Optimize loop over stress field indices

source/material_model/rheology/visco_plastic.cc

@@ -534,8 +534,8 @@ namespace aspect
             // Set a mask (false) for the 2*n_independent_components fields representing the viscoelastic
             // stress tensor components so that they are excluded from the volume fraction computation.
             const std::vector<unsigned int> &stress_field_indices = this->introspection().get_indices_for_fields_of_type(CompositionalFieldDescription::stress);
-            for (auto it = stress_field_indices.begin(); it != stress_field_indices.end(); ++it)
-              composition_mask.set(*it, false);
+            for (unsigned int field_index: stress_field_indices)
+              composition_mask.set(field_index, false);
           }
 
         return composition_mask;