yield - using element stored previously in list [Scala]

Question

I will give a simplified code to explain my question easily:

• I have a user balance i.e. val balance = 10
• I have a list of data to iterate through using for(elem <- dataList) yield (interest(elem, balance)).
  • In my case interest is a method which will perform calculations on the given arguments and return a number which will be: provided balance+the calculated interest.

How can I use this newly returned value for the next element in my data list in yield so I could keep up-to-date my balance for the next elements in the list?

i.e. for(elem <- dataList) yield (interest(elem, newBalanceFromPreviousYield))

Answer 1

I think, there is no easy way to use it just with yield. Your use-case reminds fold pattern, so, let's use it. We accumulate balance and use it in interest() invocation:

dataList.foldLeft(List.empty[Int]) { (acc, elem) =>
  acc match {
    case Nil =>
      // if no balances accumulated yet, use default balance = 10
      List(interest(elem, balance))
    case all@newBalance :: _ =>
      // else use accumulated balance
      interest(elem, newBalance) :: all
  }
}.reverse // we need to reverse result list due to :: usage