TTree LoadTree

ROOT
1

Hi,

I’m confused about the TTree LoadTree function. What exactly does this do and when do I need to use it? It seems like it is misnamed since it doesn’t actually load anything into memory, but just sets fReadEntry equal to the argument.

Thanks.

Andrew

2

Hi Andrew,

This is correct but only if you have a TTree, if instead you have a TChain, LoadTree will not only move the cursor but also potentially move to a new file (closing the current one, opening the next one and load the TTree object out of the new file).

Typically you use LoadTree within your main loop:for(Long64_t entry = 0; entry < fChain->GetEntriesFast(); ++entry) { // GetEntriesFast in case of chain will not return the real number of entries until the last file in the chain // is accessed at least once, use LoadTree to move the cursor and also give us an early indication that we // reached the last entry and to give use the current entry number within the current TTree (i.e. the // entry number we should pass to TBranch::GetEntry since it does not know about the TChain. Long64_t local_entry = LoadTree(entry); if (local_entry < 0) break; ....

Cheers,
Philippe.

3

So, is there ever a reason to call LoadTree for a TTree?

Thanks.

Andrew

4

Hi Andrew,

It can be useful to make the code ‘TChain’ ready and in case where you want to decentralize the calls to TBranch::GetEntry but do not want to pass around the entry number (after LoadTree, TTree::GetReadEntry returns the ‘current’ entry number).

Cheers,
Philippe.

5

Hi,

I don’t understand what you mean “make the code ‘TChain’ ready”.

Andrew

6

Hi,

Even though in for(Long64_t entry = 0; entry < fChain->GetEntriesFast(); ++entry) { Long64_t local_entry = LoadTree(entry); if (local_entry < 0) break; when fChain is a TTree, the last two lines do not do too much, they have the advantage of also being the most efficient when fChain points to a TChain. In other words, even if your code might not be using / being passed a TChain for now, it does not hurt to write in such a way that it eventually could be re-used with a TChain.

Cheers,
Philippe.