TTree* mytree = new TTree("mytree", "My Tree"); Int_t nEntries = mytree->GetEntriesFast(); std::cout << "Number of entries in mytree: " << nEntries << std::endl;
TTree* mytree = new TTree("mytree", "My Tree"); for (Int_t i = 0; i < mytree->GetEntriesFast(); i++) { if (i%2 == 0) { mytree->GetEntry(i); // Do something with the entry here } }Package Library: The TTree class and the GetEntriesFast method are part of the ROOT package library, which is a popular software framework for building scientific applications in high-energy physics and other fields. ROOT is written primarily in C++ and provides a wide range of tools and functionality for processing and analyzing large datasets.