How to skip entry when using TSelector?

ROOT Version: 6.26.04
Platform: Ubuntu 20.04
Compiler: g++ 9.4.0

Hello, I wonder how to skip some entries that do not meet criteria when using TSelector?

Below is an example when using traditional event loop:

TFile* file = new TFile("test.root","read");
TTree* tree = (TTree*)file->Get("TestTree");
Int_t x;

Int_t nEntries = tree->GetEntries();
for (int entry = 0; entry < nEntries; entry++)
    if(x>=10) continue; // skip events that x>=10;

When using TSelector, I do not have control of the event loop. I tried to do something in the Bool_t Selector::Process(Long64_t entry) like this:

Bool_t Selector::Process(Long64_t entry)
    // The Process() function is called for each entry in the tree (or possibly
    // keyed object in the case of PROOF) to be processed. The entry argument
    // specifies which entry in the currently loaded tree is to be processed.
    // It can be passed to either Selector::GetEntry() or TBranch::GetEntry()
    // to read either all or the required parts of the data. When processing
    // keyed objects with PROOF, the object is already loaded and is available
    // via the fObject pointer.
    // This function should contain the "body" of the analysis. It can contain
    // simple or elaborate selection criteria, run algorithms on the data
    // of the event and typically fill histograms.
    // The processing can be stopped by calling Abort().
    // Use fStatus to set the return value of TTree::Process().
    // The return value is currently not used.
    if (x >= 10)
        return kFALSE;
    printf("x=%d\n", x);

    return kTRUE;

Is this the proper way to skip some entries, or do the “preselection”?


Looks fine to me.

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