如何计算与Apache星火精确的中间？ [英] How can I calculate exact median with Apache Spark?

问题描述

<一个href=\"https://spark.apache.org/docs/0.7.0/api/core/spark/api/java/JavaDoubleRDD.html\">https://spark.apache.org/docs/0.7.0/api/core/spark/api/java/JavaDoubleRDD.html

本页面包含了一些统计功能（平均值，标准偏差，方差等），但它不包含中位数。我怎样才能精确计算中位数？

This page contains some statistics functions(mean, stdev, variance etc.) but it does not contains median. How can I calculate exact median ?

感谢

推荐答案

您需要梳理RDD并采取元素的两个元素的中间或平均值。这里是例如使用RDD [INT]：

You need to sort RDD and take element in the middle or average of two elements. Here is example with RDD[Int]:

  import org.apache.spark.SparkContext._

  val rdd: RDD[Int] = ???

  val sorted = rdd.sortBy(identity).zipWithIndex().map {
    case (v, idx) => (idx, v)
  }

  val count = sorted.count()

  val median: Double = if (count % 2 == 0) {
    val l = count / 2 - 1
    val r = l + 1
    (sorted.lookup(l).head + sorted.lookup(r).head).toDouble / 2
  } else sorted.lookup(count / 2).head.toDouble

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