Write 2d variable size arrays into TTree

[FoxWise]

Hi,
is there a way of storing 2d array with variable dimensions into the TTree?

If no, is there any workaround for that?

cheers

---

ROOT Version: 6.18

[pcanal]

For 1 variable and 1 fixed there are several options. For 2 variables dimensions use std::vector<std::vector<type_name>>

[FoxWise]

Dear pcanal.
I doubt this works.
I have tried the following example. Which does not find any good constructor for a branch with 2 vectors.

error: no matching member function for call to 'Branch'

int test(){
    int nParticles;
    vector<int> nTracks;
    vector<vector<double>> trackPosition;

    TFile file("test.root", "RECREATE");
    TTree tree("test", "test");
    tree.Branch("nParticles", &nParticles, "nParticles/I");
    tree.Branch("nTracks", nTracks, "nTracks[nParticles]/I");
    tree.Branch("trackPosition", trackPosition,"trackPosition[nParticles][nTracks]/D");

    //Assume 2 particles
    nParticles = 2;
    //1st has 2 traks
    nTracks.push_back(2);
    //2nd has 3 tracks
    nTracks.push_back(3);

    //Fill positions of each track for 1st particle
    vector<double> temp1;
    for (size_t i = 0; i < 2; i++) temp1.push_back((i+1)*10);
    trackPosition.push_back(temp1);

    //Fill positions of each track for 2st particle
    vector<double> temp2;
    for (size_t i = 0; i < 3; i++) temp2.push_back((i+1)*100);
    trackPosition.push_back(temp2);

    tree.Fill();
    file.Write();
return 0;
}

I also tried this with variable arrays like this:

int test(){
    int nParticles;
    int nTracks[100];
    Double_t trackPosition[100][100];

    TFile file("test.root", "RECREATE");
    TTree tree("test", "test");
    tree.Branch("nParticles", &nParticles, "nParticles/I");
    tree.Branch("nTracks", nTracks, "nTracks[nParticles]/I");
    tree.Branch("trackPosition", trackPosition,"trackPosition[nParticles][nTracks]/D");

    //Assume 2 particles
    nParticles = 2;
    //1st has 2 traks
    nTracks[0] = 2;
    //2nd has 3 tracks
    nTracks[1] = 3;

    //Fill positions of each track for 1st particle
    for (size_t i = 0; i < 2; i++) trackPosition[0][i] = (i+1)*10.;

    //Fill positions of each track for 2st particle
    for (size_t i = 0; i < 3; i++) trackPosition[1][i] = (i+1)*100.;

    tree.Fill();
    file.Write();
    return 0;
}

But trackPosition values in the written file are all 0.

It only works if I put 2nd dimension as constant. But then it stores too much of empty_space information.

I looked at TBranch source file and it doesn’t seem to be implemented yet.
Is it correct? Is there a hope that it will be implemented soon? Or are there any workaround for this without storing a lot of empty data and complicating future analysis.

cheers,
Bohdan

[Michael_Kelsey]

You cannot do that with plain “C style” arrays. Only one dimension can be variable.

You might be able to do it with nested std::vectors. But then you each of the outer array elements can be an independent length, unless you constrain them in your filling code.

[FoxWise]

Dear all, thanks for the replies.

All works, I just realized I need to use another Branch method.

Doesn’t work:

    vector<int> nTracks;
    vector<vector<double>> trackPosition;
    tree.Branch("nTracks", nTracks, "nTracks[nParticles]/I");
    tree.Branch("trackPosition", trackPosition,"trackPosition[nParticles][nTracks]/D");

Works fine:

    vector<int> nTracks;
    vector<vector<double>> trackPosition;
    tree->Branch("nTracks", &nTracks);
    tree->Branch("trackPosition", &trackPosition);

I leave it here in case if someone finds it helpful