Modify the bolded code below. Instructions highlighted import os from b_tree import BTree class SimpleDatabase: def __init__(self): # before an actual table is loaded, class members are set to None # a header is a list of column names #e.g., ['name', 'id', 'grade'] self.header = None #map column name to column index in the header self.columns = None # None if table is not loaded # otherwise list b-tree indices corresponding to columns self.b_trees None # rows contains actual data; this is a list of lists # specifically, this is a list of rows, where each row # is a list of values for each column # e.g., if a table with the above header has two rows #self.rows can be [['Alice', 'a1234', 'HD'], ['Bob', 'a7654', 'D']] self.rows = None # name of the loaded table self.table_name = None def get_table_name(self): return self.table_name def load_table(self, table_name, file_name): # note our DBMS only supports loading one table at a time # as we load new table, the old one will be lost print("loading {table_name) from (file_name)...) if not os.path.isfile(file_name): print("File not found") return # note, you could use a CSV module here, also we don't check # correctness of file with open(file_name) as f self.header f.readline().rstrip().split(".") self.rows = [line.rstrip().split(",") for line in f] self.table_name = table_name self.columns = {} for i, column_name in enumerate(self.header): self.columns[column_name] = i self.b_trees [None] *len(self.header) print("... done!") def select_rows(self, table_name, column_name, column_value): # modify this code such that row selection uses index if it exists # note that our DBMS only supports loading one table at a time if table_name != self.table_name: # no such table return [], [] if column_name not in self.columns: # no such column return self.header, [] col_id = self.columns[column_name] selected rows = [] for row in self.rows: if row[col_id] == column_value: selected rows.append(row) return self.header, selected_rows