向后填充条件

Question

我想应用一个数据帧的特定列向后填充满足以下条件：我有“colum_A”可以假设只有四个值，称为A，B，C，D，向后填充应该工作，如以下：

if the first not NaN is A, then backward_filling with A; 

if the first not NaN is B, then backward_filling with B; 

if the first not NaN is C, then backward_filling with B; 

if the first not NaN is D, then backward_filling with C; 

if the column_A only contains NaN, then backward_filling with D 

例如：

输入DF：

colum_A 
NaN 
NaN 
B 
B 
C 
C 

输出DF：

colum_A 
B 
B 
C 
C 
D 
D 

请任何帮助，将不胜感激。 最好的问候， 卡罗

Answer 1

我觉得你条件需要map与bfill：

#get mask for back filling NaNs 
m = df['colum_A'].isnull() 
d = {'A':'A','B':'B','C':'B','D':'C'} 
#D if all values NaN 
df['colum_B'] = 'D' if m.all() else np.where(m, df['colum_A'].map(d).bfill(),df['colum_A']) 
#alternative 
#df['colum_B'] = 'D' if m.all() else df['colum_A'].mask(m, df['colum_A'].map(d).bfill()) 
print (df) 
    colum_A colum_B 
0  NaN  B 
1  NaN  B 
2  B  B 
3  A  A 
4  NaN  B 
5  C  C 
6  C  C 
7  NaN  C 
8  NaN  C 
9  NaN  C 
10  D  D 
11  D  D 
12  A  A 
13  C  C 
14  NaN  A 
15  A  A 
16  NaN  NaN