[ROOT] TChain:: switch file method and Tree :: Change File method

From: Ben Kilminster (bjk@fnal.gov)
Date: Wed Mar 03 2004 - 17:42:08 MET


I'm looking for a simple way to keep a one to one correspondence 
between a file in a TChain and an output file generated from the 

Here is the application:

1)  I have a TTree called DataTree with branches representing data.
2)  I want to resimulate part of the DataTree and write it into another 
TTree called SimTree, storing the results in a separate file.
3)  Later,  the analysis code will loop over both DataTree and SimTree 
on an event by event basis (using TFriend) and make comparisons.

The above steps are straight forward.  The complicated parts are :
1)  DataTree is stored in a chain of multiple files.
2)  I want to keep a one-to-one correspondence between the number of 
files in DataTree and SimTree.  This is to avoid confusion when the 
user loops over different sets of files when they are simulating as 
compared to when they are analyzing.

It seems like the way to do this is something like this (excuse my 
sloppy pseudo-code) :

1)  Loop through DataTree to simulate SimTree, and for each event save 
the name of each filename that is encountered in DataTree by something 
	oldfile = DataTree->GetCurrentFile()->GetName()
2)  At the start of each event, check whether a new file has been 
opened with
	if (DataTree->GetCurrentFile()->GetName() != oldfile)
		call method SwitchFile()
3)  Run a SwitchFile method to close the SimTree file, open a new one, 
and add a construction for a new tree - something like this :
SwitchFile() {
	// store structure of SimTree
	// follow example at : 
	char newDatafile[100] = DataTree->GetCurrentFile()->GetName();
	char  newSimFile[104];
	MySimFile = new TFile(newSimFile,"WRITE");
	TTree *SimTree = new TTree("SimTree","simulated tree");
	// remake structure of SimTree and store in MySimFile
	// follow example at : 

Is there a better way to do this ?

Ideally, I think this would be fairly easy if in the code there was a :
TChain::SwitchFile() method which was called whenever a file was closed 
and a new one opened,  and a new ChangeFile method where you could 
specify the new file name, as in   TTree::ChangeFile(TTree *file, 
TString *newFileName).


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