[Python] Pandas: Creating indicator column after condition

import numpy as np
import pandas as pd
df = pd.DataFrame({
'cond': ['A', 'A', 'A', 'A', 'A', 'A', 'A', 'A', 'A', 'B', 'B','B', 'B', 'B', 'B', 'B','B','B'],
'Array': ['S', 'S', 'TT', 'TT','S', 'S', 'TT', 'TT','S', 'S', 'TT', 'TT','S', 'S', 'TT', 'TT','SS','TT'],
'Area': [3.0, 2.0, 2.88, 1.33, 2.44, 1.25, 1.53, 1.0, 0.156, 2.0, 2.4, 6.3, 6.9, 9.78, 10.2, 3.0, 16.0, 19.0]
})
print(df)


I am trying to make an indicator column that indicates if the area being a certain size has already happened. So for example, if the cond. is A, then I want to indicate the first time that the area is <=1.5 (and for all data points after), and if the cond. is B, then to indicate the first time the area >10 (and for all points after). The final result should look like:

cond Array Area Indicator
0 A S 3.000 0
1 A S 2.000 0
2 A TT 2.880 0
3 A TT 1.330 1
4 A S 2.440 1
5 A S 1.250 1
6 A TT 1.530 1
7 A TT 1.000 1
8 A S 0.156 1
9 B S 2.000 0
10 B TT 2.400 0
11 B TT 6.300 0
12 B S 6.900 0
13 B S 9.780 0
14 B TT 10.200 1
15 B TT 3.000 1
16 B SS 16.000 1
17 B TT 19.000 1


A lot of the other examples I looked at were to indicate if the area for A was <=1.5, or indicate the first time it happens, but not to indicate the first time it happens AND indicate all the datapoints after. The idea is that once my condition hits a certain area, it enters a different "phase" and I'm trying to indicate when "A" enters and stays in that phase (and the equivalent for B).

Continue reading...