[{'role': 'system', 'content': "You are an expert in machine learning and your task is to classify code snippets into one of the following categories: ['helper_functions', 'load_data', 'data_exploration', 'data_preprocessing', 'evaluation', 'modelling', 'prediction', 'result_visualization', 'save_results', 'comment_only']."}, {'role': 'user', 'content': "Identify the single most appropriate category to which the provided code belongs. Provide your response as a single category label without any additional information, commentary, or personal opinions.\\nPython Code:def user_pref_vectorization(users, features_list):\nall_users_dict = dict()\nfor user in users:\none_user = big_df[big_df.user_id == user]\nnormalize_by = len(one_user.user_id)*5\nuser_ratings = one_user.stars_x\nuser_ratings_indexes = user_ratings.index\nuser_categories_indexes = one_user.categories.index\nuser_indexes = user_categories_indexes\nuser_pref_dict = dict()\nfor feature in features_list:\nuser_pref_dict[feature] = 0\nfor index in user_indexes:\nuser_review_rating = user_ratings[user_ratings.index == index].values[0]\nuser_review_categories = one_user.categories[one_user.index == index].values[0]\nfor user_review_category in user_review_categories:\nuser_pref_dict[user_review_category] = (user_pref_dict[user_review_category] + user_review_rating)/normalize_by\nall_users_dict[user] = user_pref_dict\nusers_pref_df = pd.DataFrame.from_dict(all_users_dict, orient='index', dtype=None)\nreturn users_pref_df"}, {'role': 'assistant', 'content': 'data_preprocessing'}]