[MRG] EHN add voting paramter for ClusterCentroids

doc/introduction.rst

@@ -9,7 +9,8 @@ Introduction
 API's of imbalanced-learn samplers
 ----------------------------------
 
-The available samplers follows the scikit-learn API using the base estimator and adding a sampling functionality throw the ``sample`` method::
+The available samplers follows the scikit-learn API using the base estimator
+and adding a sampling functionality throw the ``sample`` method::
 
 :Estimator:
 

doc/whats_new.rst

@@ -40,6 +40,10 @@ Bug fixes
 New features
 ~~~~~~~~~~~~
 
+- :class:`under_sampling.ClusterCentroids` accepts a parameter ``voting``
+  allowing to use nearest-neighbors of centroids instead of centroids
+  themselves. It is more efficient for sparse input. By `Guillaume Lemaitre`_.
+
 - Turn off steps in :class:`pipeline.Pipeline` using the `None`
   object. By `Christos Aridas`_.
 

examples/under-sampling/plot_cluster_centroids.py

@@ -24,7 +24,7 @@
 X, y = make_classification(n_classes=2, class_sep=2, weights=[0.1, 0.9],
                            n_informative=3, n_redundant=1, flip_y=0,
                            n_features=20, n_clusters_per_class=1,
-                           n_samples=200, random_state=10)
+                           n_samples=50, random_state=10)
 
 # Instanciate a PCA object for the sake of easy visualisation
 pca = PCA(n_components=2)
@@ -34,25 +34,46 @@
 # Apply Cluster Centroids
 cc = ClusterCentroids()
 X_resampled, y_resampled = cc.fit_sample(X, y)
-X_res_vis = pca.transform(X_resampled)
+X_res_vis_soft = pca.transform(X_resampled)
+
+# Use hard voting instead of soft voting
+cc = ClusterCentroids(voting='hard')
+X_resampled, y_resampled = cc.fit_sample(X, y)
+X_res_vis_hard = pca.transform(X_resampled)
 
 # Two subplots, unpack the axes array immediately
-f, (ax1, ax2) = plt.subplots(1, 2)
+f, (ax1, ax2, ax3) = plt.subplots(1, 3, figsize=(15, 5))
 
 c0 = ax1.scatter(X_vis[y == 0, 0], X_vis[y == 0, 1], label="Class #0",
                  alpha=0.5)
 c1 = ax1.scatter(X_vis[y == 1, 0], X_vis[y == 1, 1], label="Class #1",
                  alpha=0.5)
 ax1.set_title('Original set')
 
-ax2.scatter(X_res_vis[y_resampled == 0, 0], X_res_vis[y_resampled == 0, 1],
+ax2.scatter(X_res_vis_soft[y_resampled == 0, 0],
+            X_res_vis_soft[y_resampled == 0, 1],
+            label="Class #0", alpha=.5)
+ax2.scatter(X_res_vis_soft[y_resampled == 1, 0],
+            X_res_vis_soft[y_resampled == 1, 1],
+            label="Class #1", alpha=.5)
+c2 = ax2.scatter(X_vis[y == 1, 0],
+                 X_vis[y == 1, 1], label="Original #1",
+                 alpha=0.2)
+ax2.set_title('Cluster centroids with soft voting')
+
+ax3.scatter(X_res_vis_hard[y_resampled == 0, 0],
+            X_res_vis_hard[y_resampled == 0, 1],
             label="Class #0", alpha=.5)
-ax2.scatter(X_res_vis[y_resampled == 1, 0], X_res_vis[y_resampled == 1, 1],
+ax3.scatter(X_res_vis_hard[y_resampled == 1, 0],
+            X_res_vis_hard[y_resampled == 1, 1],
             label="Class #1", alpha=.5)
-ax2.set_title('Cluster centroids')
+ax3.scatter(X_vis[y == 1, 0],
+            X_vis[y == 1, 1],
+            alpha=0.2)
+ax3.set_title('Cluster centroids with hard voting')
 
 # make nice plotting
-for ax in (ax1, ax2):
+for ax in (ax1, ax2, ax3):
     ax.spines['top'].set_visible(False)
     ax.spines['right'].set_visible(False)
     ax.get_xaxis().tick_bottom()
@@ -62,7 +83,8 @@
     ax.set_xlim([-6, 8])
     ax.set_ylim([-6, 6])
 
-plt.figlegend((c0, c1), ('Class #0', 'Class #1'), loc='lower center',
-              ncol=2, labelspacing=0.)
+plt.figlegend((c0, c1), ('Class #0', 'Class #1', 'Original Class #1'),
+              loc='lower center',
+              ncol=3, labelspacing=0.)
 plt.tight_layout(pad=3)
 plt.show()

imblearn/under_sampling/prototype_generation/cluster_centroids.py

@@ -12,10 +12,13 @@
 from scipy import sparse
 
 from sklearn.cluster import KMeans
+from sklearn.neighbors import NearestNeighbors
 from sklearn.utils import safe_indexing
 
 from ..base import BaseUnderSampler
 
+VOTING_KIND = ('auto', 'hard', 'soft')
+
 
 class ClusterCentroids(BaseUnderSampler):
     """Perform under-sampling by generating centroids based on
@@ -58,6 +61,18 @@ class ClusterCentroids(BaseUnderSampler):
     estimator : object, optional(default=KMeans())
         Pass a :class:`sklearn.cluster.KMeans` estimator.
 
+    voting : str, optional (default='auto')
+        Voting strategy to generate the new samples:
+
+        - If ``'hard'``, the nearest-neighbors of the centroids found using the
+          clustering algorithm will be used.
+        - If ``'soft'``, the centroids found by the clustering algorithm will
+          be used.
+        - If ``'auto'``, if the input is sparse, it will default on ``'hard'``
+          otherwise, ``'soft'`` will be used.
+
+        .. versionadded:: 0.3.0
+
     n_jobs : int, optional (default=1)
         The number of threads to open if possible.
 
@@ -91,10 +106,12 @@ def __init__(self,
                  ratio='auto',
                  random_state=None,
                  estimator=None,
+                 voting='auto',
                  n_jobs=1):
         super(ClusterCentroids, self).__init__(
             ratio=ratio, random_state=random_state)
         self.estimator = estimator
+        self.voting = voting
         self.n_jobs = n_jobs
 
     def _validate_estimator(self):
@@ -108,6 +125,22 @@ def _validate_estimator(self):
             raise ValueError('`estimator` has to be a KMeans clustering.'
                              ' Got {} instead.'.format(type(self.estimator)))
 
+    def _generate_sample(self, X, y, centroids, target_class):
+        if self.voting_ == 'hard':
+            nearest_neighbors = NearestNeighbors(n_neighbors=1)
+            nearest_neighbors.fit(X, y)
+            indices = nearest_neighbors.kneighbors(centroids,
+                                                   return_distance=False)
+            X_new = safe_indexing(X, np.squeeze(indices))
+        else:
+            if sparse.issparse(X):
+                X_new = sparse.csr_matrix(centroids)
+            else:
+                X_new = centroids
+        y_new = np.array([target_class] * centroids.shape[0])
+
+        return X_new, y_new
+
     def _sample(self, X, y):
         """Resample the dataset.
 
@@ -131,28 +164,37 @@ def _sample(self, X, y):
         """
         self._validate_estimator()
 
-        idx_under = np.empty((0, ), dtype=int)
-        centroids, y_resampled = [], []
+        if self.voting == 'auto':
+            if sparse.issparse(X):
+                self.voting_ = 'hard'
+            else:
+                self.voting_ = 'soft'
+        else:
+            if self.voting in VOTING_KIND:
+                self.voting_ = self.voting
+            else:
+                raise ValueError("'voting' needs to be one of {}. Got {}"
+                                 " instead.".format(VOTING_KIND, self.voting))
+
+        X_resampled, y_resampled = [], []
         for target_class in np.unique(y):
             if target_class in self.ratio_.keys():
                 n_samples = self.ratio_[target_class]
                 self.estimator_.set_params(**{'n_clusters': n_samples})
                 self.estimator_.fit(X[y == target_class])
-                centroids.append(self.estimator_.cluster_centers_)
-                y_resampled += [target_class] * n_samples
-
+                X_new, y_new = self._generate_sample(
+                    X, y, self.estimator_.cluster_centers_, target_class)
+                X_resampled.append(X_new)
+                y_resampled.append(y_new)
             else:
                 target_class_indices = np.flatnonzero(y == target_class)
-                idx_under = np.concatenate(
-                    (idx_under, target_class_indices), axis=0)
-
-        X_resampled = np.concatenate((centroids))
+                X_resampled.append(safe_indexing(X, target_class_indices))
+                y_resampled.append(safe_indexing(y, target_class_indices))
 
         if sparse.issparse(X):
-            X_resampled = sparse.vstack([sparse.csr_matrix(X_resampled),
-                                         safe_indexing(X, idx_under)])
+            X_resampled = sparse.vstack(X_resampled)
         else:
-            X_resampled = np.vstack((X_resampled, safe_indexing(X, idx_under)))
-        y_resampled = np.hstack((y_resampled, safe_indexing(y, idx_under)))
+            X_resampled = np.vstack(X_resampled)
+        y_resampled = np.hstack(y_resampled)
 
         return X_resampled, np.array(y_resampled)

imblearn/under_sampling/prototype_generation/tests/test_cluster_centroids.py

@@ -4,11 +4,11 @@
 from collections import Counter
 
 import numpy as np
+from scipy import sparse
 from pytest import raises
 
 from sklearn.utils.testing import assert_allclose
 from sklearn.utils.testing import assert_array_equal
-
 from sklearn.cluster import KMeans
 
 from imblearn.under_sampling import ClusterCentroids
@@ -23,17 +23,26 @@
 R_TOL = 1e-4
 
 
+def test_fit_sample_check_voting():
+    cc = ClusterCentroids(random_state=RND_SEED)
+    cc.fit_sample(X, Y)
+    assert cc.voting_ == 'soft'
+    cc = ClusterCentroids(random_state=RND_SEED)
+    cc.fit_sample(sparse.csr_matrix(X), Y)
+    assert cc.voting_ == 'hard'
+
+
 def test_fit_sample_auto():
     ratio = 'auto'
     cc = ClusterCentroids(ratio=ratio, random_state=RND_SEED)
     X_resampled, y_resampled = cc.fit_sample(X, Y)
-    X_gt = np.array([[0.06738818, -0.529627],
-                     [0.17901516, 0.69860992],
-                     [0.094035, -2.55298982],
-                     [0.92923648, 0.76103773],
+    X_gt = np.array([[0.92923648, 0.76103773],
                      [0.47104475, 0.44386323],
-                     [0.13347175, 0.12167502]])
-    y_gt = np.array([1, 1, 1, 0, 0, 0])
+                     [0.13347175, 0.12167502],
+                     [0.06738818, -0.529627],
+                     [0.17901516, 0.69860992],
+                     [0.094035, -2.55298982]])
+    y_gt = np.array([0, 0, 0, 1, 1, 1])
     assert_allclose(X_resampled, X_gt, rtol=R_TOL)
     assert_array_equal(y_resampled, y_gt)
 
@@ -42,16 +51,16 @@ def test_fit_sample_half():
     ratio = .5
     cc = ClusterCentroids(ratio=ratio, random_state=RND_SEED)
     X_resampled, y_resampled = cc.fit_sample(X, Y)
-    X_gt = np.array([[0.09125309, -0.85409574],
+    X_gt = np.array([[0.92923648, 0.76103773],
+                     [0.47104475, 0.44386323],
+                     [0.13347175, 0.12167502],
+                     [0.09125309, -0.85409574],
                      [0.19220316, 0.32337101],
                      [0.094035, -2.55298982],
                      [0.20792588, 1.49407907],
                      [0.04352327, -0.20515826],
-                     [0.12372842, 0.6536186],
-                     [0.92923648, 0.76103773],
-                     [0.47104475, 0.44386323],
-                     [0.13347175, 0.12167502]])
-    y_gt = np.array([1, 1, 1, 1, 1, 1, 0, 0, 0])
+                     [0.12372842, 0.6536186]])
+    y_gt = np.array([0, 0, 0, 1, 1, 1, 1, 1, 1])
     assert_allclose(X_resampled, X_gt, rtol=R_TOL)
     assert_array_equal(y_resampled, y_gt)
 
@@ -75,21 +84,48 @@ def test_fit_sample_object():
         ratio=ratio, random_state=RND_SEED, estimator=cluster)
 
     X_resampled, y_resampled = cc.fit_sample(X, Y)
-    X_gt = np.array([[0.06738818, -0.529627],
+    X_gt = np.array([[0.92923648, 0.76103773],
+                     [0.47104475, 0.44386323],
+                     [0.13347175, 0.12167502],
+                     [0.06738818, -0.529627],
                      [0.17901516, 0.69860992],
-                     [0.094035, -2.55298982],
-                     [0.92923648, 0.76103773],
+                     [0.094035, -2.55298982]])
+    y_gt = np.array([0, 0, 0, 1, 1, 1])
+    assert_allclose(X_resampled, X_gt, rtol=R_TOL)
+    assert_array_equal(y_resampled, y_gt)
+
+
+def test_fit_hard_voting():
+    ratio = 'auto'
+    voting = 'hard'
+    cluster = KMeans(random_state=RND_SEED)
+    cc = ClusterCentroids(
+        ratio=ratio, random_state=RND_SEED, estimator=cluster,
+        voting=voting)
+
+    X_resampled, y_resampled = cc.fit_sample(X, Y)
+    X_gt = np.array([[0.92923648, 0.76103773],
                      [0.47104475, 0.44386323],
-                     [0.13347175, 0.12167502]])
-    y_gt = np.array([1, 1, 1, 0, 0, 0])
+                     [0.13347175, 0.12167502],
+                     [0.09125309, -0.85409574],
+                     [0.12372842, 0.6536186],
+                     [0.094035, -2.55298982]])
+    y_gt = np.array([0, 0, 0, 1, 1, 1])
     assert_allclose(X_resampled, X_gt, rtol=R_TOL)
     assert_array_equal(y_resampled, y_gt)
+    for x in X_resampled:
+        assert np.any(np.all(x == X, axis=1))
 
 
-def test_fit_sample_wrong_object():
+def test_fit_sample_error():
     ratio = 'auto'
     cluster = 'rnd'
     cc = ClusterCentroids(
         ratio=ratio, random_state=RND_SEED, estimator=cluster)
     with raises(ValueError, match="has to be a KMeans clustering"):
         cc.fit_sample(X, Y)
+
+    voting = 'unknown'
+    cc = ClusterCentroids(ratio=ratio, voting=voting, random_state=RND_SEED)
+    with raises(ValueError, match="needs to be one of"):
+        cc.fit_sample(X, Y)

imblearn/utils/estimator_checks.py

@@ -271,6 +271,7 @@ def check_samplers_sparse(name, Sampler):
     elif isinstance(Sampler(), ClusterCentroids):
         # set KMeans to full since it support sparse and dense
         samplers = [Sampler(random_state=0,
+                            voting='soft',
                             estimator=KMeans(random_state=1,
                                              algorithm='full'))]
     else: