Create an inverted index from a file

inverted_index.scala

/** 
 * From a file that contains
 * doc_id w1 w2 w3 ... lines, separated by tabs
 * return an inverted index Map of w -> Set(doc_id) 
 * 
 * @param filename well isn't it obvious
 * @return Map[String,Set[String]]
 */
import scala.collection.immutable.Map

def invertedIndex(filename:String) = {
  io.Source.fromFile(filename).getLines.  // this is an iterator over lines
  map(_.split("\t")).               // split at tabs
  filter(_.size > 0).               // make sure there is at least one item
  map(x => x.drop(1).map(y => (y,x(0)) )). // get inverted pairs for all lines
  toList.                           // ? required but i'm not sure why...
  flatMap(x => x).                  // flatten to pairs -- you could filter on these
  groupBy(_._1).                    // group by the first key
  map(p => (p._1,p._2.map(_._2).toSet)) // map over groups values, turning 2nd value into sets
}

Ah, but it is defined on Streams:

def invertedIndex(fn:String) = {
  io.Source.fromFile(fn).getLines.        // this is an iterator over lines
  map(_.split("\t")).                     // split at tabs
  filter(_.size > 0).                     // make sure there is at least one item
  map(x => x.drop(1).map(y => (y,x(0)) )) // get inverted pairs for all lines
  toStream.                               // convert to Stream
  flatten.                                // flatten to pairs -- you could filter on these
  groupBy(_._1).                          // group by the first key
  map(p => (p._1,p._2.map(_._2).toSet))   // map over groups values, turning 2nd value into sets
}