Question

At the moment I have

val orders = new HashMap[Int, Int]
orders.put(36, 110)
orders.put(35, 90)
orders.put(34, 80)
orders.put(33, 60)

I would like to keep a running so that the end mapping appears as follows

36 -> 110
35 -> 200
34 -> 280
33 -> 340

At the moment I do this imperatively as follows

val keys = orders.keys.toList.sortBy(x => -x)
val accum = new HashMap[Int, Int]
accum.put(keys.head, orders(keys.head))
for (i <- 1 to keys.length - 1) {
  accum.put(keys(i), orders(keys(i)) + accum(keys(i-1)))
}
accum.foreach {
  x => println(x._1, x._2)
}

Is there a more functional way of doing this using mapping, folding etc? I would be able to do it with a straight List but the can’t quite wrap my head around how to do this with HashMap

Edit: Ordering is important. The left column (36, 35, 34, 33) needs to be in descending order

Answer 1

For the record, here is a solution using the inits method:

 import scala.collection.mutable._

 // use a LinkedHashMap to keep the order
 val orders = new LinkedHashMap[Int, Int]
 orders.put(36, 110)
 orders.put(35, 90)
 orders.put(34, 80)
 orders.put(33, 60)


// create a list of init sequences with no empty element
orders.toSeq.inits.toList.dropRight(1).

  // > this returns
  // ArrayBuffer((36,110), (35,90), (34,80), (33,60)) 
  // ArrayBuffer((36,110), (35,90), (34,80)) 
  // ArrayBuffer((36,110), (35,90)) 
  // ArrayBuffer((36,110)) 

  // now take the last key of each sequence and sum the values of the sequence
  map(init => (init.last._1, init.map(_._2).sum)).reverse.toMap.mkString("\n")

  36 -> 110
  35 -> 200
  34 -> 280
  33 -> 340