[{'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 make_rel_time(meta_df):\n"""Convert exact timestamp relative to start time and sort."""\ndef compute_rel_time(col):\n"""Relative time elapse compared with start time in seconds"""\nreturn [(val - start_time).total_seconds() for val in col]\ndf_rel_sec = meta_df[[\'engine_id\']].copy()\nrel_columns = [\'rel_started\', \'rel_completed\']\nfor col in rel_columns:\ndf_rel_sec[col] = np.empty(len(df_rel_sec), np.float)\nstart_time = meta_df.started.min()\nabs_time_data = meta_df[[\'started\', \'completed\']]\nrel_time_data = abs_time_data.apply(compute_rel_time, axis=0).values\ndf_rel_sec.loc[:, rel_columns] = rel_time_data\nsorted_df = df_rel_sec.sort([\'engine_id\', \'rel_started\', \'rel_completed\'])\nsorted_df[\'order\'] = np.empty(len(sorted_df), np.int)\nfor grp_key, grp_df in sorted_df.groupby([\'engine_id\']):\nsorted_df.loc[grp_df.index, \'order\'] = np.arange(len(grp_df))\nreturn sorted_df'}, {'role': 'assistant', 'content': 'data_preprocessing'}]