private void printCentroids(Matrix features, List<Cluster> clusters) {
		for (Cluster cluster : clusters) {
			System.out.print("Centroid " + cluster._id + " = ");
			for (int i = 1; i < features.cols(); i++) {
				
				double val = cluster._centroid[i];
				if (val == MISSING)
					System.out.print("?");
				else {
					int attributeValues = features.valueCount(i);
					if (attributeValues == 0)
						System.out.print(df.format(val));
					else
						System.out.print(features.attrValue(i, (int) val));
				}
				
				if (i == features.cols()-1)
					System.out.println();
				else
					System.out.print(", ");
				
			}
		}
	}