Histogram empty after scaling

_ROOT Version: 6.19
_Platform: OS 10.14.3

I have 10 histograms, I add them, everything works fine. I then decide to scale the resulting histogram.


But the histogram ends up empty. If I simply delete the line where I scale the histogram everything is fine again, except everything’s 10 times larger than it should.

I’m using pyROOT in case that’s relevant

Hi @Frigorifico,
uhm hard to tell what’s wrong with just this information…can you please provide a small, self-contained reproducer for your issue that we can play with/debug?


how about this:

root [0] TPython::Prompt()
>>> k=1/10
>>> print k


1 Like

Fair! If you are using python2 you want 1./10 (note the dot) or from __future__ import division to get the python3 division behavior (/ always converts to floating point).


Hello, this is simplified version of my code that exhibits the same behaviour, you just need to feed it a file with many histograms inside

import os, sys
import ROOT as root
from ROOT import TFile, TIter, TKey, gROOT

def average(directory, saveAs):
    infile = TFile.Open(saveAs + '.root') #read root file with histograms
    keyList = TIter(infile.GetListOfKeys()) #generate pointers to the histograms
    key0 = TKey() #we need to sum the histograms, and so we need to get the parameters of a first histogram, but TIter ddoesn't allow indexing
    for key in keyList:
        key0 = key
    h0 = key0.ReadObj() #use this to generate a blank histogram where to add all the others
    keyList = TIter(infile.GetListOfKeys()) #for some reason the keys are "spent" when they are called, so we rebuild keyList
    h = root.TH1F('average','Average',h0.GetNbinsX(),h0.GetXaxis().GetXmin(),h0.GetXaxis().GetXmax())
    n = 0
    for key in keyList: #loop through histograms adding them up
        n = n+1 # to know the size of keylist since it doesn't wotk like most lists in python
        cl = gROOT.GetClass(key.GetClassName()) #if something there is not a histogram, ignore it
        if (not cl.InheritsFrom("TH1")):
        h = h + key.ReadObj() #add histograms
    outfile = root.TFile('average.root', 'recreate') #create file for the final product
    h.Write() #save it

Hey, I did what you said and it works!, thanks a lot, but now my histogram is a bunch of dots instead of lines, not sure how that happened

You can do Draw("HIST") instead of Draw() to force the drawing style to bars :slight_smile:

Thanks a lot!

This works if I wanna save it as an image, but what if I want to see it in the ROOT browser like this?

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