CERN ROOT Extract Data from TNtuple

I am using CERN's ROOT framework (required), and I want to take data from a TNtuple and grap开发者_运维百科h it. I can either graph the data when I create the TNtuple, or after I write it to a .root file. Some of the support documentation suggested that I create a TTree, but that seemed like it might be overkill/roundabout since I wouldn't be using it for anything else (and the TNtuple fulfills all of my other requirements). Does anyone have a better suggestion for how to extract data from the TNtuple and graph it?

As TNtuple inherits from TTree, you can use all the methods presented in the support documentation for TTrees directly on the TNtuple. This especially means that you can use TTree::Draw() which is typically more than sufficient for quickly graphing the data. This function is documented here.

For more elaborate plots you will have to read the data from the TNtuple event by event and feed it to your favorite graphing tool in ROOT. This again follows the basic principles from a tree. The best example I could find on the ROOT homepage is in the user manual, section trees in the paragraph "Reading the Tree".

The methods used to create histograms and plots for TNtuples is essentially the same as TTrees. The code:

ntuple->Draw("var");

will create a histogram of the variable var stored in the Ntuple. If you want to plot one variable in the Ntuple as a function of another, use

ntuple->Draw("xVar:yVar");

You can do fancier things such as creating plots only when a logical condition is satisfied. For example, suppose you want a histogram of var1 only when var2 is greater than 2 and var3 is less than 0.

ntuple->Draw("var","var2 > 2 && var3 < 0");

By plotting in this way, ROOT automatically sets the binning and range for the x-axis. If you wish to control these features yourself, use

ntuple->Draw("var >> hist(Nbins,xmin,xmax)");

This creates the object hist, which you treat as a usual histogram object in ROOT. As stated in the previous post, this is documented in the ROOT manual along with several other features and tools. Unfortunately, the manual doesn't always give clear explanations.

{
  ntuple->Draw("py:px","px>py","goff");
  TGraph *gr = new TGraph(ntuple->GetSelectedRows(),ntuple->GetV2(), ntuple->GetV1());
  gr->Draw("AP");
}