通过分组减少/折叠scala序列

Question

In scala, given an Iterable of pairs, say Iterable[(String, Int]) , is there a way to accumulate or fold over the ._2 s based on the ._1 s? Like in the following, add up all the #s that come after A and separately the # after B

List(("A", 2), ("B", 1), ("A", 3))

I could do this in 2 steps with groupBy

val mapBy1 = list.groupBy( _._1 )
for ((key,sublist) <- mapBy1) yield (key, sublist.foldLeft(0) (_+_._2))

but then I would be allocating the sublists, which I would rather avoid.

Answer 1

You could do something like:

list.foldLeft(Map[String, Int]()) {
   case (map, (k,v)) => map + (k -> (map.getOrElse(k, 0) + v))
}

Answer 2

You could build the Map as you go and convert it back to a List after the fact.

listOfPairs.foldLeft(Map[String,Int]().withDefaultValue(0)){
      case (m,(k,v)) => m + (k -> (v + m(k)))
    }.toList

Answer 3

You could also use groupBy with mapValues :

list.groupBy(_._1).mapValues(_.map(_._2).sum).toList
res1: List[(String, Int)] = List((A,5), (B,1))