Loop over histograms in ROOT file

Looping over all histograms in a ROOT file using pyROOT

from ROOT import TFile
tfile = TFile.Open("histos.root")
tfile.cd()
for h in tfile.GetListOfKeys():
    h = h.ReadObj()
    print h.ClassName(), h.GetName()

Loop over all histograms in pyROOT

Here’s a code snippet to loop over all histograms (and get their names) in pyROOT:

from ROOT import TFile

fname = 'histos.root'
hnames = []

tfile = TFile.Open(fname)
for key in tfile.GetListOfKeys():
    h = key.ReadObj()
    if h.ClassName() == 'TH1F' or h.ClassName() == 'TH2F':
        hnames.append(h.GetName())

Build ROOT 6.06 on Linux Mint 17.3

To build ROOT 6.06/00 (released on 2015-12-09) on Linux Mint 17.3 “Rosa”, follow these instructions:

sudo apt-get install git dpkg-dev make g++ gcc binutils libx11-dev libxpm-dev \
libxft-dev libxext-dev cmake libpng12-dev libjpeg-dev libgif-dev libxml2-dev python-dev

wget https://root.cern.ch/download/root_v6.06.00.source.tar.gz
tar -xzf root_v6.06.00.source.tar.gz

mkdir root-build
cd root-build
cmake ../root-6.06.00/

# Edit CMakeCache.txt to enable roofit, minuit2, fortran, xrootd

cmake --build .
cmake -DCMAKE_INSTALL_PREFIX=/tmp/root -P cmake_install.cmake

(based on https://root.cern.ch/building-root.)

The instructions should also work for Ubuntu 14.04 “Trusty Tahr” and other distributions derived from it.

TGraph in pyROOT

pyROOT is a python binding of ROOT, which allows you to do a lot of pythonic things. The example below shows how to make a TGraph in pyROOT:

from ROOT import TGraph, gPad
from array import array
from math import exp

def func(x):
    y = 100*exp(-x)
    return y


# Method 1
if True:
    n = 100
    gr1 = TGraph(n)

    for i in range(n):
        x = 0.01*i
        gr1.SetPoint(i, x, func(x))

    gr1.SetMarkerStyle(20)
    gr1.SetMarkerSize(1)
    gr1.Draw("AP")


# Method 2
if False:
    n = 100
    xarr = []
    yarr = []

    for i in range(n):
        x = 0.01*i
        y = func(x)
        xarr.append(x)
        yarr.append(y)

    gr2 = TGraph(len(xarr), array('d', xarr), array('d', yarr))

    gr2.SetMarkerStyle(20)
    gr2.SetMarkerSize(1)
    gr2.Draw("AP")

RooFit tutorial by Wouter

RooFit tutorial by Wouter Verkerke at School of Statistics (SOS ’08):

https://indico.in2p3.fr/contributionDisplay.py?contribId=15&confId=750

(in PDF)