Member Avatar for JohnPool

Can somebody give me advice on how to create a recursive version of GetEnumerator()?
The well-known Towers of Hanoi problem (see e.g. http://en.wikipedia.org/wiki/Tower_of_Hanoi) may serve as an example that is comparable to the actual problem I have. A simple algorithm to show all moves for a stacjk of disks of height n is:

void MoveTower0 (int n, Needle start, Needle finish, Needle temp)
{
  if (n > 0)
  {
    MoveTower0 (n - 1, start, temp, finish);
    Console.WriteLine ("Moving disk from {0} to {1}", start, finish);
    MoveTower0 (n - 1, temp, finish, start);
  }
}

What I actually want to do is set up a class HanoiTowerMoves that implements IEnumerable and that enables me to iterate over all moves as follows:

foreach (Move m in HanoiTowerMoves) Console.WriteLine (m);

The first step towards a GetEnumerator() implementation seems to get rid of the MoveTower parameters. This can easily be done by using a stack. I also introduced a class Move that combines the parameters into a single variable.

class Move
{
  public int N { private set; get; }
  public Needle Start { private set; get; }
  public Needle Finish { private set; get; }
  public Needle Temp { private set; get; }

  public Move (int n, Needle start, Needle finish, Needle temp)
  {
    N = n;
    Start = start;
    Finish = finish;
    Temp = temp;
  }

  public override string ToString ()
  {
    return string.Format ("Moving disk from {0} to {1}", Start, Finish);
  }
}

Now MoveTower can be rewritten as follows:

void MoveTower1 ()
{
  Move m = varStack.Pop ();

  if (m.N > 0)
  {
    varStack.Push (new Move (m.N - 1, m.Start, m.Temp, m.Finish));
    MoveTower1 ();
    Console.WriteLine (m);
    varStack.Push (new Move (m.N - 1, m.Temp, m.Finish, m.Start));
    MoveTower1 ();
  }
}

This version must be called as follows:

varStack.Push (new Move (n, Needle.A, Needle.B, Needle.Temp));
MoveTower1 ();

The next step towards an iterable version is to implement the class:

class HanoiTowerMoves : IEnumerable<Move>
{
  Stack<Move> varStack;
  int n; // number of disks

  public HanoiTowerMoves (int n)
  {
    this.n = n;
    varStack = new Stack<Move> ();
  }
  
  public IEnumerator<Move> GetEnumerator ()
  {
    [B]// ????????????????????????????[/B]  }
    
  // required by the compiler:
  IEnumerator IEnumerable.GetEnumerator ()
  {
    return GetEnumerator ();
  }
}

Now the big question to me is: what does the body of GetEnumerator () look like?
Can somebody solve this mystery for me?

The code of Program.cs of the console application I created can be found in the attachment.

using System;
using System.Collections.Generic;
using System.Collections;

/* Towers of Hanoi
 * ===============
 * Suppose you have a tower of N disks on needle A, which are supposed to end up on needle B.
 * The big picture is to first move the entire stack of the top N-1 disks to the Temp needle,
 * then move the N-th disk to B, then move the Temp stack to B using A as the new Temp needle.
 * This is reflected in the way the recursion is set up.
 */

namespace ConsoleApplication1
{
  static class main
  {
    static void Main (string [] args)
    {
      int n;
      Console.WriteLine ("Towers of Hanoi");

      while (true)
      {
        Console.Write ("\r\nEnter number of disks: ");

        if (!int.TryParse (Console.ReadLine (), out n))
        {
          break;
        }

        HanoiTowerMoves moves = new HanoiTowerMoves (n);
        moves.Run (1); // algorithm version number, see below
      }
    }
  }

  class Move
  {
    public int N { private set; get; }
    public Needle Start { private set; get; }
    public Needle Finish { private set; get; }
    public Needle Temp { private set; get; }

    public Move (int n, Needle start, Needle finish, Needle temp)
    {
      N = n;
      Start = start;
      Finish = finish;
      Temp = temp;
    }

    public override string ToString ()
    {
      return string.Format ("Moving disk from {0} to {1}", Start, Finish);
    }
  }

  enum Needle { A, B, Temp }

  class HanoiTowerMoves : IEnumerable<Move>
  {
    Stack<Move> varStack;
    int n;            // number of disks

    public HanoiTowerMoves (int n)
    {
      this.n = n;
      varStack = new Stack<Move> ();
    }

    public void Run (int version)
    {
      switch (version)
      {
        case 0: // Original version
          MoveTower0 (n, Needle.A, Needle.B, Needle.Temp);
          break;

        case 1: // No parameters (i.e. argument values passed via stack)
          varStack.Push (new Move (n, Needle.A, Needle.B, Needle.Temp));
          MoveTower1 ();
          break;

        case 2: // Enumeration
          foreach (Move m in this)
          {
            Console.WriteLine (m);
          }

          break;
      }
    }

    void MoveTower0 (int n, Needle start, Needle finish, Needle temp)
    {
      if (n > 0)
      {
        MoveTower0 (n - 1, start, temp, finish);
        Console.WriteLine ("Moving disk from {0} to {1}", start, finish);
        MoveTower0 (n - 1, temp, finish, start);
      }
    }

    void MoveTower1 ()
    {
      Move m = varStack.Pop ();

      if (m.N > 0)
      {
        varStack.Push (new Move (m.N - 1, m.Start, m.Temp, m.Finish));
        MoveTower1 ();
        Console.WriteLine (m);
        varStack.Push (new Move (m.N - 1, m.Temp, m.Finish, m.Start));
        MoveTower1 ();
      }
    }

    public IEnumerator<Move> GetEnumerator ()
    {
      yield break; // ????????????????????????????
    }

    /*
      void MoveTower1 ()
      {
        Move m = varStack.Pop ();

        if (m.N > 0)
        {
          varStack.Push (new Move (m.N - 1, m.Start, m.Temp, m.Finish));
          MoveTower1 ();
          Console.WriteLine (m); ? yield return m;
          varStack.Push (new Move (m.N - 1, m.Temp, m.Finish, m.Start));
          MoveTower1 ();
        }
      }
    */

    // required by the compiler:
    IEnumerator IEnumerable.GetEnumerator ()
    {
      return GetEnumerator ();
    }
  }
}
Edited by JohnPool because: n/a
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  • Latest Post Latest Post by darkagn
Member Avatar for darkagn

So you want the GetEnumerator class to return the enumerator for your internal Stack object, varStack ? I would do this:

public IEnumerator<Move> GetEnumerator()
{
  if (varStack == null)
    return null;
  else
    return varStack.GetEnumerator();
}
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