Reads a CSV file with header and rows where first column is the key, writes into new file without header and where lines with duplicated key are merged into a single one

JoinDuplicatedLines.scala

import java.io.File
import java.io.PrintWriter

import scala.annotation.migration
import scala.collection.immutable.ListMap
import scala.collection.mutable.Map

object JoinDuplicatedLines {
  def main(args: Array[String]) {
    val input = io.Source fromFile "input.csv"
    val map = Map.empty[String, String]

    try {
      for (line <- input.getLines drop 1) {
        val array = line.split(";") map (_.trim)
        map get (array(0)) match {
          case Some(str) => map += array(0) -> str.concat(";").concat(array.drop(1).mkString(";"))
          case None      => map += array(0) -> array.mkString(";")
        }
      }
    } finally { input close }

    val output = new PrintWriter(new File("output.csv"))
    val sortedMap = ListMap(map.toSeq sortBy (_._1): _*)

    try { sortedMap.values foreach { v => output println (v) } }
    finally { output close }
  }
}