RDD-Transformation
转换（Transformation）算子就是对RDD进行操作的接口函数，其作用是将一个或多个RDD变换成新的RDD。
使用Spark进行数据计算，在利用创建算子生成RDD后，数据处理的算法设计和程序编写的最关键部分，就是利用变换算子对原始数据产生的RDD进行一步一步的变换，最终得到期望的计算结果。
对于变换算子可理解为分两类：1，对Value型RDD进行变换的算子；2，对Key/Value型RDD进行变换算子。在每个变换中有仅对一个RDD进行变换的，也有是对两个RDD进行变换的。
对单个Value型的RDD进行变换
- map
- filter
- distinct
- flatMap
- sample
- union
- intersection
- groupByKey
对于上面列出的几个RDD转换算子因为在前面的文章里有介绍了，这里就不进行示例展示了。详见
coalesce——重新分区
将当前RDD进行重新分区，生成一个以numPartitions参数指定的分区数存储的新RDD。参数shuffle为true时在变换过程中进行shuffle操作，否则不进行shuffle。
def coalesce(numPartitions: Int, shuffle: Boolean = false, partitionCoalescer: Option[PartitionCoalescer] = Option.empty)(implicit ord: Ordering[T] = null): RDD[T]
Note:
With shuffle = true, you can actually coalesce to a larger number of partitions. This is useful if you have a small number of partitions, say 100, potentially with a few partitions being abnormally large. Calling coalesce(1000, shuffle = true) will result in 1000 partitions with the data distributed using a hash partitioner. The optional partition coalescer passed in must be serializable.
scala> val rdd = sc.parallelize(List(1,2,3,4,5,6,7,8), 4)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[7] at parallelize at <console>:24
scala> rdd.partitions
res13: Array[org.apache.spark.Partition] = Array(org.apache.spark.rdd.ParallelCollectionPartition@7b0, org.apache.spark.rdd.ParallelCollectionPartition@7b1, org.apache.spark.rdd.ParallelCollectionPartition@7b2, org.apache.spark.rdd.ParallelCollectionPartition@7b3)
scala> rdd.partitions.length
res14: Int = 4
scala> rdd.collect
res15: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8)
scala> rdd.glom.collect
res16: Array[Array[Int]] = Array(Array(1, 2), Array(3, 4), Array(5, 6), Array(7, 8))
scala> val newRDD = rdd.coalesce(2, false)
newRDD: org.apache.spark.rdd.RDD[Int] = CoalescedRDD[9] at coalesce at <console>:26
scala> newRDD.partitions.length
res17: Int = 2
scala> newRDD.collect
res18: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8)
scala> newRDD.glom.collect
res19: Array[Array[Int]] = Array(Array(1, 2, 3, 4), Array(5, 6, 7, 8))
coalesce-test
coalesce
pipe——调用Shell命令
# Return an RDD created by piping elements to a forked external process.
def pipe(command: String): RDD[String]
在Linux系统中，有许多对数据进行处理的shell命令，我们可能通过pipe变换将一些shell命令用于Spark中生成新的RDD。
scala> val rdd = sc.parallelize(0 to 7, 4)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[11] at parallelize at <console>:24
scala> rdd.glom.collect
res20: Array[Array[Int]] = Array(Array(0, 1), Array(2, 3), Array(4, 5), Array(6, 7))
scala> rdd.pipe("head -n 1").collect #提取每一个分区中的第一个元素构成新的RDD
res21: Array[String] = Array(0, 2, 4, 6)
pipe-RDD
pipe
sortBy——排序
对原RDD中的元素按照函数f指定的规则进行排序，并可通过ascending参数进行升序或降序设置，排序后的结果生成新的RDD，新的RDD的分区数量可以由参数numPartitions指定，默认与原RDD相同的分区数。
# Return this RDD sorted by the given key function.
def sortBy[K](f: (T) ⇒ K, ascending: Boolean = true, numPartitions: Int = this.partitions.length)(implicit ord: Ordering[K], ctag: ClassTag[K]): RDD[T]
sortBy
scala> val rdd = sc.parallelize(List(2,1,4,3),1)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[19] at parallelize at <console>:24
scala> rdd.glom.collect
res24: Array[Array[Int]] = Array(Array(2, 1, 4, 3))
scala> rdd.sortBy(x=>x, true).collect
res25: Array[Int] = Array(1, 2, 3, 4)
scala> rdd.sortBy(x=>x, false).collect
res26: Array[Int] = Array(4, 3, 2, 1)
对两个Value型RDD进行变换
cartesian——笛卡尔积
输入参数为另一个RDD，返回两个RDD中所有元素的笛卡尔积。
# Return the Cartesian product of this RDD and another one,
# that is, the RDD of all pairs of elements (a, b) where a is in this and b is in other.
def cartesian[U](other: RDD[U])(implicit arg0: ClassTag[U]): RDD[(T, U)]
cartesian-RDD
scala> val rdd1 = sc.parallelize(List("a", "b", "c"),1)
rdd1: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[27] at parallelize at <console>:24
scala> val rdd2 = sc.parallelize(List(1,2,3), 1)
rdd2: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[28] at parallelize at <console>:24
scala> rdd1.cartesian(rdd2).collect
res27: Array[(String, Int)] = Array((a,1), (a,2), (a,3), (b,1), (b,2), (b,3), (c,1), (c,2), (c,3))
subtract——补集
输入参数为另一个RDD，返回原始RDD与输入参数RDD的补集，即生成由原始RDD中而不在输入参数RDD中的元素构成新的RDD，参数numPartitions指定新RDD分区数。
#Return an RDD with the elements from this that are not in other.
defsubtract(other: RDD[T], numPartitions: Int): RDD[T]
subtract-RDD
scala> val rdd1 = sc.parallelize(0 to 5, 1)
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[30] at parallelize at <console>:24
scala> val rdd2 = sc.parallelize(0 to 2,1)
rdd2: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[31] at parallelize at <console>:24
scala> rdd1.subtract(rdd2).collect
res28: Array[Int] = Array(3, 4, 5)
subtract
union——并集
返回原始RDD与另一个RDD的并集。
# Return the union of this RDD and another one.
def union(other: RDD[T]): RDD[T]
def ++(other: RDD[T]): RDD[T]
#Return the union of this RDD and another one.
union-RDD
zip——联结
生成由原始RDD的值为Key，另一个RDD的值为Value依次配对构成的所有Key/Value对，并返回这些Key/Value对集合构成的新RDD
Paste_Image.png
对Key/Value型RDD进行变换
对单个Key-Value型RDD进行变换
combineByKey——按Key聚合
def combineByKey[C](createCombiner: (V) => C, mergeValue: (C, V) => C, mergeCombiners: (C, C) => C): RDD[(K, C)]
combineByKey-RDD
scala> val pair = sc.parallelize(List(("fruit", "Apple"), ("fruit", "Banana"), ("vegetable", "Cucumber"), ("fruit", "Cherry"), ("vegetable", "Bean"), ("vegetable", "Pepper")),2)
pair: org.apache.spark.rdd.RDD[(String, String)] = ParallelCollectionRDD[41] at parallelize at <console>:24
scala> val combinePair = pair.combineByKey(List(_), (x:List[String], y:String) => y::x, (x:List[String], y:List[String]) => x:::y)
combinePair: org.apache.spark.rdd.RDD[(String, List[String])] = ShuffledRDD[42] at combineByKey at <console>:26
scala> combinePair.collect
res31: Array[(String, List[String])] = Array((fruit,List(Banana, Apple, Cherry)), (vegetable,List(Cucumber, Pepper, Bean)))
flatMapValues——对所有Value进行flatMap
# Pass each value in the key-value pair RDD through a flatMap function without changing the keys;
# this also retains the original RDD's partitioning.
def flatMapValues[U](f: (V) =>TraversableOnce[U]): RDD[(K, U)]
flatMapValue-RDD
scala> val rdd = sc.parallelize(List("a", "boy"), 1).keyBy(_.length)
rdd: org.apache.spark.rdd.RDD[(Int, String)] = MapPartitionsRDD[44] at keyBy at <console>:24
scala> rdd.collect
res32: Array[(Int, String)] = Array((1,a), (3,boy))
scala> rdd.flatMapValues(x=>"*" + x + "*").collect
res33: Array[(Int, Char)] = Array((1,*), (1,a), (1,*), (3,*), (3,b), (3,o), (3,y), (3,*))
keys——提取Key
将Key/Value型RDD中的元素的Key提取出来，所有Key值构成一个序列形成新的RDD。
# Return an RDD with the keys of each tuple.
def keys: RDD[K]
keys-RDD
scala> val pairs = sc.parallelize(List("wills", "aprilchang","kris"),1).keyBy(_.length)
pairs: org.apache.spark.rdd.RDD[(Int, String)] = MapPartitionsRDD[47] at keyBy at <console>:24
scala> pairs.collect
res34: Array[(Int, String)] = Array((5,wills), (10,aprilchang), (4,kris))
scala> pairs.keys.collect
res35: Array[Int] = Array(5, 10, 4)
mapValues——对Value值进行变换
将Key/Value型RDD中的元素的Value值使用输入参数函数f进行变换构成一个新的RDD。
# Pass each value in the key-value pair RDD through a map function without changing the keys;
# this also retains the original RDD's partitioning.
def mapValues[U](f: (V) => U): RDD[(K, U)]
mapValues-RDD
partitionBy——按Key值重新分区
def partitionBy(partitioner: Partitioner): RDD[(K, V)]
#Return a copy of the RDD partitioned using the specified partitioner.
partitionBy-RDD
scala> val pairs = sc.parallelize(0 to 9, 2).keyBy(x=>x)
pairs: org.apache.spark.rdd.RDD[(Int, Int)] = MapPartitionsRDD[51] at keyBy at <console>:24
scala> pairs.collect
res37: Array[(Int, Int)] = Array((0,0), (1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (7,7), (8,8), (9,9))
scala> import org.apache.spark.HashPartitioner
import org.apache.spark.HashPartitioner
scala> val partitionPairs = pairs.partitionBy(new HashPartitioner(2)) #按每个key的Hash值进行分区的
partitionPairs: org.apache.spark.rdd.RDD[(Int, Int)] = ShuffledRDD[52] at partitionBy at <console>:27
scala> partitionPairs.glom.collect
res38: Array[Array[(Int, Int)]] = Array(Array((0,0), (2,2), (4,4), (6,6), (8,8)), Array((1,1), (3,3), (5,5), (7,7), (9,9)))
reduceByKey——按Key值进行Reduce操作
def reduceByKey(func: (V, V) => V): RDD[(K, V)]
## Merge the values for each key using an associative and commutative reduce function.
def reduceByKey(func: (V, V) => V, numPartitions: Int): RDD[(K, V)]
## Merge the values for each key using an associative and commutative reduce function.
## This will also perform the merging locally on each mapper before sending results to a reducer,
## similarly to a "combiner" in MapReduce. Output will be hash-partitioned with numPartitions partitions.
def reduceByKey(partitioner: Partitioner, func: (V, V) => V): RDD[(K, V)]
## Merge the values for each key using an associative and commutative reduce function.
