Subtract one histogram from another histogram

ROOT
1

I have two histograms, let’s say one named "“NAME1” and the other “NAME2”. How do I subtract one histogram from the other and find the difference between the two histograms in a new histogram called “DIFFERENCE”?

2

Hi @ERYI20 and welcome on the root forum.
You can use the TH1::Add() method of TH1 for this task.
The code should be like this.

difference->Add(name1,name2,1,-1);

Best,
Dilicus

3

Thanks for your answer. I used your command for a 2D hsitogram, and I am getting an unexpected output from the subtracted histogram. Could you tell me if the bins or something else should be considered in the subtracted histograms?

4

Could you please be more specific on the unexpected result you are obtaining?
In any case the 3 histogram you are using should have the same binning

5

I mean, the number of entries in the subtracted histogram is greater than the individual histograms. I also used the same bins for all three histograms.

6

This is how the number of entries of the new histogram is computed.

 Double_t nEntries = TMath::Abs( c1*h1->GetEntries() + c2*h2->GetEntries() );

I do not know if this explain your issue.

7

In this case, are we adding rather than subtracting the two histograms (h1 and h2)?

8

It depends from c1 and c2. Since you pass c1 equal to 1 and c2 equal to -1, the instruction in your case becomes

TMath::Abs (1* name1->GetEntries() + (-1)*name2->GetEntries() )

So your expected number of entries should be equal to the absolute value of the entries of name1 minus the entries of name2.

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9

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