Fix the building reference index when ref_index is None in DISE (#…

…264)

* Fix the return type when calculating medoid in `get_initial_selection`

* Add test case for ref_index is None

* Remove redundant checking of ref_index

selector/methods/distance.py

@@ -507,8 +507,6 @@ def __init__(
         """
         self.r0 = r0
         self.r = r0
-        if ref_index is not None and ref_index < 0:
-            raise ValueError(f"ref_index must be a non-negative integer, got {ref_index}.")
         self.ref_index = ref_index
         self.tol = tol
         self.n_iter = n_iter
@@ -677,7 +675,7 @@ def get_initial_selection(x=None, x_dist=None, ref_index=None, fun_dist=None):
         if x_dist is None:
             x_dist = fun_dist(x)
         # calculate the medoid center
-        initial_selections = [np.argmin(np.sum(x_dist, axis=0))]
+        initial_selections = [int(np.argmin(np.sum(x_dist, axis=0)))]
 
     # the length of the distance matrix is the number of samples
     if x_dist is not None:

selector/methods/tests/test_distance.py

@@ -301,6 +301,16 @@ def test_directed_sphere_same_number_of_pts():
     assert_equal(collector.r, 1)
 
 
+def test_directed_sphere_same_number_of_pts_None():
+    """Test DirectSphereExclusion with `size` = number of points in dataset with the ref_index None."""
+    # None as the reference point
+    x = np.array([[0, 0], [0, 1], [0, 2], [0, 3], [0, 4]])
+    collector = DISE(r0=1, tol=0, ref_index=None)
+    selected = collector.select(x, size=3)
+    assert_equal(selected, [2, 0, 4])
+    assert_equal(collector.r, 1)
+
+
 def test_directed_sphere_exclusion_select_more_number_of_pts():
     """Test DirectSphereExclusion on points on the line with `size` < number of points in dataset."""
     # (0,0) as the reference point