Spark API 详解/大白话解释 之 map、mapPartitions、mapValues、mapWith、flatMap、flatMapWith、fla

map(function) 
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。

举例：

val a = sc.parallelize(1 to 9, 3)
val b = a.map(x => x*2)//x => x*2是一个函数，x是传入参数即RDD的每个元素，x*2是返回值
a.collect
//结果Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9)
b.collect
//结果Array[Int] = Array(2, 4, 6, 8, 10, 12, 14, 16, 18)

 

当然map也可以把Key变成Key-Value对

val a = sc.parallelize(List("dog", "tiger", "lion", "cat", "panther", " eagle"), 2)
val b = a.map(x => (x, 1))
b.collect.foreach(println(_))
/*
(dog,1)
(tiger,1)
(lion,1)
(cat,1)
(panther,1)
( eagle,1)
*/

 

---

mapPartitions(function) 
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

package test

import scala.Iterator

import org.apache.spark.SparkConf
import org.apache.spark.SparkContext

object TestRdd {
  def sumOfEveryPartition(input: Iterator[Int]): Int = {
    var total = 0
    input.foreach { elem =>
      total += elem
    }
    total
  }
  def main(args: Array[String]) {
    val conf = new SparkConf().setAppName("Spark Rdd Test")
    val spark = new SparkContext(conf)
    val input = spark.parallelize(List(1, 2, 3, 4, 5, 6), 2)//RDD有6个元素，分成2个partition
    val result = input.mapPartitions(
      partition => Iterator(sumOfEveryPartition(partition)))//partition是传入的参数，是个list，要求返回也是list，即Iterator(sumOfEveryPartition(partition))
    result.collect().foreach {
      println(_)//6 15
    }
    spark.stop()
  }
}

 

---

mapValues(function) 
原RDD中的Key保持不变，与新的Value一起组成新的RDD中的元素。因此，该函数只适用于元素为KV对的RDD

val a = sc.parallelize(List("dog", "tiger", "lion", "cat", "panther", " eagle"), 2)
val b = a.map(x => (x.length, x))
b.mapValues("x" + _ + "x").collect

 

//"x" + _ + "x"等同于everyInput =>"x" + everyInput + "x" 
//结果 
Array( 
(3,xdogx), 
(5,xtigerx), 
(4,xlionx), 
(3,xcatx), 
(7,xpantherx), 
(5,xeaglex) 
)

---

mapWith和flatMapWith 
感觉用得不多，参考http://blog.csdn.net/jewes/article/details/39896301

---

flatMap(function) 
与map类似，区别是原RDD中的元素经map处理后只能生成一个元素，而原RDD中的元素经flatmap处理后可生成多个元素

val a = sc.parallelize(1 to 4, 2)
val b = a.flatMap(x => 1 to x)//每个元素扩展
b.collect
/*
结果    Array[Int] = Array( 1, 
                           1, 2, 
                           1, 2, 3, 
                           1, 2, 3, 4)
*/

 

---

flatMapValues(function)

val a = sc.parallelize(List((1,2),(3,4),(5,6)))
val b = a.flatMapValues(x=>1 to x)
b.collect.foreach(println(_))
/*
(1,1)
(1,2)
(3,1)
(3,2)
(3,3)
(3,4)
(5,1)
(5,2)
(5,3)
(5,4)
(5,5)
(5,6)
*/