Clone empty TH1

ROOT
1

Hi,

Is there a way to clone an empty histogram (i.e. clone only its dimensions) as there’s a way to clone an empty TTree (CloneTree(0))?
I’d like to avoid the overhead of cloning a histogram and then resetting it.

Thanks,
N.

2

[quote=“Nir”]Hi,

Is there a way to clone an empty histogram (i.e. clone only its dimensions) as there’s a way to clone an empty TTree (CloneTree(0))?
I’d like to avoid the overhead of cloning a histogram and then resetting it.

Thanks,
N.[/quote]

Strange question, can you, please, give more details? Why do you need to clone the “void”, if you can simply create the histogram with required dimensions?

3

Hi,

I can see the point but no, the only interface we offer is Clone() + Reset(). Maybe you can Clone() before filling the original histogram?

Cheers, Axel.

4

tpochep: Yes, I was asking if there already was a function which created an empty histogram according to the given dimensions of a given histogram. This is a pretty common operation for efficiency studies.

I ended up doing this:

TH1F* cloneDims1d(TH2F* hist)
{
  if (hist == NULL) return NULL;
  TH1F* cloneHist = new TH1F(Form("%s_clone", hist->GetName()), 
                             hist->GetTitle(), 
                             hist->GetNbinsX(),
                             hist->GetXaxis()->GetXbins()->GetArray());
  return cloneHist;
}

But it would be quite nice if there was a build-in function that did it - especially for higher dimension hists.

N.

5

[quote=“Nir”]
I ended up doing this:

TH1F* cloneDims1d(TH2F* hist)
{
  if (hist == NULL) return NULL;
  TH1F* cloneHist = new TH1F(Form("%s_clone", hist->GetName()), 
                             hist->GetTitle(), 
                             hist->GetNbinsX(),
                             hist->GetXaxis()->GetXbins()->GetArray());
  return cloneHist;
}

But it would be quite nice if there was a build-in function that did it - especially for higher dimension hists.

N.[/quote]

Hmm. What you can do, is just copy taxis. Like this:

.....
TH1F * h1 = .... //hist to copy dimensions from.

....
TH1F h2  = ...;

Now:

h1->GetXaxis()->Copy(*h2.GetXaxis());

And you’ll have both number of bins and range from h1 in h2. Is it what you want?