# Cumulative sum within group, reset when encounters a 0

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Namrata Goswami on 17 Dec 2020
Answered: Eric Sofen on 6 Jan 2021
I have a table and I want to get the cumulative sum within a group(by ID), but the cumulative count should reset if the counter is 0 at any point within a group and again start the cumulative count from 1.
Input
ID Counter
A 1
A 0
A 1
A 1
B 1
B 0
B 1
Expected output:
ID Counter Cumulative
A 1 1
A 0 0
A 1 1
A 1 2
B 1 1
B 0 0
B 1 1

Eric Sofen on 6 Jan 2021
It took me a while to come up with a way to do this without looping, then it clicked! You need to create groups based on where the 0s are. To do that, flip the logic of the counter and cumsum that:
t.ResetGroup = cumsum(~t.Counter);
Then, you can use varfun to group by both ID and ResetGroup and do the cumsum.
t = varfun(@cumsum, t, "GroupingVariable",["ID","ResetGroup"])

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