Spark Transformations介绍
背景
本文介绍是基于Spark 1.3源码
如何创建RDD？
RDD可以从普通数组创建出来，也可以从文件系统或者HDFS中的文件创建出来。
举例：从普通数组创建RDD，里面包含了1到9这9个数字，它们分别在3个分区中。
scala> val a = sc.parallelize(1 to 9, 3)
a: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[103] at parallelize at <console>:21
举例：读取文件README.md来创建RDD，文件中的每一行就是RDD中的一个元素
scala> val file = sc.textFile("README.md")
file: org.apache.spark.rdd.RDD[String] = README.md MapPartitionsRDD[3] at textFile at <console>:21虽然还有别的方式可以创建RDD，但在本文中我们主要使用上述两种方式来创建RDD以说明Transformations。
map
map是对RDD中的每个元素都执行一个指定的函数来产生一个新的RDD。任何原RDD中的元素在新RDD中都有且只有一个元素与之对应。
定义
def map[U: ClassTag](f: T => U): RDD[U]举例：
scala> val a = sc.parallelize(List("dog", "salmon", "salmon", "rat", "elephant"), 3)
a: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[4] at parallelize at <console>:21
scala> val b = a.map(_.length)
b: org.apache.spark.rdd.RDD[Int] = MapPartitionsRDD[5] at map at <console>:23
scala> val c=a.zip(b)
c: org.apache.spark.rdd.RDD[(String, Int)] = ZippedPartitionsRDD2[6] at zip at <console>:25
scala> c.collect
res0: Array[(String, Int)] = Array((dog,3), (salmon,6), (salmon,6), (rat,3), (elephant,8))filter
filter作用于原RDD中每个元素，过滤掉原RDD中f返回值为false的元素
定义
def filter(f: T => Boolean): RDD[T]举例
scala> val file=sc.textFile("README.md")
file: org.apache.spark.rdd.RDD[String] = README.md MapPartitionsRDD[19] at textFile at <console>:21
scala> file.filter(line=>line.contains("spark")).count
res5: Long = 11
scala> file.filter(line=>line.contains("spark")).collect
res6: Array[String] = Array(<http://spark.apache.org/>, guide, on the [project web page](http://spark.apache.org/documentation.html), ["Building Spark"](http://spark.apache.org/docs/latest/building-spark.html)., " ./bin/spark-shell", " ./bin/pyspark", "examples to a cluster. This can be a mesos:// or spark:// URL, ", " MASTER=spark://host:7077 ./bin/run-example SparkPi", Testing first requires [building Spark](#building-spark). Once Spark is built, tests, ["Specifying the Hadoop Version"](http://spark.apache.org/docs/latest/building-with-maven.html#specifying-the-hadoop-version), ["Third Party Hadoop Distributions"](http://spark.apache.org/docs/latest/hadoop-third-party-distributions.html), Please refer to the [Configuration guide](http://spark.apache.org/docs/latest/configurat...flatMap
flatMap和map的区别是作用于map的函数只会返回一个元素，作用后元素个数不变，而作用于flatMap的函数返回包含0个或多个元素list的迭代器
定义
def flatMap[U: ClassTag](f: T => TraversableOnce[U]): RDD[U]
举例
scala> val file=sc.textFile("README.md")
file: org.apache.spark.rdd.RDD[String] = README.md MapPartitionsRDD[25] at textFile at <console>:21
scala> file.flatMap(_.split(" ")).take(5)
res11: Array[String] = Array(#, Apache, Spark, "", Spark)
scala> file.map(_.split(" ")).take(5)
res12: Array[Array[String]] = Array(Array(#, Apache, Spark), Array(""), Array(Spark, is, a, fast, and, general, cluster, computing, system, for, Big, Data., It, provides), Array(high-level, APIs, in, Scala,, Java,, and, Python,, and, an, optimized, engine, that), Array(supports, general, computation, graphs, for, data, analysis., It, also, supports, a))
scala> file.map(_.length).take(5)
res1: Array[Int] = Array(14, 0, 78, 72, 73)我们在统计一个文件中有多少单词时，应该使用flatMap，如果使用map分词，每行返回一个数组。如果计算每行的长度应该使用map
mapPartitions
mapPartitions是map的一个变种。map的输入函数是应用于RDD中每个元素，而mapPartitions的输入函数是应用于每个分区，也就是把每个分区中的内容作为整体来处理的。
定义
def mapPartitions[U: ClassTag](
f: Iterator[T] => Iterator[U], preservesPartitioning: Boolean = false): RDD[U]f即为输入函数，它处理每个分区里面的内容。每个分区中的内容将以Iterator[T]传递给输入函数f，f的输出结果是Iterator[U]。最终的RDD由所有分区经过输入函数处理后的结果合并起来的。
举例
scala> :paste
// Entering paste mode (ctrl-D to finish)
val nums=sc.parallelize(1 to 9,3)
nums.mapPartitions(iter=>{
var res = List[(Int, Int)]()
var pre = iter.next
while (iter.hasNext) {
val cur = iter.next;
res ::= (pre, cur)
pre = cur
}
res.iterator
}).collect()
// Exiting paste mode, now interpreting.
nums: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[7] at parallelize at <console>:21
res1: Array[(Int, Int)] = Array((2,3), (1,2), (5,6), (4,5), (8,9), (7,8))上述例子经过mapPartitions把分区中一个元素和它的下一个元素组成一个Tuple。因为分区中最后一个元素没有下一个元素了，所以(3,4)和(6,7)不在结果中。
mapPartitions还有些变种，比如mapPartitionsWithIndex、mapPartitionsWithContext、
mapPartitionsWithSplit，但是从1.2开始mapPartitionsWithContext、mapPartitionsWithSplit这些已作废，下面介绍mapPartionsWithIndex。
mapPartionsWithIndex
定义
def mapPartitionsWithIndex[U: ClassTag](
f: (Int, Iterator[T]) => Iterator[U], preservesPartitioning: Boolean = false): RDD[U]mapPartitionsWithIndex类似于mapPartitions,只是作用函数是两个参数，多了partition的索引。
举例
scala> :paste
// Entering paste mode (ctrl-D to finish)
val nums=sc.parallelize(1 to 9,3)
nums.mapPartitionsWithIndex((index,iter)=>{
if(index == 0)
iter.toList.map(_*2).iterator
else
iter
}).collect()
// Exiting paste mode, now interpreting.
nums: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:21
res0: Array[Int] = Array(2, 4, 6, 4, 5, 6, 7, 8, 9)
参考
http://homepage.cs.latrobe.edu.au/zhe/ZhenHeSparkRDDAPIExamples.html
https://spark.apache.org/docs/latest/programming-guide.html