[quote]Is it possible to store and Int_t as a key and a TLorentzVector as a value using TMap? [/quote]No.
Function members of TMap class:virtual void Add(TObject* obj)
void Add(TObject* key, TObject* value)
Jan
use a vector (a natural way of mapping ints to something else), e.g. a TObjArray / TClonesArray, or a std::vector. If that doesn’t do it because your ints have a range too high then use a std::map<int, TLorentzVector>.