3.4 Spark RDD Action操作2-take、top、takeOrdered

1 take

def take(num: Int): Array[T]

take用于获取RDD中从0到num-1下标的元素。

例子：

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)

2 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。

例子：

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)

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。

例子：

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)