Branch & corresponding histogram

[ajaydeo]

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ROOT Version: 6.23/01
Platform: 64 bit Fedora 32
Compiler: gcc version 10.1.1 20200507 (Red Hat 10.1.1-1) (GCC)

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Hello,

I have a conceptual problem.

I have a data file from an experiment which contains several signals. I am converting entire data into a ROOT Tree format as follows:

CandleTree->Branch(sig_name[br_loop], &signals[br_loop],sig_format[br_loop]);

Now, when I display (say, using TBrowser or TTreeViewer ) one of the signals, I get the following:

But, this is not how actual the signal should look. Therefore, from the tree generated above, I sorted the entries into histogram as below:

        Long64_t nentries = t1->GetEntriesFast();

        for (Int_t i=0; i<nentries; i++) {
                t1->GetEntry(i);
                for(int j = 0; j < n_signals; j++){
                        if(signals[j]>0) his[j]->Fill(signals[j]);
                }
        }

then the histogram corresponding to that branch looks as follows:

Okay, I understand the difference in the entries. The second histogram is filled excluding zeros.
But, apart from entries - Why the two histograms are different when I am filling the data in both the cases?

Please can you help me understand what is happening.

Thank you.

[couet]

Looking at the plots, it seems the two histogram do not have the same bin width. The one below seems to have many more bins than the upper one.

[ajaydeo]

Thank you @couet for you reply. Yes, that possibly could be the reason. Then, how to create a branch to contain histograms with desired bin width?

The second histogram in my first post is made using TH1F(nm,nm,16384,0.,16384.); while the first histogram (from) has no specific width.

[couet]

Yes it has 16384 bins … try:

TH1F(nm,nm,100,0.,16384.);

[ajaydeo]

Thank you.

Actually, I want second histogram (because that correctly represents energy spectrum from the detector) directly from TTreeViewer.

Or, if at all possible, to generate branches with 16384 bins, with bindiwth=1.

Is this possible?

[couet]

The bin width is not a parameter you can set when you book an histogram. It is computed from the histogram limits and the number of bins: (xmax-xmin)/nbins

[ajaydeo]

Okay. May be I was not clear enough earlier. Let me try to rephrase my question.

Is it at all possible to create a branch to store histogram with say nbins = 16384?

[couet]

@ajaydeo

Is it at all possible to create a branch to store histogram with say nbins = 16384 ?

“brand creation” and “number of bins for the resulting histogram” are two unrelated things. You specify the number of bins at the histogram creation time. Let say:

auto h = new TH1F(nm,nm, 16384,0.,16384.);
t-Draw("x >> h");

I hope that answer your question

[ajaydeo]

That is exactly how I have generated the second histogram in the first post.

One last question: can I generate the second histogram directly from TTreeViewer by using corresponding branch? If yes, the please explain how?

[couet]

In the TTree Viewer there is a field called “histogram”. I guess you can put your histogram there.

[ajaydeo]

I dragged SIG007 branch to x, and then clocked Draw current selection box.
With this, I still do not get the behaviour that I am expecting. Please see below

[couet]

it works for me:

[ajaydeo]

It worked! Thank you very much @couet. Your time and efforts are much appreciated.