Re: [ROOT] Random access to the entries of a TChain

From: Rene Brun (Rene.Brun@cern.ch)
Date: Tue Jun 04 2002 - 21:29:03 MEST


Hi David,

A TChain is designed mainly for sequential processing. You do not need to
read all the entries for each file, but you better process each file
sequentially.
If you need to do random access in a collection of files, it is much
better to open all files once, get the coresponding Trees in a array
of Trees (eg a TObjArray). Then you can select any Tree randomly
and any entry in any Tree randomly.

Rene Brun

On Tue, 4 Jun 2002 david.chamont@poly.in2p3.fr wrote:

> Hi I/O rooters,
> 
> I was wondering, when one read randomly the entries of a TChain
> which has many files attached, does it affect performance a lot ?
> (in most cases, each new entry will be taken from a different file, and I
> guess it implies many open/close of the files attached to the Chain).
> 
> David C.
> 



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