Getting a HashMap from Scala's HashMap.mapValues?

Question

The example below is a self-contained example I've extracted from my larger app.

Is there a better way to get a HashMap after calling mapValues below? I'm new to Scala, so it's very likely that I'm going about this all wrong, in which case feel free to suggest a completely different approach. (An apparently obvious solution would be to move the logic in the mapValues to inside the accum but that would be tricky in the larger app.)

#!/bin/sh
exec scala "$0" "$@"
!#

import scala.collection.immutable.HashMap

case class Quantity(val name: String, val amount: Double)

class PercentsUsage {
  type PercentsOfTotal = HashMap[String, Double]

  var quantities = List[Quantity]()

  def total: Double = (quantities map { t => t.amount }).sum

  def addQuantity(qty: Quantity) = {
    quantities = qty :: quantities
  }

  def percentages: PercentsOfTotal = {
    def accum(m: PercentsOfTotal, qty: Quantity) = {
      m + (qty.name -> (qty.amount + (m getOrElse (qty.name, 0.0))))
    }
    val emptyMap = new PercentsOfTotal()

    // The `emptyMap ++` at the beginning feels clumsy, but it does the
    // job of giving me a PercentsOfTotal as the result of the method.
    emptyMap ++ (quantities.foldLeft(emptyMap)(accum(_, _)) mapValues (dollars => dollars / total))
  }
}

val pu = new PercentsUsage()

pu.addQuantity(new Quantity("A", 100))
pu.addQuantity(new Quantity("B", 400))

val pot = pu.percentages
println(pot("A")) // prints 0.2
println(pot("B")) // prints 0.8

Answer 1

Rather than using a mutable HashMap to build up your Map, you can just use scala collections' built in groupBy function. This creates a map from the grouping property to a list of the values in that group, which can then be aggregated, e.g. by taking a sum:

def percentages: Map[String, Double] = {
   val t = total
   quantities.groupBy(_.name).mapValues(_.map(_.amount).sum / t)
}

This pipeline transforms your List[Quantity] => Map[String, List[Quantity]] => Map[String, Double] giving you the desired result.