Identify first block in a pandas series based on switching values

May 12, 2023

Consider the following pandas series:

import pandas as pd
s = pd.Series([0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 0, 1])

I want to identify the first block of 1 values. The block starts when 0 switches to 1 for the first time and ends when it switches back (don’t has to). The rest should just equal zero. One restriction: no iteration allowed, only pure pandas.

Expected output:

s_new = pd.Series([0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0])

>Solution :

You can identify the first block of 1 by first identifying the 1s that follow a 0, computing the cumsum and keeping the block equal to 1:

out = s.where(s.diff().eq(1).cumsum().eq(1), 0)

Output:

0     0
1     0
2     0
3     1
4     1
5     1
6     1
7     0
8     0
9     0
10    0
11    0
12    0
13    0
14    0
15    0
dtype: int64

Intermediates:

    s  diff  eq(1)  cumsum
0   0   NaN  False       0
1   0   0.0  False       0
2   0   0.0  False       0
3   1   1.0   True       1
4   1   0.0  False       1
5   1   0.0  False       1
6   1   0.0  False       1
7   0  -1.0  False       1
8   1   1.0   True       2
9   0  -1.0  False       2
10  0   0.0  False       2
11  1   1.0   True       3
12  1   0.0  False       3
13  1   0.0  False       3
14  0  -1.0  False       3
15  1   1.0   True       4