Getting a histogram from a tree in pyroot

Newbie
1

Hi,

I have a root file with a tree “btree” inside and few branches in that tree, e.g. “bb”. I can open the file and print number of entries in the tree, but don’t how to get the histogram from “bb” and draw it. I guess I am missing one line at the end:

import ROOT

File1 = ROOT.TFile.Open(“test.root” ,“READ”)
btree = File1. btree
entries = btree.GetEntries()
print(“entries:”, entries) # this works
#now i need to get the bb histogram from the btree somehow and the I want to do
bb.Draw()

Thanks for help, seems super basic, but I couldn’t find a way…
Marek

2

Hi Marek,

One way to plot the histogram is the TBrowser. You can call

rootbrowse /path/to/file.root`

navigate through the branches of the tree and double click bb to get the histogram.

In your Python code, you can use

btree.Draw("bb")

Cheers,
Jakob

3

Hi Jakob,

thanks, “btree.Draw(“bb”)” works when I run it from the command line, when it’s in the code, it doesn’t do anything, although I get:

Info in TCanvas::MakeDefCanvas: created default TCanvas with name c1

But what I really want is tho have a TH1 in my code, so that I can do something with it (change title and how it looks) and then save it as pdf. So I need to retrieve this as a TH1, how do I do that?

Thanks!
Marek

4

OK, I found what I needed:

histo1 = ROOT.TH1F(‘histo1’,‘histo1’ ,100 ,-2 ,18)
btree.Project(“histo1”, “bb”)
histo1.Draw()