我正在尝试找到一种方法,来计算给定数据框的中位数。
val df = sc.parallelize(Seq(("a",1.0),("a",2.0),("a",3.0),("b",6.0), ("b", 8.0))).toDF("col1", "col2")
+----+----+
|col1|col2|
+----+----+
| a| 1.0|
| a| 2.0|
| a| 3.0|
| b| 6.0|
| b| 8.0|
+----+----+
现在我想要做类似这样的事情:
df.groupBy("col1").agg(calcmedian("col2"))
结果应该看起来像这样:+----+------+
|col1|median|
+----+------+
| a| 2.0|
| b| 7.0|
+----+------+`
因此calcmedian()必须是一个UDAF,但问题是,UDAF的“evaluate”方法只接受一行数据,而我需要整个表来排序并返回中位数...
// Once all entries for a group are exhausted, spark will evaluate to get the final result
def evaluate(buffer: Row) = {...}
有没有什么办法可以实现这个?或者有没有其他好的解决方案?我想强调一下,我知道如何在“一个组”数据集上计算中位数。但是我不想在“foreach”循环中使用此算法,因为这样效率低下!
谢谢!
编辑:
这是我到目前为止尝试过的:
object calcMedian extends UserDefinedAggregateFunction {
// Schema you get as an input
def inputSchema = new StructType().add("col2", DoubleType)
// Schema of the row which is used for aggregation
def bufferSchema = new StructType().add("col2", DoubleType)
// Returned type
def dataType = DoubleType
// Self-explaining
def deterministic = true
// initialize - called once for each group
def initialize(buffer: MutableAggregationBuffer) = {
buffer(0) = 0.0
}
// called for each input record of that group
def update(buffer: MutableAggregationBuffer, input: Row) = {
buffer(0) = input.getDouble(0)
}
// if function supports partial aggregates, spark might (as an optimization) comput partial results and combine them together
def merge(buffer1: MutableAggregationBuffer, buffer2: Row) = {
buffer1(0) = input.getDouble(0)
}
// Once all entries for a group are exhausted, spark will evaluate to get the final result
def evaluate(buffer: Row) = {
val tile = 50
var median = 0.0
//PROBLEM: buffer is a Row --> I need DataFrame here???
val rdd_sorted = buffer.sortBy(x => x)
val c = rdd_sorted.count()
if (c == 1){
median = rdd_sorted.first()
}else{
val index = rdd_sorted.zipWithIndex().map(_.swap)
val last = c
val n = (tile/ 100d) * (c*1d)
val k = math.floor(n).toLong
val d = n - k
if( k <= 0) {
median = rdd_sorted.first()
}else{
if (k <= c){
median = index.lookup(last - 1).head
}else{
if(k >= c){
median = index.lookup(last - 1).head
}else{
median = index.lookup(k-1).head + d* (index.lookup(k).head - index.lookup(k-1).head)
}
}
}
}
} //end of evaluate
groupByKey
对数据进行分组,将聚合后的数据转换为Buffer
,可以使用一些UDF
来实现这个过程,然后创建一个 UDF 来计算中位数。 - Alberto BonsantoUserDefinedAggregateFunction
基类有许多成员需要实现,而不仅仅是evaluate
。传递给evaluate
的Row
缓冲区是最后一步。您是否尝试过任何实现?如果是,能否展示一下您目前的代码? - mattinbitspercentile_approx
/percentile
是什么。数据框上的groupBy
不会物理移动数据(这里洗牌没有区别)。它是aggregate(ByKey)
的等效形式,这在 API 中清晰地反映出来。无论如何,您都无法在 UDAF 内部访问数据框。 - zero323