scala 如何在RDD中展平列表？
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How to flatten list inside RDD?
提问by zork
Is it possible to flatten list inside RDD? For example convert:
是否可以在 RDD 中展平列表？例如转换：
val xxx: org.apache.spark.rdd.RDD[List[Foo]]
to:
到：
val yyy: org.apache.spark.rdd.RDD[Foo]
How to do this?
这个怎么做？
回答by Shyamendra Solanki
val rdd = sc.parallelize(Array(List(1,2,3), List(4,5,6), List(7,8,9), List(10, 11, 12)))
// org.apache.spark.rdd.RDD[List[Int]] = ParallelCollectionRDD ...
val rddi = rdd.flatMap(list => list)
// rddi: org.apache.spark.rdd.RDD[Int] = FlatMappedRDD ...
// which is same as rdd.flatMap(identity)
// identity is a method defined in Predef object.
// def identity[A](x: A): A
rddi.collect()
// res2: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
回答by maasg
You just need to flatten it, but as there's no explicit 'flatten' method on RDD, you can do this:
您只需要展平它，但由于 RDD 上没有明确的“展平”方法，您可以这样做：
rdd.flatMap(identity)
回答by Xavier Guihot
You could pimpthe RDDclass to attach a .flattenmethod (in order to follow the Listapi):
你可以拉皮条这个RDD类来附加一个.flatten方法（为了遵循Listapi）：
object SparkHelper {
implicit class SeqRDDExtensions[T: ClassTag](val rdd: RDD[Seq[T]]) {
def flatten: RDD[T] = rdd.flatMap(identity)
}
}
which can then simply be used as such:
然后可以简单地使用它：
rdd.flatten
