Aggregate list values in Scala

Question

Starting with a list of objects containing two parameters notional and currency, how can I aggregate the total notional per currency?

Given:

case class T         


        

Answer 1

Starting Scala 2.13, most collections are provided with the groupMapReduce method which is (as its name suggests) an equivalent (more efficient) of a groupBy followed by mapValues and a reduce step:

trades.groupMapReduce(_.currency)(_.amount)(_ + _)
// immutable.Map[String,Int] = Map(JPY -> 10000100, USD -> 10010000, GBP -> 10001000)

This:

• groups elements based on their currency (group part of groupMapReduce)

• maps grouped values to their amount (map part of groupMapReduce)

• reduces values (_ + _) by summing them (reduce part of groupMapReduce).

This is an equivalent version performed in one pass through the List of:

trades.groupBy(_.currency).mapValues(_.map(_.amount).reduce(_+_))