拜托，我想在數(shù)據(jù)框中找到客戶端中的最后一個(gè)值，我該怎么做？例子 ：df = pd.DataFrame({'date':     ['2018-06-13', '2018-06-14', '2018-06-15', '2018-06-16'],    'gain': [[10, 12, 15],[14, 11, 15],[9, 10, 12], [6, 4, 2]],    'how':  [['customer1', 'customer2', 'customer3'],             ['customer4','customer5','customer6' ],            ['customer7', 'customer8', 'customer9'],            ['customer5', 'customer6', 'customer10'] ]}   df :            date       gain                    how    0 2018-06-13  [10, 12, 15]    [customer1, customer2, customer3]    1 2018-06-14  [14, 11, 15]    [customer4, customer5, customer6]    2 2018-06-15  [9, 10, 12]     [customer7, customer8, customer9]    3 2018-06-16  [6, 4, 2]       [customer5, customer6, customer10]I want to do a function that returns the last gain in the dataframe.example : for the customer5 = 6 for the customer4 = 14 for the customer20 = 'not found' 太感謝了