1、first：def first(): T

該函式返回RDD的第一個元素，不排序。

scala> var rdd1 = sc.makeRDD(Array(("A","1"),("B","2"),("C","3")),2)
rdd1: org.apache.spark.rdd.RDD[(String, String)] = ParallelCollectionRDD[33] at makeRDD at :21
 
scala> rdd1.first
res14: (String, String) = (A,1)
 
scala> var rdd1 = sc.makeRDD(Seq(10, 4, 2, 12, 3))
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at makeRDD at :21
 
scala> rdd1.first
res8: Int = 10
 

2、count：def count(): Long

該函式返回RDD中的元素總數量。

scala> var rdd1 = sc.makeRDD(Array(("A","1"),("B","2"),("C","3")),2)
rdd1: org.apache.spark.rdd.RDD[(String, String)] = ParallelCollectionRDD[34] at makeRDD at :21
 
scala> rdd1.count
res15: Long = 3

3、reduce：def reduce(f: (T, T) ⇒ T): T

該函式根據對映函式f對RDD中的元素進行二元計算，返回計算結果。

scala> var rdd1 = sc.makeRDD(1 to 10,2)
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[36] at makeRDD at :21
 
scala> rdd1.reduce(_ + _)
res18: Int = 55
 
scala> var rdd2 = sc.makeRDD(Array(("A",0),("A",2),("B",1),("B",2),("C",1)))
rdd2: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[38] at makeRDD at :21
 
//字元相串，數字相加
scala> rdd2.reduce((x,y) => {
     |       (x._1 + y._1,x._2 + y._2)
     |     })
res21: (String, Int) = (CBBAA,6)