python pandas conditional cumulative sum

Question

Consider my dataframe df

data  data_binary  sum_data
  2       1            1
  5       0            0
  1       1            1
  4       1            2
  3       1            3
  10      0            0
  7       0            0
  3       1            1

I want to calculate the cumulative sum of data_binary within groups of contiguous 1 values.

The first group of 1's had a single 1 and sum_data has only a 1. However, the second group of 1's has 3 1's and sum_data is [1, 2, 3].

I've tried using np.where(df['data_binary'] == 1, df['data_binary'].cumsum(), 0) but that returns

array([1, 0, 2, 3, 4, 0, 0, 5])

Which is not what I want.

Answer 1

you want to take the cumulative sum of data_binary and subtract the most recent cumulative sum where data_binary was zero.

b = df.data_binary
c = b.cumsum()
c.sub(c.mask(b != 0).ffill(), fill_value=0).astype(int)

0    1
1    0
2    1
3    2
4    3
5    0
6    0
7    1
Name: data_binary, dtype: int64

---

Explanation

Let's start by looking at each step side by side

cols = ['data_binary', 'cumulative_sum', 'nan_non_zero', 'forward_fill', 'final_result']
print(pd.concat([
        b, c,
        c.mask(b != 0),
        c.mask(b != 0).ffill(),
        c.sub(c.mask(b != 0).ffill(), fill_value=0).astype(int)
    ], axis=1, keys=cols))


   data_binary  cumulative_sum  nan_non_zero  forward_fill  final_result
0            1               1           NaN           NaN             1
1            0               1           1.0           1.0             0
2            1               2           NaN           1.0             1
3            1               3           NaN           1.0             2
4            1               4           NaN           1.0             3
5            0               4           4.0           4.0             0
6            0               4           4.0           4.0             0
7            1               5           NaN           4.0             1

The problem with cumulative_sum is that the rows where data_binary is zero, do not reset the sum. And that is the motivation for this solution. How do we "reset" the sum when data_binary is zero? Easy! I slice the cumulative sum where data_binary is zero and forward fill the values. When I take the difference between this and the cumulative sum, I've effectively reset the sum.