Raise Error whenever incorrect bounds are given

Author: MichalisPanayides

ciw/tests/test_state_tracker.py

@@ -1199,4 +1199,19 @@ def test_compare_state_probabilities_to_analytical(self):
             self.assertEqual(round(state_probs[state], 2), round(expected_probs[state], 2))
 
         error_squared = sum([(state_probs[i] - expected_probs[i])**2 for i in sorted(state_probs.keys())])
-        self.assertEqual(round(error_squared, 4), 0)
+        self.assertEqual(round(error_squared, 4), 0)
+
+    def test_error_checking_for_state_probabilities(self):
+        ciw.seed(0)
+        N = ciw.create_network(
+            arrival_distributions=[ciw.dists.Exponential(1)],
+            service_distributions=[ciw.dists.Exponential(2)],
+            number_of_servers=[1]
+        )
+        Q = ciw.Simulation(N, tracker=ciw.trackers.SystemPopulation())
+        Q.simulate_until_max_time(10)
+
+        self.assertRaises(ValueError, Q.statetracker.state_probabilities, (-1, 5))
+        self.assertRaises(ValueError, Q.statetracker.state_probabilities, (4, 2))
+        self.assertRaises(ValueError, Q.statetracker.state_probabilities, (-1, -4))
+        self.assertRaises(ValueError, Q.statetracker.state_probabilities, (3, 3))

ciw/trackers/state_tracker.py

@@ -59,6 +59,9 @@ def state_probabilities(self, observation_period=(0, float("Inf"))):
         prev_date = self.history[0][0]    
         prev_state = self.history[0][1]
 
+        if start < 0 or end <= start:
+            raise ValueError('Observation period need to be a positive interval above zero')
+
         for event in self.history:
             date = event[0]
             state = event[1]