reduceByKey函数ＡＰＩ：

def reduceByKey(partitioner: Partitioner, func: JFunction2[V, V, V]): JavaPairRDD[K, V]

def reduceByKey(func: JFunction2[V, V, V], numPartitions: Int): JavaPairRDD[K, V]

 该函数利用映射函数将每个K对应的V进行运算。 

其中参数说明如下：
– func：映射函数，根据需求自定义；
– partitioner：分区函数；
– numPartitions：分区数，默认的分区函数是HashPartitioner。

返回值：可以看出最终是返回了一个ＫＶ键值对。

使用示例：

ｌｉｎｕｘ:/$ spark-shell
。。。
17/10/28 20:33:54 WARN SparkConf: In Spark 1.0 and later spark.local.dir will be overridden by the value set by the cluster manager (via SPARK_LOCAL_DIRS in mesos/standalone and LOCAL_DIRS in YARN).
17/10/28 20:33:55 WARN MetricsSystem: Using default name DAGScheduler for source because spark.app.id is not set.
Spark context available as sc.
17/10/28 20:33:57 WARN SessionState: load mapred-default.xml, HIVE_CONF_DIR env not found!
17/10/28 20:33:58 WARN SessionState: load mapred-default.xml, HIVE_CONF_DIR env not found!
SQL context available as sqlContext.

scala> val x = sc.parallelize(List(
     |     ("a", "b", 1),
     |     ("a", "b", 1),
     |     ("c", "b", 1),
     |     ("a", "d", 1))
     | )
x: org.apache.spark.rdd.RDD[(String, String, Int)] = ParallelCollectionRDD[0] at parallelize at <console>:21

scala> val byKey = x.map({case (id,uri,count) => (id,uri)->count})
byKey: org.apache.spark.rdd.RDD[((String, String), Int)] = MapPartitionsRDD[1] at map at <console>:23

scala> val reducedByKey = byKey.reduceByKey(_ + _)
reducedByKey: org.apache.spark.rdd.RDD[((String, String), Int)] = ShuffledRDD[2] at reduceByKey at <console>:25


scala> reducedByKey.collect.foreach(println)
((c,b),1)
((a,d),1)
((a,b),2)

 使用reduceByKey实现group by:

假设有一张表：my_table,按照key进行group by并统计出((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp))*((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp)),以及count个数：

create table if not exists my_table(
key string,
l_scrsrp int,
l_ncrsrp int,
scrsrp int,
ncrsrp int
)
insert into my_table(key,l_scrsrp,l_ncrsrp,scrsrp,ncrsrp)values("key1",1,0,2,0);
insert into my_table(key,l_scrsrp,l_ncrsrp,scrsrp,ncrsrp)values("key1",1,0,2,0);
insert into my_table(key,l_scrsrp,l_ncrsrp,scrsrp,ncrsrp)values("key2",1,0,2,0);
insert into my_table(key,l_scrsrp,l_ncrsrp,scrsrp,ncrsrp)values("key3",1,0,3,0);
insert into my_table(key,l_scrsrp,l_ncrsrp,scrsrp,ncrsrp)values("key2",1,0,3,0);
0: jdbc:hive2://xx.xx.xx.xx:xxxx/> 
0: jdbc:hive2://xx.xx.xx.xx:xxxx/> select key,sum(((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp))*((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp))),count(0) myvalue 
0: jdbc:hive2://xx.xx.xx.xx:xxxx/> from my_table
0: jdbc:hive2://xx.xx.xx.xx:xxxx/> group by key;

+-------+------+----------+--+
|  key  | _c1  | myvalue  |
+-------+------+----------+--+
| key1  | 2    | 2        |
| key2  | 5    | 2        |
| key3  | 4    | 1        |
+-------+------+----------+--+

使用reduceByKey实现的代码如下：

scala> val y=sc.parallelize(List(
     | ("key1",1,0,2,0),
     | ("key1",1,0,2,0),
     | ("key2",1,0,2,0),
     | ("key3",1,0,3,0),
     | ("key2",1,0,3,0)
     | ))
y: org.apache.spark.rdd.RDD[(String, Int, Int, Int, Int)] = ParallelCollectionRDD[0] at parallelize at <console>:21

scala> val byKey = y.map({case (key,scrsrp,ncrsrp,l_scrsrp,l_ncrsrp) => (key)->((((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp))*((l_scrsrp-l_ncrsrp)-(scrsrp-ncrsrp))),(1))})
byKey: org.apache.spark.rdd.RDD[(String, (Int, Int))] = MapPartitionsRDD[1] at map at <console>:23
scala> byKey.foreach(println)
(key3,(4,1))
(key1,(1,1))
(key1,(1,1))
(key2,(1,1))
(key2,(4,1))

scala> val reducedByKey = byKey.reduceByKey((x1, x2) =>(x1._1 + x2._1,x1._2 + x2._2))
reducedByKey: org.apache.spark.rdd.RDD[(String, (Int, Int))] = ShuffledRDD[3] at reduceByKey at <console>:25

scala> reducedByKey.collect.foreach(println)
(key1,(2,2))                                                                    
(key2,(5,2))
(key3,(4,1))

 实现统计字符个数：

scala> val x = sc.parallelize(List("a", "b", "a", "a", "b", "b", "b", "b"))  
x: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[4] at parallelize at <console>:21

scala>  val s = x.map((_, 1))  
s: org.apache.spark.rdd.RDD[(String, Int)] = MapPartitionsRDD[5] at map at <console>:23

scala> val result = s.reduceByKey((pre, after) => pre + after)  
result: org.apache.spark.rdd.RDD[(String, Int)] = ShuffledRDD[6] at reduceByKey at <console>:25

scala> println(result.collect().toBuffer)  
ArrayBuffer((a,3), (b,5))

scala> result.foreach(println)
(a,3)
(b,5)