scala - Apache Flink - groupBy -

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i trying follow first exercise on http://dataartisans.github.io/flink-training/exercises/.

now come following problem. groupby function give me back? , how foldleft method transform - me unknown - result?

the code following:

mails.map {           m => (m._1.substring(0, 7), m._2.substring(m._2.lastindexof("<") + 1, m._2.length - 1))         }         .groupby(0, 1)         .reducegroup( ms => ms.foldleft("", "", 0)( (c, m) => (m._1, m._2, c._3 + 1) ) )

regards, kevin

groupby returns grouped data set: https://ci.apache.org/projects/flink/flink-docs-release-1.0/apis/batch/dataset_transformations.html#groupreduce-on-grouped-dataset

foldleft defines folding (or reducing) order. see here: https://en.wikipedia.org/wiki/fold_%28higher-order_function%29


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