def take(num: Int): Array[T]
take用于获取RDD中从0到num-1下标的元素，不排序。
scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21 scala> rdd1.take(1)res0: Array[Int] = Array(10) scala> rdd1.take(2)res1: Array[Int] = Array(10, 4)def top(num: Int)(implicit ord: Ordering[T]): Array[T]
top函数用于从RDD中，按照默认（降序）或者指定的排序规则，返回前num个元素。
scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21 scala> rdd1.top(1)res2: Array[Int] = Array(12) scala> rdd1.top(2)res3: Array[Int] = Array(12, 10) //指定排序规则scala> implicit val myOrd = implicitly[Ordering[Int]].reversemyOrd: scala.math.Ordering[Int] = scala.math.Ordering$$anon$4@767499ef scala> rdd1.top(1)res4: Array[Int] = Array(2) scala> rdd1.top(2)res5: Array[Int] = Array(2, 3)def takeOrdered(num: Int)(implicit ord: Ordering[T]): Array[T]
takeOrdered和top类似，只不过以和top相反的顺序返回元素。
scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21 scala> rdd1.top(1)res4: Array[Int] = Array(2) scala> rdd1.top(2)res5: Array[Int] = Array(2, 3) scala> rdd1.takeOrdered(1)res6: Array[Int] = Array(12) scala> rdd1.takeOrdered(2)res7: Array[Int] = Array(12, 10)