Module hw06
Package provided.listFW.visitors

Class FoldRAlgo

java.lang.Object
provided.listFW.visitors.FoldRAlgo
All Implemented Interfaces:
IListAlgo
public class FoldRAlgo extends Object implements IListAlgo
Performs the fold-right (reverse accumulation) algorithm using the given IAccumulator. The general reverse accumulation algorithm would not take an accumulator as a top-level input parameter, instead the accumulator would be hidden inside the algorithm. But since the fold process, makes the folding process invariant and the accumulator the variant piece, the accumulator is thus necessarily an input parameter here.
Author:
swong

  • Constructor Details

    • FoldRAlgo

      public FoldRAlgo()

  • Method Details

    • emptyCase

      public Object emptyCase​(MTList host, Object... acc)
      Description copied from interface: IListAlgo
      Operates on MTList, the empty list. Since IEmptyList has only one method, execute(), there is not much the host list can do here. So the code for the empty case usually involves:
      • inp, and perhaps
      • host.execute(..., ...).
      Specified by:
      emptyCase in interface IListAlgo
      Parameters:
      host - the MTList that is executing this algorithm.
      acc - a variable number of input parameters that can be used for any purpose.
      Returns:
      result from calling this method. The type of the result is problem-specific and may be null.

    • nonEmptyCase

      public Object nonEmptyCase​(NEList host, Object... acc)
      Description copied from interface: IListAlgo
      Operates on NEList, a non-empty list. The host list now has a first and a rest. So the code here usually involves what host.getFirst() and host.getRest() can do to accomplish the task at hand.
      • host.getFirst() is simply a data Object that the host list holds. It is problem-specific, and thus what it can do depends on the problem the current algorithm is trying to solve.
      • host.getRest() in contrast is an IList! What can an IList do? execute an alogrithm IListAlgo with some input. What IListAlgo can that be? The current algorithm that is being executed is as good a candidate as any other IListAlgo. In Java, we use the key word this to reference the current receiver of the method call. Having the rest of host (recursively) execute the current algorithm is expressed in Java as: host.getRest().execute(this, ...).

      To summarize, the code for the non-empty case usually involves:

      • host.getFirst(), and the recursive call
      • host.getRest().execute(this, something involving inp).
      Specified by:
      nonEmptyCase in interface IListAlgo
      Parameters:
      host - the NEList that is executing this algorithm
      acc - a variable number of input parameters that can be used for any purpose.
      Returns:
      result from calling this method. The type of the result is problem-specific and may be null.