Set parameters for pre-defined functions

[caforio]

I would like to fit an histogram with the pre-defined “gaus” function by setting the parameters. In the user guide it is said that one should use option “B” “[…] to fix one or more parameters [when] the fitting function is a predefined one, like polN , expo , landau , gaus”, but it does not say how to set the parameters.
Of course the syntax
gaus->SetParameters(par);
where “par” is an array, does not work.
How should I proceed?

Many thanks in advance

Davide

ROOT Version: 6.14
Platform: Linux 4.15.0-70-generic #79-Ubuntu SMP Tue Nov 12 10:36:11 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Compiler: Not Provided

---

[Wile_E_Coyote]

TF1 *f = ((TF1*)(gROOT->GetFunction("gaus")));
f->SetParameters(...);

[caforio]

Thanks for your reply.
Anyway it does not seem to work.
I have an histogram with more (Gaussian) peaks, and I want to fit each peak individually. Therefore first I search for the peaks with the TSpectrum() function, and then I perform the fit. Here is the piece of code that is supposed to do that (vOffset[station] is an array of TH1D histograms):

 // Searching for the peak(s), and fitting them
 peaks = s->Search(vOffset[station], 0.001, "noMarkov", 0.2);
 cout << "peaks: " << peaks << "\n";
 peak_pos = s->GetPositionX();
 for ( Int_t p = 0 ; p < peaks ; p++ ) {
   par[0] = vOffset[station]->GetBinContent(vOffset[station]->GetXaxis()->FindBin(peak_pos[p])); // "height"
   par[1] = peak_pos[p]; // "mean"
   par[2] = 0.02; // "sigma"
   cout << "peak position: " << peak_pos[p] << "\tpeak height: " << vOffset[station]->GetBinContent(vOffset[station]->GetXaxis()->FindBin(peak_pos[p])) << "\n";
   vOffset[station]->Fit("gaus", "B");
   TF1 *f = ((TF1*)(gROOT->GetFunction("gaus")));
   f->SetParameters(par);
 }

According to the output it seems that the fit is actually performed as many times as many peaks are found, but each time the parameters are overridden:
peaks: 2
peak position: 0.31 peak height: 81
FCN=114.609 FROM MIGRAD STATUS=CONVERGED 358 CALLS 359 TOTAL
EDM=3.45044e-09 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 Constant 5.26362e+01 3.57764e+00 1.57355e-02 -1.18895e-05
2 Mean 3.44236e-01 3.80586e-03 1.51409e-05 1.62027e-02
3 Sigma 4.77900e-02 2.13055e-03 7.50300e-06 -5.79584e-02
peak position: 0.39 peak height: 54
FCN=114.609 FROM MIGRAD STATUS=CONVERGED 118 CALLS 119 TOTAL
EDM=9.61076e-07 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 Constant 5.26361e+01 3.57759e+00 1.57454e-02 -4.28919e-05
2 Mean 3.44238e-01 3.80586e-03 1.51260e-05 4.80336e-01
3 Sigma 4.77900e-02 2.13049e-03 7.48970e-06 -5.99565e-01

The output histogram is also attached.

Many thanks for any further help!

Davide

[Wile_E_Coyote]

You first fit it and then set the values of parameters

[caforio]

Right (I thought that Fit() worked like Draw(), one first instantiates it and then changes the attributes), but even if I set the parameters before the fit the result does not change:

that part of the code:

       TF1 *f = ((TF1*)(gROOT->GetFunction("gaus")));
       f->SetParameters(par);
       vOffset[station]->Fit("gaus", "B");

the results:

peaks: 2
peak position: 0.31	peak height: 82
 FCN=105.475 FROM MIGRAD    STATUS=CONVERGED     111 CALLS         112 TOTAL
                     EDM=1.42505e-08    STRATEGY= 1      ERROR MATRIX ACCURATE 
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  Constant     5.21955e+01   3.51238e+00   1.45538e-02  -2.44286e-05
   2  Mean         3.43834e-01   4.16302e-03   1.63895e-05  -4.57769e-02
   3  Sigma        5.23988e-02   2.65127e-03   9.08220e-06   3.31301e-02
peak position: 0.39	peak height: 49
 FCN=105.475 FROM MIGRAD    STATUS=CONVERGED      83 CALLS          84 TOTAL
                     EDM=4.3694e-09    STRATEGY= 1  ERROR MATRIX UNCERTAINTY   3.6 per cent
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  Constant     5.21954e+01   3.51408e+00  -2.78515e-03  -3.67083e-05
   2  Mean         3.43834e-01   4.14783e-03  -1.96062e-06   1.83239e-02
   3  Sigma        5.23988e-02   2.64274e-03   2.06310e-06  -3.10724e-02

[Wile_E_Coyote]

Problem with TSpectrum: GetPositionX()

So, see the two source code files (peaks.C and SpectrumFit.cxx) given in the old topic from the link in my previous post here. BTW. When you post “source code” or “output” here, do remember to enclose them into two lines which contain just three characters ``` (see how your post has been edited above).

[caforio]

Thanks for the links, actually I already knew them and I used them to write my code, just in a simpler fashion. It looks like I have to use the full version
I will read them more deeply and maybe I will understand why I cannot set the fit parameters in my code.

[Wile_E_Coyote]

The problem is that your peaks are not well separated so, in the last step, you need to fit them all together.

[caforio]

Setting also the fit range helped a lot:

vOffset[station]->Fit("gaus", "B", "", peak_pos[p]-0.04, peak_pos[p]+0.04);

Regards