Hello ROOTers,

Is it possible to do TChain::Merge applying selections on the entries? In alternative, I can first do TChain::Merge and then apply selection: how do I select entries in a TTree (or create a new TTree selecting entries from a previous one)?

I am doing a loop on different selections and inside the loop just asking the number of entries of a TChain for the given selection (code posted below). However, it takes a lot because the entries are O(10^7), or at least I think this is the motivation.

I’d like to create offline a TTree with only the entries that already pass a baseline selection, and then just loop on this TTree varying the selections. I think this could speed up the whole loop.

This is the code I am using:

```
TChain t("merged");
t.Add("~/mydir/*_ntuple.root");
Int_t R2cuts[5] = {50, 40, 30, 25, 20};
Int_t TBTOcuts[5] = {92, 90, 87, 85, 80};
Int_t dim = 25;
Int_t counter = 0;
Int_t globalIndex[dim];
TString Scut;
TString Bcut;
Float_t S[dim];
Float_t B[dim];
for(Int_t iTBTO=0; iTBTO<5; iTBTO++){
for(Int_t iR2=0; iR2<5; iR2++){
globalIndex[counter] = R2cuts[iR2] + TBTOcuts[iTBTO]*100;
Scut.Form("Mbc>5.27 && abs(deltaE)<0.06 && R2<%i/100 && cosTBTO<%i/100 && K_PID_bin_kaon>0.6 && pi0_InvM>0.1 && pi0_InvM<0.15 && abs(cosHelicityAngleMomentum)<0.98 && ((pi0_daughter0clusterReg==1 && pi0_daughter0E>0.0225) || (pi0_daughter0clusterReg==2 && pi0_daughter0E>0.020) || (pi0_daughter0clusterReg==3 && pi0_daughter0E>0.020)) && ((pi0_daughter1clusterReg==1 && pi0_daughter1E>0.0225) || (pi0_daughter1clusterReg==2 && pi0_daughter1E>0.020) || (pi0_daughter1clusterReg==3 && pi0_daughter1E>0.020)) && isSignal>0", R2cuts[iR2],TBTOcuts[iTBTO]);
Bcut.Form("Mbc>5.27 && abs(deltaE)<0.06 && R2<%i/100 && cosTBTO<%i/100 && K_PID_bin_kaon>0.6 && pi0_InvM>0.1 && pi0_InvM<0.15 && abs(cosHelicityAngleMomentum)<0.98 && ((pi0_daughter0clusterReg==1 && pi0_daughter0E>0.0225) || (pi0_daughter0clusterReg==2 && pi0_daughter0E>0.020) || (pi0_daughter0clusterReg==3 && pi0_daughter0E>0.020)) && ((pi0_daughter1clusterReg==1 && pi0_daughter1E>0.0225) || (pi0_daughter1clusterReg==2 && pi0_daughter1E>0.020) || (pi0_daughter1clusterReg==3 && pi0_daughter1E>0.020)) && isSignal<1", R2cuts[iR2],TBTOcuts[iTBTO]);
S[counter] = t.GetEntries(Scut);
B[counter] = t.GetEntries(Bcut);
counter++;
}
}
```

Thanks a lot for your help!

Cheers,

Riccardo