[pyclustering.cluster.xmeans] MNDL score

[Nibbles4]

Hi,
I'm currently implementing X-means with MNDL criterion myself and had a look at your implementation for MNDL scores and noticed 2 things:

• You use addition instead of multiplication in your term for Ks:
  https://github.com/annoviko/pyclustering/blob/master/pyclustering/cluster/xmeans.py#L330
  According to [1] (11) it should be Ks = ( 2.0 * alpha * sigma / (N ** 0.5) ) * ( (alpha ** 2.0) * sigma_sqrt / N + W - Kw / 2.0 ) ** 0.5;
• This for-loop seems to effectively multiply the final MNDL score by the number of clusters:
  https://github.com/annoviko/pyclustering/blob/master/pyclustering/cluster/xmeans.py#L328-L343

[1] Mahdi Shahbaba and Soosan Beheshti. "Improving X-means clustering with MNDL." 2012 11th International Conference on Information Science, Signal Processing and their Applications (ISSPA).

[annoviko]

Hello, @Nibbles4 ,

Thank you for the bug reporting! And thank you for your comment about the for-loop. I will correct the problem and optimize the code.

[annoviko]

Have done at the revision 7c264be .