How to implement LDA in Spark and get the topic distributions of new documents
import org.apache.spark.rdd._
import org.apache.spark.mllib.clustering.{LDA, DistributedLDAModel, LocalLDAModel}
import org.apache.spark.mllib.linalg.{Vector, Vectors}
import scala.collection.mutable
val stopWordsInput = sc.textFile("stopwords.csv")
val stopWords = stopWordsInput.collect()
//create training document set
val input = sc.textFile("training_documents/*").collect().map(s=>s.mkString);
val corpus: RDD[Array[String]] = sc.parallelize(input.map{
doc => doc.split("\\s")
})
val termCounts: Array[(String, Long)] = corpus.flatMap(_.map(_ -> 1L)).reduceByKey(_ + _).collect().sortBy(-_._2)
val vocabArray: Array[String] = termCounts.takeRight(termCounts.size).map(_._1)
val vocab: Map[String, Int] = vocabArray.zipWithIndex.toMap
// Convert training documents into term count vectors
val documents: RDD[(Long, Vector)] =
corpus.zipWithIndex.map { case (tokens, id) =>
val counts = new mutable.HashMap[Int, Double]()
tokens.foreach { term =>
if (vocab.contains(term)) {
val idx = vocab(term)
counts(idx) = counts.getOrElse(idx, 0.0) + 1.0
}
}
(id, Vectors.sparse(vocab.size, counts.toSeq))
}
// Set LDA parameters and create model
val numTopics = 10
val ldaModel: DistributedLDAModel = new LDA().setK(numTopics).setMaxIterations(20).run(documents).asInstanceOf[DistributedLDAModel]
val localLDAModel: LocalLDAModel = ldaModel.toLocal
//create test input, convert to term count, and get its topic distribution
val test_input = Seq("this is my test document")
val test_document:RDD[(Long,Vector)] = sc.parallelize(test_input.map(doc=>doc.split("\\s"))).zipWithIndex.map{ case (tokens, id) =>
val counts = new mutable.HashMap[Int, Double]()
tokens.foreach { term =>
if (vocab.contains(term)) {
val idx = vocab(term)
counts(idx) = counts.getOrElse(idx, 0.0) + 1.0
}
}
(id, Vectors.sparse(vocab.size, counts.toSeq))
}
val topicDistributions = localLDAModel.topicDistributions(test_document)
println("first topic distribution:"+topicDistributions.first._2.toArray.mkString(", "))
Hi @medoidai , good question! I've never tried using an LDA model on text with words it hasn't seen before. I am no expert in LDA, just using off-the-shelf implementations. Making new types of LDA that allow for infinite vocabularies (new words) does seem to be an area of research though
http://proceedings.mlr.press/v28/zhai13.pdf