Cumulative sum of a dataframe column with restart

Question

I would like to perform the following function on a dataframe.

Calculate the cumulative sum of a column, notice:

1. It looks at the previous index only, not including the current one, e.g. the very first one will be zero as there is no previous data to look at.

2. When it doesn't cumulate, e.g the increment is zero, it restarts the count.

         Number  Cumulative
      0       1           0
      1       1           1
      2       1           2
      3       0           3
      4       0           0
      5       1           0
      6       1           1
      7       0           2
   

I know there is an expanding function, but it doesnt restart when it sees zero

Answer 1

IIUC, this works by making groups according to whether the previous row was 0, then getting the cumulative count:

>>> df
   Number
0       1
1       1
2       1
3       0
4       0
5       1
6       1
7       0

df['Cumulative'] = df.groupby(df.Number.shift().eq(0).cumsum()).cumcount()

>>> df
   Number  Cumulative
0       1           0
1       1           1
2       1           2
3       0           3
4       0           0
5       1           0
6       1           1
7       0           2

Alternatively, if it really is cumsum you want, then apply cumsum with the same grouping as above, and shift it 1 down:

df['Cumulative '] = df.groupby(df.Number.eq(0).cumsum()).cumsum().shift().fillna(0)
>>> df
   Number  Cumulative 
0       1          0.0
1       1          1.0
2       1          2.0
3       0          3.0
4       0          0.0
5       1          0.0
6       1          1.0
7       0          2.0