Error: attempt to access a null-pointer, while plotting histo

ROOT
1

Dear ROOT experts,

I have a root file with several histograms in it. I am trying to access those histograms and plot them, but getting the error attempt to access a null pointer . I opened the file with TBrowser and all TH1F histograms can be displayed. Here is part of the code and below the error message.

	histo_file = ru.TFile(outfile, "READ")   
	
	# call each histo
        h0 = ru.TH1F()
        h0 = histo_file.Get("ZZ4Mu")
       
	final_histo = ru.TFile("final_histo.root", "RECREATE")
	
	# define canvas 
	cnv = ru.TCanvas("canv", "canv", 400, 400)

        final_histo.cd()
        cnv.cd()

        h0.Draw("HIST")

Error message:

  File "plotHisto.py", line 156, in <module>
    h0.Draw("HIST")
ReferenceError: attempt to access a null-pointer

Thanks in advance,
Aya

_ROOT Version: 6.18.04
_Platform: Ubuntu.18.04

2

After the line you better check that h0 is really set. It might be that Get did not work. For instance if ZZ4Mu does not exist in your file.

3

Thanks for answering, how could I be able to check it?

4

rootls -l final_histo.root

5

Just test if the returned pointer exits, ie: is not null.

6

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