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。

1. scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))
2. rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21
3.  
4. scala> rdd1.take(1)
5. res0: Array[Int] = Array(10)
6.  
7. scala> rdd1.take(2)
8. res1: Array[Int] = Array(10, 4)
9.  

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。

1. scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))
2. rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21
3.  
4. scala> rdd1.top(1)
5. res2: Array[Int] = Array(12)
6.  
7. scala> rdd1.top(2)
8. res3: Array[Int] = Array(12, 10)
9.  
10. //指定排序规则
11. scala> implicit val myOrd = implicitly[Ordering[Int]].reverse
12. myOrd: scala.math.Ordering[Int] = scala.math.Ordering$$anon$4@767499ef
13.  
14. scala> rdd1.top(1)
15. res4: Array[Int] = Array(2)
16.  
17. scala> rdd1.top(2)
18. res5: Array[Int] = Array(2, 3)
19.  

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。

1. scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))
2. rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[40] at makeRDD at :21
3.  
4. scala> rdd1.top(1)
5. res4: Array[Int] = Array(2)
6.  
7. scala> rdd1.top(2)
8. res5: Array[Int] = Array(2, 3)
9.  
10. scala> rdd1.takeOrdered(1)
11. res6: Array[Int] = Array(12)
12.  
13. scala> rdd1.takeOrdered(2)
14. res7: Array[Int] = Array(12, 10)
15.