pandas 处理缺失值（连续值取平均，离散值fillna"<unk>"）

# 2.1处理缺失值，连续值用均值填充
continuous_fillna_number = []
 
for i in train_null_ix:
    if(i in continuous_ix):
        mean_v = df_train[i].mean()
        continuous_fillna_number.append(mean_v)
        df_train[i] = df_train[i].fillna(mean_v)
 
np.save("continuous_fillna_number.npy", continuous_fillna_number) #保存下来，test时候用到（增量test也可以用）

# 2.2离散值Nan映射成"<unk>"
for i in train_null_ix:
    if(i in categorical_ix):
        df_train[i].fillna("<unk>")