title: Spark Scala Understanding reduceByKey(_ + _)
body: <p>I can't understand reduceByKey(_ + _) in the first example of spark with scala  </p>
  
  <pre><code>object WordCount {
  def main(args: Array[String]): Unit = {
  val inputPath = args(0)
  val outputPath = args(1)
  val sc = new SparkContext()
  val lines = sc.textFile(inputPath)
  val wordCounts = lines.flatMap {line =&gt; line.split(" ")}
  .map(word =&gt; (word, 1))
  .reduceByKey(_ + _)  **I cant't understand this line**
  wordCounts.saveAsTextFile(outputPath)
  }
  }
  </code></pre>