12.1. Importing data from ROOT files

Importing data from ROOT files is easy: Points and Histogram/Histogram2d objects may be constructed directly from TMatrixD/TMatrixF/TH1/TH2 objects. An example may be found in the following file misc_importing_from_root.py.

Here we will discuss only main functions and points. Firstly let us create TH1D, TH2D and TMatrixD.

# Create ROOT objects
roothist1d = R.TH1D('testhist', 'testhist', 10, -5, 5)
roothist2d = R.TH2D('testhist', 'testhist', 20, 0, 10, 24, 0, 12)

Then, after ROOT objects are filled, or just read from the ROOT file, the GNA objects may be created with their regular constructors:

# Create Points
p1 = C.Points(roothist1d)
p2 = C.Points(roothist2d)
# Create Histograms
h1d = C.Histogram(roothist1d)

The single precision versions TH1F, TH2F and TMatrixF are valid as well.

It should be noted, that

  • Points loose any information on bin edges.

  • Internally TH2 keeps the data in transposed order, i.e. first index represents y-coordinate, while second index represents x-coordinate. When converted to Histogram2d the memory order is changed implicitly to x/y for first/second index.

The difference between array-like and histogram-like representation may be seen from the following figures.

../_images/misc_importing_from_root_1d.png

1d histograms plotted from ROOT.TH1D, GNA.Histogram and GNA.Points.

When Points object is plotted as histogram, it is assumed that first bin starts at 0 and bin width is 1.

../_images/misc_importing_from_root_2d.png

2d histograms plotted from ROOT.TH2D, GNA.Histogram2d and GNA.Points.

The same applies to Points object is plotted as 2d histogram.