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How do I make a recursive GetEnumerator() ?

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1 post since Aug 2008
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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.

Attachments Program.txt (3.48KB)
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1,199 posts since Aug 2007
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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;
    return varStack.GetEnumerator();
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