condition is column 'A' > 0.5
I want to calculate the index of the last condition established and assign it to column 'cond_index'
A cond_index
0 0.001566 NaN
1 0.174676 NaN
2 0.553506 2
3 0.583377 3
4 0.418854 3
5 0.836482 5
6 0.927756 6
7 0.800908 7
8 0.277646 7
9 0.388323 7
Use Index.to_series with replace missing values if not match condition in Series.where with comapre for greater like 0.5 and last forward filling missing values:
df['new'] = df.index.to_series().where(df['A'].gt(0.5)).ffill()
print (df)
A cond_index new
0 0.001566 NaN NaN
1 0.174676 NaN NaN
2 0.553506 2.0 2.0
3 0.583377 3.0 3.0
4 0.418854 3.0 3.0
5 0.836482 5.0 5.0
6 0.927756 6.0 6.0
7 0.800908 7.0 7.0
8 0.277646 7.0 7.0
9 0.388323 7.0 7.0