TH1::Scale issue (maybe?)

ROOT
1

Hello rooters,

I have an issue with TH1::Scale(). I’ve been looking at the code and at previous posts in the forum but I can’t find the solution to this one.
The thing is that I want to use Scale() to, well, scale a TH1 up to the same normalization value as another TH1 that I have. The thing is that after I perform the scaling the integrals of both histograms are still not equal, but slightly different. Here’s a bit of code:

  Double_t h1Integral = h1->Integral();
  Double_t h2Integral = h2->Integral();
  Double_t scale = h2Integral / h1Integral;
  h1->Scale(scale);
  Double scale2 = h2->Integral() / h1->Integral() - 1;

In this case, I was expecting scale2 to be exactly 0, but it isn’t. Am I doing things right?

Thanks once again for your help!

Marcos

2

if your TH1 are TH1S or TH1C, the bin contents could overflow when you scale them.

3

Hi oxon,

Thanks for the fast reply, I’m working with TH1D so hopefully overflowing is not an issue.

Marcos

4

I think it would be better to attache a minimum reproducible script. The following example works as you expected.

root [0] TH1D h1("h1", "h1", 100, -5, 5)
root [1] TH1D h2("h2", "h2", 100, -5, 5)
root [2] h1.FillRandom("gaus", 1000)
root [3] h2.FillRandom("gaus", 2000)
root [4] Double_t h1Integral = h1.Integral()
root [5] Double_t h2Integral = h2.Integral()
root [6] Double_t scale = h2Integral / h1Integral
root [7] h1.Scale(scale)
root [8] Double_t scale2 = h2.Integral() / h1.Integral() - 1
root [9] scale2
(Double_t)0.00000000000000000e+00
5

Mmm, I was wrong, I was using TH1F. The thing is that I have ~ 10^9 entries in the histo, maybe I’m hitting a floating precision problem? I’ll try to regenerate the histos with TH1D and see if that works.

Thanks for your help!

Marcos

6

Picking up this old post, to ask about my issue: After scaling here, the number of entries in h1 didn’t change, which is strange given the description of Scale() here. Can someone please explain?

I am using root 6.32.02

7

Hi,
scaling the histogram does not change the number of entries in it. Why would you think it should?

8

It is scaling the bin contents. So, I wondered why does it not reflect in the entries. I wanted to make a plot where I was also displaying the stats. But, since stats did not reflect the updated bin contents, I was puzzled about the Scale().

9

Maybe a simple example can help see what’s going on; try this, check the stats box and think about it:

TH1D h1("h1", "h1", 10, -5, 5);
h1.Fill(3, 3.2);
h1.SetBinContent(2, 5.1);
h1.Draw("hist");
10

Yes, the entries depend on how many times I use SetBinContent()/Fill(). That is what you wanted to convey, right?

So, this makes me feel like a TH1F’s entries should display something like the area, shouldn’t it? That would seem like a more appropriate description of the diagram?