Summing over vectors with RDF

ROOT
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1

Hi,

I have been using RDataframe for some time and was wondering if there is an efficient method to sum over all events for vector objects.

For example, one could have a vector of floats in a TTree x = <a, b, c>. I am wondering if it is possible to sum over all the 0th index values (i.e. “a”) across the events in a TTree? I do know that it is possible to flatten the vector and then sum over the individual float.

Best Regards,
Sahibjeet

2

Dear Sahibjeet,

Thanks for the post and welcome to the ROOT community!

Yes, there is, and it’s quite easy. If I understand correctly your question and make my guesswork right, it should be

auto first_element_sum = mydf.Define("first_element", "myVectorBranch[0]").Sum("first_element");
std::cout << *first_element_sum << std::endl;

You can be more efficient by specifying the type of the data stored in the vector. For example, supposing that the type stored in the vector is a float:

auto first_element_sum = mydf.Define("first_element", "myVectorBranch[0]").Sum<float>("first_element");
std::cout << *first_element_sum << std::endl;

I hope this helps!

Cheers,
D

3

Dear Danilo,

Thanks a lot for your reply!

Your code snippet definitely works, since it flattens the array. However, I was wondering if there was any feature/function in RDF that could exactly emulate the effect “np.sum” of this snippet:

import numpy as np
M = np.array([[1,2,3], [4,5,6]])
np.sum(M, axis = 0)

In this situation, one gets an output of sums as a vector. I am not sure how hard this would be to implement but such a feature seems quite natural to have in RDF

Best Regards,
Sahibjeet

4

Hi,

I think I do not understand well the question.
Do you have a branch with a vector<int> and you want to obtain per event the sum of the elements in such vector?

D

5

Dear Danilo,

Apologies for the confusion. For each element in the vector, I would like to get the sum of the element for all events in a tree.

Best Regards,
Sahibjeet Singh

6

Hi,

It’s basically the same syntax:

auto df = ROOT::RDataFrame(8);
auto dfVectorColumn = df.Define("v", [](){return std::vector<double>({1.,2.2,.32.2});});

auto sum = dfVectorColumn.Sum("v");
std::cout << *sum << std::endl;

Cheers,
D

7

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