问题描述
我有一个两列的 DataFrame,Int
类型的 ID
和 Vector
类型的 Vec
(org.apache.spark.mllib.linalg.Vector).
I have a DataFrame of two columns, ID
of type Int
and Vec
of type Vector
(org.apache.spark.mllib.linalg.Vector
).
DataFrame 如下所示:
The DataFrame looks like follow:
ID,Vec
1,[0,0,5]
1,[4,0,1]
1,[1,2,1]
2,[7,5,0]
2,[3,3,4]
3,[0,8,1]
3,[0,0,1]
3,[7,7,7]
....
我想做一个 groupBy($"ID")
然后通过对向量求和对每个组内的行应用聚合.
I would like to do a groupBy($"ID")
then apply an aggregation on the rows inside each group by summing the vectors.
上面例子的期望输出是:
The desired output of the above example would be:
ID,SumOfVectors
1,[5,2,7]
2,[10,8,4]
3,[7,15,9]
...
可用的聚合函数将不起作用,例如df.groupBy($"ID").agg(sum($"Vec")
将导致 ClassCastException.
The available aggregation functions will not work, e.g. df.groupBy($"ID").agg(sum($"Vec")
will lead to an ClassCastException.
如何实现自定义聚合函数,允许我对向量或数组求和或任何其他自定义操作?
How to implement a custom aggregation function that allows me to do the sum of vectors or arrays or any other custom operation?
推荐答案
Spark >= 3.0
您可以将 Summarizer
与 sum
import org.apache.spark.ml.stat.Summarizer
df
.groupBy($"id")
.agg(Summarizer.sum($"vec").alias("vec"))
火花
就我个人而言,我不会打扰 UDAF.不仅仅是冗长而且不是很快(Spark UDAF 与 ArrayType 作为 bufferSchema 性能问题)相反,我会简单地使用 reduceByKey
/foldByKey
:
Personally I wouldn't bother with UDAFs. There are more than verbose and not exactly fast (Spark UDAF with ArrayType as bufferSchema performance issues) Instead I would simply use reduceByKey
/ foldByKey
:
import org.apache.spark.sql.Row
import breeze.linalg.{DenseVector => BDV}
import org.apache.spark.ml.linalg.{Vector, Vectors}
def dv(values: Double*): Vector = Vectors.dense(values.toArray)
val df = spark.createDataFrame(Seq(
(1, dv(0,0,5)), (1, dv(4,0,1)), (1, dv(1,2,1)),
(2, dv(7,5,0)), (2, dv(3,3,4)),
(3, dv(0,8,1)), (3, dv(0,0,1)), (3, dv(7,7,7)))
).toDF("id", "vec")
val aggregated = df
.rdd
.map{ case Row(k: Int, v: Vector) => (k, BDV(v.toDense.values)) }
.foldByKey(BDV.zeros[Double](3))(_ += _)
.mapValues(v => Vectors.dense(v.toArray))
.toDF("id", "vec")
aggregated.show
// +---+--------------+
// | id| vec|
// +---+--------------+
// | 1| [5.0,2.0,7.0]|
// | 2|[10.0,8.0,4.0]|
// | 3|[7.0,15.0,9.0]|
// +---+--------------+
只是为了比较一个简单的"UDAF.所需的导入:
And just for comparison a "simple" UDAF. Required imports:
import org.apache.spark.sql.expressions.{MutableAggregationBuffer,
UserDefinedAggregateFunction}
import org.apache.spark.ml.linalg.{Vector, Vectors, SQLDataTypes}
import org.apache.spark.sql.types.{StructType, ArrayType, DoubleType}
import org.apache.spark.sql.Row
import scala.collection.mutable.WrappedArray
类定义:
class VectorSum (n: Int) extends UserDefinedAggregateFunction {
def inputSchema = new StructType().add("v", SQLDataTypes.VectorType)
def bufferSchema = new StructType().add("buff", ArrayType(DoubleType))
def dataType = SQLDataTypes.VectorType
def deterministic = true
def initialize(buffer: MutableAggregationBuffer) = {
buffer.update(0, Array.fill(n)(0.0))
}
def update(buffer: MutableAggregationBuffer, input: Row) = {
if (!input.isNullAt(0)) {
val buff = buffer.getAs[WrappedArray[Double]](0)
val v = input.getAs[Vector](0).toSparse
for (i <- v.indices) {
buff(i) += v(i)
}
buffer.update(0, buff)
}
}
def merge(buffer1: MutableAggregationBuffer, buffer2: Row) = {
val buff1 = buffer1.getAs[WrappedArray[Double]](0)
val buff2 = buffer2.getAs[WrappedArray[Double]](0)
for ((x, i) <- buff2.zipWithIndex) {
buff1(i) += x
}
buffer1.update(0, buff1)
}
def evaluate(buffer: Row) = Vectors.dense(
buffer.getAs[Seq[Double]](0).toArray)
}
还有一个用法示例:
df.groupBy($"id").agg(new VectorSum(3)($"vec") alias "vec").show
// +---+--------------+
// | id| vec|
// +---+--------------+
// | 1| [5.0,2.0,7.0]|
// | 2|[10.0,8.0,4.0]|
// | 3|[7.0,15.0,9.0]|
// +---+--------------+
另见:如何在 Spark SQL 中找到分组向量列的平均值?.
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