Re: Drawing histogram from Ntuple with error bars

From: Rene Brun (Rene.Brun@cern.ch)
Date: Thu Jul 15 1999 - 14:26:42 MEST


Marco van Leeuwen wrote:
> 
> Hi all,
> 
> Strangely enough, I didn't find an answer to this question in the Roottalk
> digest, although it must have been mentioned... My question is the
> following: is is possible to use a TNtuple containing x, y and dy values to
> draw a graph (histogram) of points x,y with errorbars y-dy to y+dy ?
> 

Hi Marco,
See comments in TTree::draw at URL:
  http://root.cern.ch/root/html/src/TTree.cxx.html#TTree:Draw

This includes the following section:
//
//      How to obtain more info from TTree::Draw
//      ========================================
//
//  Once TTree::Draw has been called, it is possible to access useful
//  information still stored in the TTree object via the following
functions:
//    -GetSelectedRows()    // return the number of entries accepted by
the
//                          //selection expression. In case where no
selection
//                          //was specified, returns the number of
entries processed.
//    -GetV1()              //returns a pointer to the float array of V1
//    -GetV2()              //returns a pointer to the float array of V2
//    -GetV3()              //returns a pointer to the float array of V3
//    -GetW()               //returns a pointer to the double array of
Weights
//                          //where weight equal the result of the
selection expression.
//   where V1,V2,V3 correspond to the expressions in
//   TTree::Draw("V1:V2:V3",selection);
//
//   Example:
//    Root > ntuple->Draw("py:px","pz>4");
//    Root > TGraph *gr = new TGraph(ntuple->GetSelectedRows(),
//                                   ntuple->GetV2(), ntuple->GetV1());
//    Root > gr->Draw("ap"); //draw graph in current pad
//    creates a TGraph object with a number of points corresponding to
the
//    number of entries selected by the expression "pz>4", the x points
of the graph
//    being the px values of the Tree and the y points the py values.


Rene Brun



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