## Issue

If I have a `pandas.core.series.Series`

named `ts`

of either 1’s or NaN’s like this:

```
3382 NaN
3381 NaN
...
3369 NaN
3368 NaN
...
15 1
10 NaN
11 1
12 1
13 1
9 NaN
8 NaN
7 NaN
6 NaN
3 NaN
4 1
5 1
2 NaN
1 NaN
0 NaN
```

I would like to calculate cumsum of this serie but it should be reset (set to zero) at the location of the NaNs like below:

```
3382 0
3381 0
...
3369 0
3368 0
...
15 1
10 0
11 1
12 2
13 3
9 0
8 0
7 0
6 0
3 0
4 1
5 2
2 0
1 0
0 0
```

Ideally I would like to have a vectorized solution !

I ever see a similar question with Matlab :

Matlab cumsum reset at NaN?

but I don’t know how to translate this line `d = diff([0 c(n)]);`

## Solution

A simple Numpy translation of your Matlab code is this:

```
import numpy as np
v = np.array([1., 1., 1., np.nan, 1., 1., 1., 1., np.nan, 1.])
n = np.isnan(v)
a = ~n
c = np.cumsum(a)
d = np.diff(np.concatenate(([0.], c[n])))
v[n] = -d
np.cumsum(v)
```

Executing this code returns the result `array([ 1., 2., 3., 0., 1., 2., 3., 4., 0., 1.])`

. This solution will only be as valid as the original one, but maybe it will help you come up with something better if it isn’t sufficient for your purposes.

Answered By – emprice

Answer Checked By – Terry (BugsFixing Volunteer)