How to Extract Variable from Branch in ROOT

[usernew]

Is there a way to extract the variables inside a Branch in ROOT.
If I write:

 ROOT::RDataFrame d("Delphes", "tag_1_delphes_events.root");
 auto df_mass = d.Define("Dimuon_mass", InvariantMass<float>, {"Muon.PT", "Muon.Eta", "Muon.Phi", "mu_mass"});

I can calculate the invariant mass by using the values inside the Muon.PT branch, etc.

Is there a way to just save the values inside the Muon.PT branch, etc to use in inside RVec.

I want to do something like this

 auto d1 = d.Define("pt", pt<float>, "Muon.PT");  RVec<RVec<size_t>>
 reco_zz_to_4l(rvec_f pt, rvec_f eta, rvec_f phi, rvec_f mu_mass,
 rvec_i charge);

but have it work.

Thanks

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[eguiraud]

Hi,
thank you for creating a post that’s smaller in scope than this, I’m not sure how to help there.

Unfortunately you can’t mix RDataFrame column definitions and standard C++, it just can’t work within the C++ (or python) programming language.

So if you want to use a quantity as an argument to a C++ function, it needs to be a C++ variable. If you want to use a quantity within RDataFrame, you can Define it.

You can use RDataFrame’s Take method to extract all values of a given RDF column into a regular std::vector. But it’s usually more efficient to do all operations you need within RDataFrame, and extract just the histograms or the final quantities you need.

Hope this clarifies things a bit.
The ROOT tutorials have a few examples of full analyses done with RDF that you can check out.
Cheers,
Enrico

[usernew]

Hello,

@eguiraud This does help as I am working through it more; I believe I understand your reply.
What I have a question about is the Define in RDataFrame. If I am eventually calculating the invariant mass (which I am using the tutorial’s code to do so) by using:

auto df_mass = df_muon_mass.Define("Dimuon_mass", InvariantMass<float>, {"Muon.PT", "Muon.Eta", "Muon.Phi", "muon_mass"});

how can I define muon_mass if I do not have that branch in my tree. I tried many different was to define it. After writing,

auto df_muon_mass = df.Define("muon_mass", ROOT::RVec<float>, {"49254", "0.1"});

where 49254 is the collection size and 0.1 is the value I want to give the muon mass. I am also defining it to be an RVec so that I can calculate the invariant mass. My other branch values (Muon.PT,etc which are already present in my tree) are floats.

I keep getting this error:

Processing invariantmass.C...
In file included from input_line_8:1:
/mnt/c/1/MG5_aMC_v2_6_6/triplet2s/Events/run_01/invariantmass.C:109:64: error: expected '(' for function-style cast or type construction
   auto df_muon_mass = df.Define("muon_mass", ROOT::RVec<float>, {"49254", "0.1"});

How can I write my Define correctly so that the Invariant mass will be calculated?

[eguiraud]

The problem with that code is two-fold: firstly, C++ constructors are a bit special and you can’t use them as factory functions: ROOT::RVec<float> is not a function that returns a ROOT::RVec<float>, hence the compilation error. Secondly, Define requires valid column names as last argument, you cannot just pass any argument to the function as in df.Define("muon_mass", ROOT::RVec<float>, {"49254", "0.1"});.

This should work (it uses a C++11 lambda expression):

// a function that returns the RVec you want
auto make_rvec = [] { return ROOT::RVec<float>(49254, 0.1); };
df.Define("muon_mass", make_rvec);

or all togheter:

df.Define("muon_mass", [] { return ROOT::RVec<float>(49254, 0.1); });

Cheers,
Enrico

[usernew]

@eguiraud Thank you for your response. I did try that however I get this error:

terminate called after throwing an instance of ‘std::runtime_error’
what(): Cannot call operator + on vectors of different sizes.

The collection size was obtained from looking at the collection size of the other muon branches, so what would be the problem here?

[eguiraud]

Maybe the collection has different sizes for different entries in the dataset?