python - pandas dataframe如何对某列的空数据位置进行update？update的函数是自定义的，参数是同一行的另外两列数据

Question

dataframe的某列fillna如何通过自定义函数的返回值进行填充？自定义函数需要以某几列数据作为参数。类似下面代码的功能怎么实现？df['resultOfab'].fillna（myFunc(df['acolumn'],df['bcolumn']))

貌似只有dataframe才有fillna函数，而且只能指定值作为填充。要么就是只能加减乘除某两列得到新列数据。如果全部重新计算效率又很低。

Answer 1

Is it like this? It's a little unclear what to do. . .

>>> df
     0    1    2
0  1.0  NaN  5.0
1  2.0  3.0  6.0
2  3.0  5.0  7.0
3  4.0  NaN  8.0
>>> def dfunc(df1, df2):
...     return df1 + 2 * df2
...
>>> df[1] = df[1].fillna(dfunc(df[0], df[2]))
>>> df
     0     1    2
0  1.0  11.0  5.0
1  2.0   3.0  6.0
2  3.0   5.0  7.0
3  4.0  20.0  8.0

---

Or like this?

>>> df
     0    1    2
0  1.0  NaN  5.0
1  2.0  3.0  6.0
2  3.0  5.0  7.0
3  4.0  NaN  8.0

>>> def d_func(df1, df2):
...     return df1 + df2 * 2
...

>>> df[1] = np.where(df[1].isnull(), d_func(df[0], df[2]), df[1])

>>> df
     0     1    2
0  1.0  11.0  5.0
1  2.0   3.0  6.0
2  3.0   5.0  7.0
3  4.0  20.0  8.0

This is vector type a?b:c