Spark算子：RDD行动Action操作(2)–take、top、takeOrdered

take

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)

top

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]].reverse
myOrd: 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)
takeOrdered

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)