Returns
		-------
		np.array

		Integer document indices

def get_doc_indices(self):
		'''
		Returns
		-------
		np.array

		Integer document indices
		'''
		if self._document_category_df is None:
			return pd.np.array([])
		categories_d = {d: i for i, d in enumerate(self.get_categories())}
		return self._document_category_df.category.apply(categories_d.get).values