Scala TreeSet: foreach vs foldLeft

Here is the best article I found to understand what is foldLeft. And here is an example on how to replace foreach with foldLeft:

class KbdKey() {
  def generateQuartets(): ListSet[Quartet] = {...}
}

class KbdMatrix() {
  var ts_keys = new TreeSet[KbdKey]()

  var l_quartets = ListSet.empty[Quartet];
  ts_keys.foreach(l_quartets ++= _.generateQuartets())

  // 2 lines above can be replaced by the line below

  val l_quartets = ts_keys.tail.foldLeft(ts_keys.head.generateQuartets)(_ ++ _.generateQuartets)
}

Scala Treeset example: partition

var ts_keys = new TreeSet[KbdKey]()(new CompareRowThenCol())

val (ts_inferiorOrEqualRowKeys,ts_superiorRowKeys) = ts_Keys.partition(_.i_row<=i_row)

class CompareRowThenCol extends Ordering[KbdKey] {
    def compare(k1: KbdKey, k2: KbdKey) = k1.value-k2.value
}

Scala Treeset example: filter, groupBy, foldLeft, sortBy, etc...

A scala example doing various operations on a TreeSet containing keys organized in rows and columns and who have a certain type. Output is an html table.

def myPrint(i_type: Int, ts_keys: TreeSet[KbdKey]) {

  val lcol = (0 to 16)

  print(lcol.tail.foldLeft("Column [" + lcol.head +"]")(_ + "Column [" + _ +"]"))

  val ts_matrixFilteredByType = ts_keys.filter((k: KbdKey) => k.i_type==i_type)

  val m_matrixGroupByRow = ts_matrixFilteredByType.groupBy((k: KbdKey) => k.i_row)

  val m_matrixGroupBySortedRow = ListMap(m_matrixGroupByRow.toList.sortBy{_._1}:_*)

  m_matrixGroupBySortedRow.foreach((p:(Int, TreeSet[KbdKey])) => 
    print(p._2.tail.foldLeft("Row["+p._1+"]" + p._2.head.myprint)(_ + _.myprint) +"-"))
}

Things to notice are:

• use of foldLeft to do some printing and not some plain summing "as usual".
• groupBy returns pairs.