".as("MovingRate"))
  .show()

给出:

+----+-----+-----------+------------------+
|Type|Month|MonthlyRate|        MovingRate|
+----+-----+-----------+------------------+
|blue|    1|       0.33|              0.33|
|blue|    2|        0.3|              0.54|
|blue|    3|        0.7|              1.03|
|blue|    4|        0.9|1.6600000000000001|
+----+-----+-----------+------------------+

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