Convert TGraph to TH1 histogram

Hello,

I want to convert a TGraph to a TH1 so that I can multiply two histograms bin by bin. What is the best way of accomplishing that?

Thank you

What do you mean “converting a TGraph to a TH1”? I don’t think there is one clear meaning attached to this conversion: a point in a TGraph has x and y coordinates, a histogram is a collection of bins and their number of entries.

Also, in principle it would be much easier to just fill an histogram as you fill the TGraph :slight_smile:
If your starting point must be a TGraph, then you can loop over all its points and fill a TH1 with those.
Assuming you want to fill TH1 with the x coordinate as histogram value and the y coordinate as weight (not tested):

TH1 h; // the histogram (you should set the number of bins, the title etc)
auto nPoints = graph.GetN(); // number of points in your TGraph
for(int i=0; i < nPoints; ++i) {
   double x,y;
   graph.GetPoint(i, x, y);
   h->Fill(x,y); // ?
}
3 Likes

Thank you for your answer.

I have some data in the form of a TGraph and some data in the form of a TH1 histogram, but i want to multiply the TGraph data with the TH1 data, and so I was wondering how to make a histogram from the TGraph.

I’m getting an error when i try to run the for loop, it says:

<Error: Can’t call TGraph::GetPoint(i,&x,&y) in current scope (tmpfile):2:
Possible candidates are…
(in TGraph)
/mnt/research/NuInt/NUISANCE_new/genie_v2_10_10/lamp/GENIESupport/root/lib/libHist.so -1:-1 0 public: virtual Int_t TGraph::GetPoint(Int_t i,Double_t& x,Double_t& y) const;
*** Interpreter error recovered ***>

Any idea how to fix this?

Call instead graph.GetPoint(i, x, y);

It worked!

Thank you both :slight_smile:

1 Like

Ah yeah my snippet had two errors: the first is the one the compiler complained about: x and not &x (the address of x) must be passed to TGraph::GetPoint.

The other problem is that I created x and y as int but you really want to make them double!!!
I edited my original snippet to reflect this changes.

1 Like

This topic was automatically closed 14 days after the last reply. New replies are no longer allowed.