[{'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:import numpy as np\ndef monthly_data(df):\nmonthly_prices, periods = [], []\nperiods = []\nmy_month = df.index[0].month #checking the first month in the data\nfor j in range(df.shape[0]):\nif df.index[j].month != my_month:\nmonthly_prices.append(df['Close'][j])\nperiods.append(df.index[j])\nmy_month = df.index[j].month\nmonthly_prices = np.array(monthly_prices).astype('float32')\nreturn periods,monthly_prices"}, {'role': 'assistant', 'content': 'data_preprocessing'}]