[MRG] FIX n_iter attribute for KMeans, algorithm=elkan (scikit-learn#11353)

Author: jeremiedbb

doc/whats_new/v0.20.rst

@@ -545,6 +545,11 @@ Decomposition, manifold learning and clustering
   by :user:`Vighnesh Birodkar <vighneshbirodkar>` and
   :user:`Olivier Grisel <ogrisel>`.
 
+- Fixed a bug in :func:`cluster.k_means_elkan` where the returned `iteration`
+  was 1 less than the correct value. Also added the missing `n_iter_` attribute
+  in the docstring of :class:`cluster.KMeans`. :issue:`11353` by
+  :user:`Jeremie du Boisberranger <jeremiedbb>`.
+
 Metrics
 
 - Fixed a bug in :func:`metrics.precision_recall_fscore_support`

sklearn/cluster/_k_means_elkan.pyx

@@ -258,4 +258,4 @@ def k_means_elkan(np.ndarray[floating, ndim=2, mode='c'] X_,
         update_labels_distances_inplace(X_p, centers_p, center_half_distances,
                                         labels, lower_bounds, upper_bounds,
                                         n_samples, n_features, n_clusters)
-    return centers_, labels_, iteration
+    return centers_, labels_, iteration + 1

sklearn/cluster/k_means_.py

@@ -859,6 +859,9 @@ class KMeans(BaseEstimator, ClusterMixin, TransformerMixin):
     inertia_ : float
         Sum of squared distances of samples to their closest cluster center.
 
+    n_iter_ : int
+        Number of iterations run.
+
     Examples
     --------
 

sklearn/cluster/tests/test_k_means.py

@@ -979,3 +979,11 @@ def test_check_sample_weight():
     assert_equal(_num_samples(X), _num_samples(checked_sample_weight))
     assert_almost_equal(checked_sample_weight.sum(), _num_samples(X))
     assert_equal(X.dtype, checked_sample_weight.dtype)
+
+
+def test_iter_attribute():
+    # Regression test on bad n_iter_ value. Previous bug n_iter_ was one off
+    # it's right value (#11340).
+    estimator = KMeans(algorithm="elkan", max_iter=1)
+    estimator.fit(np.random.rand(10, 10))
+    assert estimator.n_iter_ == 1