Viewing ROOT trees, Ntuples

Display / Browse a ROOT tree

Some simple tools are provided to histogram the variables contained in a root tree (in the simplest case just a Ntuple). Note: If a leaf is a vector with variable length the browser can only show the (1dim) histogram of all entries. This situation is marked with a v after the leafname. Clicking on a tree produces a list showing the variables (leafs) of the tree. Just clicking into a name displays a 1-dim histogram with default binning (100), the range is calculated from the first few 1000 values. A table is presented with default Nbin, LowX UpperX which may be edited. These defaults can be saved for the same or a later session activating the option Remember hist limits when showing trees from the Various HistPresent options pupup menu.

A scatter plot is produced by selecting 2 variables by pressing the grey button left of the name (which turns into green) and then clicking Show leaves. Pressing the green button deselects it.

To display not just the value itself but an expression of it press Edit expression. An expression can contain the variables names explicitly (e.g. TMath::Sqrt(var1)) Tab completion is implemented for variables of the Ntuple and some frequently used TMath:: expression like TMath::Sqrt(
The name can also be taken from a selection. This is done by putting $1 as a placeholder. (e.g. Defining an expression: $1&&0xfff and then clicking on a variables name will select only the lower 12 bit of the variables value. For 2-dim hists expressions like sqrt($1):$2&&0xfff may be used. Similiarly formula cuts may be applied by Edit formula cut. (e.g. $1 > 1000 & $2 > 2000). Applying graphical cuts (TCutG) are possible by giving the name of a memory resident TCutG and the 2 variables names used for the cut.

The command C++ Code, MakeClass generates a skeleton analysis C++ code which is described Help_on_MakeCode

Use of a root macro

A simple and powerful way to analyze data contained in a tree is achieved with the help of a ROOT macro. A skeleton code is generated with the MakeClass method of the tree class. The following interactive session demonstrates this:

root [0] TFile f("hsimple.root")
root [1] f.ls()
TFile**         hsimple.root    Demo ROOT file with histograms
 TFile*         hsimple.root    Demo ROOT file with histograms
  KEY: TH3F     hpx;1   This is the px distribution
  KEY: TH2F     hpxpy;1 py vs px
  KEY: TProfile hprof;1 Profile of pz versus px
  KEY: TNtuple  ntuple;1        Demo ntuple
root [2] ntuple->MakeClass();
Info in TTreePlayer::MakeClass: 
Files: ntuple.h and ntuple.C generated from TTree: ntuple
root [3]

ntuple.h and ntuple.C will now contain the skeleton C++ code. Detailed information can be found in the class descriptions of TTree and TCutG in ROOTs WWW site, the ROOT HOWTOS contain useful examples.