What type to use to store an in-memory mutable data table in Scala?

Question

Each time a function is called, if it\'s result for a given set of argument values is not yet memoized I\'d like to put the result into an in-memory table. One column is mea

Answer 1

In addition to Landei's answer, I also want to suggest the bottom-up (non-memoization) way of doing DP in Scala is possible, and the core idea is to use foldLeft(s).

Example for computing Fibonacci numbers

  def fibo(n: Int) = (1 to n).foldLeft((0, 1)) {
    (acc, i) => (acc._2, acc._1 + acc._2)
  }._1

Example for longest increasing subsequence

def longestIncrSubseq[T](xs: List[T])(implicit ord: Ordering[T]) = {
  xs.foldLeft(List[(Int, List[T])]()) {
    (memo, x) =>
      if (memo.isEmpty) List((1, List(x)))
      else {
        val resultIfEndsAtCurr = (memo, xs).zipped map {
          (tp, y) =>
            val len = tp._1
            val seq = tp._2
            if (ord.lteq(y, x)) { // current is greater than the previous end
              (len + 1, x :: seq) // reversely recorded to avoid O(n)
            } else {
              (1, List(x)) // start over
            }
        }
        memo :+ resultIfEndsAtCurr.maxBy(_._1)
      }
  }.maxBy(_._1)._2.reverse
}