add a new counter based on an existing counter in python pandas

Question

I have a Series that look like this

and I would like to generate a second counter that looks like this

   col  col2
0   1     1
1   2     1
2   3     1
3   4     1
4   1     2
5   2     2
6   3     2
7   1     3
8   2     3
9   3     3
10  1     4
11  2     4

How can I do that in python?

jezrael · Accepted Answer · 2020-03-25 07:23:59Z

3

If 1 is always start of groups then create mask by compare by Series.eq and then add Series.cumsum for cumulative sum:

df['col2'] = df['col'].eq(1).cumsum()
print (df)
    col  col2
0     1     1
1     2     1
2     3     1
3     4     1
4     1     2
5     2     2
6     3     2
7     1     3
8     2     3
9     3     3
10    1     4
11    2     4

answered Mar 25, 2020 at 7:23

jezrael

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