1. 複習List的操作

list操作：

val ls1 = List(1)
val ls2 = List(2)
// 追加一個元素
ls1 :+ 2 這個結果為：res15: List[Any] = List(1, 2)
// 追加一個集合
ls1 ++ ls2 這個結果為：res16: List[Int] = List(1, 2)
// 追加一個集合
ls1 ::: ls2 這個結果為：res17: List[Int] = List(1, 2)

2. combineByKey

val rdd1 = sc.parallelize(List("dog","cat","gnu","salmon","rabbit"
 
,"wolf","bear","bee"),3)

val rdd2 = sc.parallelize(List(1,2,1,2,1,2,1,2),3)

scala> val rdd3 = rdd2.zip(rdd1).collect
rdd3: Array[(Int, String)] = Array((1,dog), (2,cat), (1,gnu), (2,salmon), (1,rabbit), (2,wolf), (1,bear), (2,bee))


scala> val rdd3 = rdd2.zip(rdd1)
rdd3: org.apache.spark.rdd.
 
RDD[(Int, String)] = ZippedPartitionsRDD2[5] at zip at <console>:27

scala> rdd3.combineByKey(List(_),(a:List[String],b:String)=> a :+ b,(x:List[String],y:List[String])=> x ::: y)
res21: org.apache.spark.rdd.RDD[(Int, List[String])] = ShuffledRDD[6] at combineByKey at <console>:
 
26

scala> res21.collect
res22: Array[(Int, List[String])] = Array((1,List(gnu, rabbit, dog, bear)), (2,List(cat, wolf, bee, salmon)))

說明： List[String],b:String)=> a :+ b 作用是什麼？