Get error of TGraphAsymmError

ROOT
1

I have a TGraphAsymmError object and I need to find out its y-value and its error, given an x-value. For the y-value I can use graph->Eval(x) For the uncertainty, there is graph->GetErrorX(n) but it takes bin number n as an input and I don’t know that. I tried graph->GetXaxis()->FindBin(n)but I think this uses a different binning (?). I know that TGraph’s are unbinned, hence using a bin number is probably not a good idea. But how would I do something like graph->GetError(x)

2

Hi,
In fact Eval doesn’t return value at point but value from interpolation function. I suppose that only solution it’s making loop and checking all points e.g.

Double_t x,y;
for(int i=0;i<graph->GetN();i++){
	graph->GetPoint(i,x,y);
	if(x==what_you_want){
	return gr->GetEYhigh/low();
	}
}
3

Thanks. Yes, I did something similar now. Although x is not necessarily a point in the TGraph, so I first have to find the nearest point. Making a histogram out of this TGraphAsymmError is not possible either?

4

What do you mean by making histogram? Do you want to convert TGraphAssymErrors into TH1D? It might be problem because as far as I know TH1D doesn’t support “asymetric errors”

5

Hi,

Can you give us some context? What are you trying to do?

Axel.