Is GroupBy operation natively implemented in RDataFrame?

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ROOT Version: 6.20
Platform: Mac OS
Compiler: Clang

Dear ROOT developers,

I am currently analyzing some TTrees employing RDataFrame. Their structure is as follows:

particle info_1 info_2 … event
1 … … … 1
2 … … … 1
3 … … … 1
…
10 … … … 2
11 … … … 2

My goal is to perform a GroupBy operation grouping on the event column. Please, keep in mind that rows are unordered (e.g. particle 3 could belong to event 2 and particle 4 to event 1).

My current approach is to create a std::map where the key is the event, and the value is a user-defined struct. This map is updated, after applying some filters and definitions, using the Foreach function of RDataFrame. Is it possible to perform a GroupBy natively in RDataFrame?
Moreover, I am considering exploiting multithreading in my analysis, so having such function in RDF API would allow me to take advantage of the implicit MT capability of ROOT.

Best regards,
Loris

2

Hi Loris,
unfortunately GroupBy is not implemented in RDataFrame, but you can implement it as a custom action, also for the MT case. Returning a std::unordered_map seems like a good way to do it at a first glance.

You could even propose your implementation for inclusion in ROOT inside ROOT/RDFHelpers.hxx.

Here is a tutorial that shows how to implement a custom action in practice.

I hope this helps!
Cheers,
Enrico

3

Hi Enrico,

I see, thank you very much. I will try to polish a bit my code and have a look at it!

Cheers,
Loris

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