python - 如何在python中有条件地选择上一行的值？

Question

我想仅在满足特定条件时才选择上一行的值
例如。

df:
Value  Marker  
10  0  
12  0  
50  1  
42  1  
52  0  
23  1

我想选择前一行的值，其中 marker == 0如果当前值 marker == 1 .
结果:

df:
Value  Marker  Prev_Value  
10  0  nan
12  0  nan
50  1  12
42  1  12
52  0  nan
23  1  52

我试过: df[prev_value] = np.where(df[marker] == 1, df[Value].shift(), np.nan)但这并不像我想要的那样采用有条件的先前值。

Answer 1

condition = (df.Marker.shift() == 0) & (df.Marker == 1)
df['Prev_Value'] = np.where(condition, df.Value.shift(), np.nan)

输出:

df
   Value  Marker  Prev_Value
0     10       0         NaN
1     12       0         NaN
2     50       1        12.0
3     42       1         NaN
4     52       0         NaN
5     23       1        52.0