GetBranch from all Files in a TChain

ROOT
1

Hello,

Sorry if this is an obvious question, but I’m new to PyROOT. Our experiment has it’s data in a relatively large number of identically structured files. From what I’ve read, this is exactly what the TChain is for. I created a TChain with a subset of the data.

filelist = ['Workspace/data/all/cf/calib_Prodv5-3_lite_011208a_cf.root',
              'Workspace/data/all/cf/calib_Prodv5-3_011307a_cf.root',
              'Workspace/data/all/cf/calib_Prodv5-3_011301a_cf.root',
              'Workspace/data/all/cf/calib_Prodv5-3_011204a_cf.root']

dataChain = TChain('rrqDir/calibzip1')
for fName in filelist:
    dataChain.Add(fName)

I would like to read out a single branch for all files in the TChain, but whenever I grab a branch from the chain it just grabs from the first file. For example below: I can get the total number of entries in the chain, but I only get the entries from a single file when I run GetBranch(). How do I access the same branch from each of the rest of the files in the TChain?


In [35]: dataChain.GetEntries()
Out[35]: 1733321L

In [36]: dataChain.GetBranch('ytNF').GetEntries()
Out[36]: 166631L
2

Try:
dataChain.MakeClass(‘MyDataChain’)
then have a look at the “Loop()” method in the generated “MyDataChain.C” file.
Inside you will find explanations which show how to “read only this branch” (note the difference between “ientry” and “jentry”).

3

That example code showed me exactly what I needed. Thanks.