normalizing histograms

Richard Dubois (richard@SLAC.stanford.edu)
Tue, 01 Jul 1997 13:12:51 -0700

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I am using 2 PAW ntuples h2root-ed. I want to overplot variables from
each file, normalized by entries. I can now sort out (see attachment) how
to normalize them when I have explicitly defined the histograms.

I am curious to know if I can also do it in the 'interactive' mode,
using the h10 TTree object, viz:

TFile f("file1.root");
TFile g("file2.root");

f.cd();
h10.Draw("momentum");
g.cd();
h10.Draw("momentum","","same");

Can I get pointers to these two histograms so I can normalize them
as per Rene's solution (in the attachment)?

Thanks,

Richard

-- 
Richard Dubois
SLD, Stanford Linear Accelerator Center
Richard@slac.stanford.edu
http://www.slac.stanford.edu/~richard/
415-926-3824
415-926-2923 (FAX)


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Date: Tue, 01 Jul 1997 18:00:12 +0200
From: Rene Brun <Rene.Brun@cern.ch>
Subject: Re: root question
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Richard Dubois wrote:
> 
> Hi Rene,
> 
>  In a quick search I didn't find an address to send root questions...
> 
>  I want to do an overlay of two plots, normalized by the number of
> entries for each. I haven't been able to figure out how to do the
> normalization. How do I do it?
> 


Hi Richard,
We have two lists roottalk and rootdev. To suscribe see http://root.cern.ch
roottalk is a general discussion list (we have about 300 people now)
rootdev is the Root developers. Send mail to rootdev@hpsalo.cern.ch
for bug reports (if any).
Now the answer to your question.
Suppose you have a Root file containing two histograms h1 and h2
You can do:
  Root > float norm1 = 1/h1.GetEntries()
  Root > float norm2 = 1/h2.GetEntries()
  Root > TH1F hnorm1 = norm1*(*h1)
  Root > TH1F hnorm2 = norm2*(*h2)
  Root > hnorm1.Draw();
  Root > hnorm2.Draw("same")


Note that if h1 and h2 are two TH1F objects (not pointers) created with
  TH1F h1(...)
  TH2F h2(..)
you should replace the statement  above TH1F hnorm1 = norm1*(*h1) by
  TH1F hnorm1 = norm1*h1
(*h1) dereferences the pointer (normal C++).


Rene




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