Sort TTree entries

ROOT
1

Hi,

Is there a way to sort the TTree entries given the values stored in one branch?

For example, I have a TTree with events. One branch contains the signal-to-noise ratio (SNR) of each events. I’d like to loop over the entries (with tree->GetEntry(t)) and read the TTree by decreasing order of SNR.

Is there a way of doing that?
Thank you.

2

Hi,

You can do something like:tree->BuildIndex("SNR"); TTreeIndex *index = (TTreeIndex*)tree->GetTreeIndex(); for( int i = index->GetN() - 1; i >=0 ; --i ) { Long64_t local = tree->LoadTree( index->GetIndex()[i] ); branch->GetEntry(local); .... }

Cheers,
Philippe.

3

Hi Philippe,

This looks interesting. I’m not so familiar with the TTreeIndex class. Where is the SNR-sorting actually performed? is it at the level of the BuildIndex function?

Thanks

4

is it at the level of the BuildIndex function?

yes, BuildIndex will evaluate the formula given at each entry and then sort them in increasing order.

Cheers,
Philippe.

5

I notice that TChainIndex does not have GetIndex. Is there a way to do this using a TChain instead of a TTree? I just want to loop in index order.

6

Yes, it shouldn’t make a difference.

GetIndex is spelt GetEntryWithIndex :slight_smile: