Merge pull request #789 from MLnick/master

Adding Scala version of PageRank example

examples/src/main/scala/spark/examples/SparkPageRank.scala

+package spark.examples
+
+import spark.SparkContext._
+import spark.SparkContext
+
+
+/**
+ * Computes the PageRank of URLs from an input file. Input file should
+ * be in format of:
+ * URL         neighbor URL
+ * URL         neighbor URL
+ * URL         neighbor URL
+ * ...
+ * where URL and their neighbors are separated by space(s).
+ */
+object SparkPageRank {
+  def main(args: Array[String]) {
+    if (args.length < 3) {
+      System.err.println("Usage: PageRank <master> <file> <number_of_iterations>")
+      System.exit(1)
+    }
+    var iters = args(2).toInt
+    val ctx = new SparkContext(args(0), "PageRank", System.getenv("SPARK_HOME"), Seq(System.getenv("SPARK_EXAMPLES_JAR")))
+    val lines = ctx.textFile(args(1), 1)
+    val links = lines.map{s =>
+      val parts = s.split("\\s+")
+      (parts(0), parts(1))
+    }.distinct().groupByKey().cache()
+    var ranks = links.mapValues(v => 1.0)
+
+    for (i <- 1 to iters) {
+      val contribs = links.join(ranks).values.flatMap{ case (urls, rank) =>
+        val size = urls.size
+        urls.map(url => (url, rank / size))
+      }
+
+      ranks = contribs.groupByKey().mapValues{ranks =>
+        val sumRanks = ranks.foldLeft(0.0)(_ + _)
+        0.15 + sumRanks * 0.85
+      }
+    }
+
+    val output = ranks.collect()
+    output.foreach(tup => println(tup._1 + " has rank: " + tup._2 + "."))
+
+    ctx.stop()
+    System.exit(0)
+  }
+}
+