pins emptyset value in CHII of order 0 to the baseline value

shapiq/game_theory/exact.py

@@ -2,6 +2,7 @@
 like interaction indices or generalized values."""
 
 import copy
+import warnings
 from typing import Callable, Optional, Union
 
 import numpy as np
@@ -405,6 +406,14 @@ def base_interaction(self, index: str, order: int) -> InteractionValues:
         for i, interaction in enumerate(powerset(self._grand_coalition_set, max_size=order)):
             interaction_lookup[interaction] = i
 
+        # CHII is un-defined for empty set
+        if index == "CHII" and () in interaction_lookup:
+            warnings.warn(
+                f"CHII is not defined for the empty set. Setting to the baseline value "
+                f"{self.baseline_value}."
+            )
+            base_interaction_values[interaction_lookup[()]] = self.baseline_value
+
         # Transform into InteractionValues object and store in computed dictionary
         base_interaction = InteractionValues(
             values=base_interaction_values,

tests/tests_game_theory/test_exact_computer.py

@@ -201,7 +201,7 @@ def _interaction(arr: np.ndarray):  # dtype bool
         ("BV", 1),
         ("SII", 2),
         ("BII", 2),
-        # ("CHII", 2),  # TODO: fix this
+        ("CHII", 2),
         ("Co-Moebius", 2),
         ("SGV", 2),
         ("BGV", 2),
@@ -239,6 +239,18 @@ def permutation_game(X: np.ndarray):
         assert (value - perm_interaction_values[perm_coalition]) < 10e-7
 
 
+def test_warning_cii():
+    """Checks weather a warning is raised for the CHII index and min_order = 0."""
+    n = 5
+    soum = SOUM(n, n_basis_games=10)
+    exact_computer = ExactComputer(n_players=n, game_fun=soum)
+    with pytest.warns(UserWarning):
+        exact_computer("CHII", 0)
+
+    # check that warning is not raised for min_order > 0
+    exact_computer("CHII", 1)
+
+
 @pytest.mark.parametrize(
     "index, order",
     [