我有数据框，即InputDataframeclasssectionsubmarksschoolcity0IAEng80jghsssalem1IAMat90jghsssalem2IAEng50Nansalem3IIIAEng80gphssNan4IIIAMat45Nansalem5IIIAEng40gphssNan6IIIAEng20gphsssalem7IIIAMat55gphssNan当“class”和“section”列中的值匹配时，我需要替换“school”和“city”中的“Nan”。结果应该是，输入数据框classsectionsubmarksschoolcity0IAEng