rf101_basics.py

import ROOT


def rf101_basics():

    # S e t u p   m o d e l
    # ---------------------

    # Declare variables x,mean,sigma with associated name, title, initial
    # value and allowed range
    x = ROOT.RooRealVar("x", "x", -10, 10)
    mean = ROOT.RooRealVar("mean", "mean of gaussian", 1, -10, 10)
    sigma = ROOT.RooRealVar("sigma", "width of gaussian", 1, 0.1, 10)

    # Build gaussian p.d.f in terms of x,mean and sigma
    gauss = ROOT.RooGaussian("gauss", "gaussian PDF", x, mean, sigma)

    # Construct plot frame in 'x'
    xframe = x.frame(ROOT.RooFit.Title("Gaussian p.d.f."))  # RooPlot

    # P l o t   m o d e l   a n d   c h a n g e   p a r a m e t e r   v a l u e s
    # ---------------------------------------------------------------------------

    # Plot gauss in frame (i.e. in x)
    gauss.plotOn(xframe)

    # Change the value of sigma to 3
    sigma.setVal(3)

    # Plot gauss in frame (i.e. in x) and draw frame on canvas
    gauss.plotOn(xframe, ROOT.RooFit.LineColor(ROOT.kRed))

    # G e n e r a t e   e v e n t s
    # -----------------------------

    # Generate a dataset of 1000 events in x from gauss
    data = gauss.generate(ROOT.RooArgSet(x), 10000)  # ROOT.RooDataSet

    # Make a second plot frame in x and draw both the
    # data and the p.d.f in the frame
    xframe2 = x.frame(ROOT.RooFit.Title(
        "Gaussian p.d.f. with data"))  # RooPlot
    data.plotOn(xframe2)
    gauss.plotOn(xframe2)

    # F i t   m o d e l   t o   d a t a
    # -----------------------------

    # Fit pdf to data
    gauss.fitTo(data)

    # Print values of mean and sigma (that now reflect fitted values and
    # errors)
    mean.Print()
    sigma.Print()

    # Draw all frames on a canvas
    c = ROOT.TCanvas("rf101_basics", "rf101_basics", 800, 400)
    c.Divide(2)
    c.cd(1)
    ROOT.gPad.SetLeftMargin(0.15)
    xframe.GetYaxis().SetTitleOffset(1.6)
    xframe.Draw()
    c.cd(2)
    ROOT.gPad.SetLeftMargin(0.15)
    xframe2.GetYaxis().SetTitleOffset(1.6)
    xframe2.Draw()

    c.SaveAs("rf101_basics.png")


if __name__ == "__main__":
    rf101_basics()