fix: best_solution for multi-objective optimization

Crylab · web-flow · commit 7e068ddd25ed · 2025-07-08T01:42:10.000+02:00
This fix allows to optimize both single-objective and multi-objective optimization cases
diff --git a/pygad/pygad.py b/pygad/pygad.py
@@ -2227,8 +2227,19 @@ def best_solution(self, pop_fitness=None):
                 raise ValueError(f"The type of the 'pop_fitness' parameter is expected to be list, tuple, or numpy.ndarray but ({type(pop_fitness)}) found.")
 
             # Return the index of the best solution that has the best fitness value.
-            best_match_idx = numpy.where(
-                pop_fitness == numpy.max(pop_fitness))[0][0]
+            # For multi-objective optimization: find the index of the solution with the maximum fitness in the first objective,
+            # break ties using the second objective, then third, etc.
+            pop_fitness_arr = numpy.array(pop_fitness)
+            # Get the indices that would sort by all objectives in descending order
+            sorted_indices = numpy.lexsort([ -pop_fitness_arr[:,i] for i in reversed(range(pop_fitness_arr.shape[1])) ])
+            best_match_idx = sorted_indices[0]
+            maximum_fitness_value = pop_fitness_arr[best_match_idx]
+
+            best_match_list = numpy.where(
+                pop_fitness == maximum_fitness_value)
+            
+            best_match_idx = best_match_list[0][0]  # Get the first index of the best match.
+
 
             best_solution = self.population[best_match_idx, :].copy()
             best_solution_fitness = pop_fitness[best_match_idx]
@@ -2494,4 +2505,4 @@ def load(filename):
     except:
         # raise BaseException("Error loading the file. If the file already exists, please reload all the functions previously used (e.g. fitness function).")
         raise BaseException("Error loading the file.")
-    return ga_in
+    return ga_in
