RDD键值转换操作(3)–groupByKey、reduceByKey、reduceByKeyLocally
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groupByKey
def groupByKey(): RDD[(K, Iterable[V])]
def groupByKey(numPartitions: Int): RDD[(K, Iterable[V])]
def groupByKey(partitioner: Partitioner): RDD[(K, Iterable[V])]
该函数用于将RDD[K,V]中每个K对应的V值,合并到一个集合Iterable[V]中,
参数numPartitions用于指定分区数;
参数partitioner用于指定分区函数;
reduceByKey
def reduceByKey(func: (V, V) => V): RDD[(K, V)]
def reduceByKey(func: (V, V) => V, numPartitions: Int): RDD[(K, V)]
def reduceByKey(partitioner: Partitioner, func: (V, V) => V): RDD[(K, V)]
该函数用于将RDD[K,V]中每个K对应的V值根据映射函数来运算。
参数numPartitions用于指定分区数;
参数partitioner用于指定分区函数;
reduceByKeyLocally
def reduceByKeyLocally(func: (V, V) => V): Map[K, V]
该函数将RDD[K,V]中每个K对应的V值根据映射函数来运算,运算结果映射到一个Map[K,V]中,而不是RDD[K,V]。
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