val keys = List('a','b','a')
val values = List(45,65,12)
val finalMap:Map(char:Int) =
  scala.collection.mutable.Map().withDefaultValue(0)
  0 until keys.length map (w => finalMap(keys(w)) += values(w))

scala> (keys zip values) groupBy (_._1) map { case (k,v) => (k, (v map (_._2)).sum) }
res0: scala.collection.immutable.Map[Char,Int] = Map(b -> 65, a -> 57)

scala> keys.zip(values).groupBy(_._1).mapValues(l => l.map { v =>
     | println("OK mapping")
     | v._2
     | }.sum)
OK mapping
OK mapping
OK mapping
res2: scala.collection.immutable.Map[Char,Int] = Map(b -> 65, a -> 57)

scala> res2('a')   // recomputes
OK mapping
OK mapping
res4: Int = 57

val m = keys.zip(values).groupBy(_._1).mapValues(l => l.map(_._2).sum)

keys.zip(values) = List((a, 45), (b, 65), (a, 12))

keys.zip(values).groupBy(_._1) = Map((a, List((a, 45), (a, 12))), (b, List((b, 65))))

List((a, 45), (a, 12)).map(_._2) = List(45,12)

List(45, 12).sum = 57

( Map[Char,Int]() /: (keys,values).zipped ) ( (m,kv) =>
  m + ( kv._1 -> ( m.getOrElse( kv._1, 0 ) + kv._2 ) )
)