Just to compile with the forum rules:
- ROOT Version: 6.24/08
- Platform: CentOS Linux release 7.9.2009
- Compiler: GCC 4.8.5
Hello everyone,
I would like to ask about a problem I encountered while working on my analysis using RDataFrame. For simplicity I will skip the Whys and Whens and jump straight to the point.
I have a std::vector<float> particleEnergies stored in a TTree for every event that passes my selection and in this vector I store energies of particle. It is the same particle, same true energy but different values because there are multiple reconstruction algorithms based on which the value is calculated. My goal is to compare them. Therefore I need an access to each element of vector separately. In the first step I would like to do something like this:
auto df = ROOT::RDataFrame("myTree", "mytree.root");
auto algo0 = df.Histo1D("particleEnergies[0]");
auto algo1 = df.Histo1D("particleEnergies[1]");
auto algo2 = df.Histo1D("particleEnergies[2]");
.......
Basically I would like to create a histogram which is not filled by every value in vector (as is the case in documentation) but only by 1 element. I tried this and it is not possible as branch with particleEnergies[] does not exist.
There is of course possibility to store every element in separate branch like float E1, E2, E3, E4,… already in selection and then access each branch individually. However I do not like this approach as I am hardcoding everything and in case an algorithm is added in the future or taken away I would need to change it almost from scratch.
I already discussed this with your ROOT colleague and she came up with a very nice solution where I firstly Define the branch from vector and then access it. Something like this:
auto df = ROOT::RDataFrame("myTree", "mytree.root");
auto loopDf = df.Define("NOOP", "0");
for (int i = 0; i < 9; i++) {
loopDf = loopDf.Define("particleEnergies" + std::to_string(i), "particleEnergies[i]");
}
// Now every element has a new separate branch and I can access it and for example fill histos
for (int i = 0; i < 9; i++) {
loopDf.Histo1D(Form("particleEnergies%i", i));
........
}
The question I have is this. Is this the correct approach or is there a better one or different one where one can fully use functionalities of RDataFrame and access individual elements of vector and fill histograms “easily” without creating/duplocating branch? And if it is at all possible or maybe completely stupid to make possible my first approach in the future? (example below)
auto df = ROOT::RDataFrame("myTree", "mytree.root");
auto algo3 = df.Histo1D("particleEnergies[3]");