GetParameter from a TF1 after fitting a graph

ROOT
1

I have a set of data pressx that is an array of doubles representing different pressures, and ratey, which is a 3-d array of rates which all correspond with those pressures.
This code works fine, it gives the expected output.

for(int i = 0; i < 80; i++) for(int j = 0; j < 4; j++) { TGraph *gr = new TGraph(count, pressx, ratey[i][j]); gr->Fit("pol1", "N"); TF1 *fit = gr->GetFunction("pol1"); // doesn't do anything yet }
Which is something like

Fitting results: Parameters: NO. VALUE ERROR 0 1.394513 5.089739 1 -0.001425 0.007489
For each one of the 320 arrays.

This doesn’t work:

for(int i = 0; i < 80; i++) for(int j = 0; j < 4; j++) { TGraph *gr = new TGraph(count, pressx, ratey[i][j]); gr->Fit("pol1", "N"); TF1 *fit = gr->GetFunction("pol1"); cout << fit->GetParameter(0) << endl; cout << fit->GetParameter(1) << endl; }

I get output like [code]Fitting results:
Parameters:
NO. VALUE ERROR
0 1972353841828572710836978851435764041775644420634645785532212557621689350216983729690258820091815214520707018459778846229014730463689795283159686675440788434341446857549396297720976863864596521606573053437174786929002242367013541837766078679220224.000000 5.089739
1 20379705714213766341736252165763112630883593503138327473248784208480583523907746432706736109011669045963268457781549463239180946361389043090791873471552147397816692953832362689851848513703833775709707432007304360176606345363332255706174004920320.000000 0.007489

*** Break *** segmentation violation
Generating stack trace…
Aborted
[/code]

And if I use “QN” for the fit’s options, I just get no output other than the segfault.

I assume that I’m not doing it right, but what’s wrong about my method?

2

The answer is quite obvious (see doc of TGraph::Fit). When you specify the option “N” the function is not stored in the graph object. So you should get a null pointer when trying to access it.

Rene

3

I’d have to disagree that it’s obvious, but you know how it works, so thank you very much!