ROOT::Fit::FitResult fixed / bound parameter states not updated

[AntoniMarcinek]

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Description

If one runs fitting several times fixing/releasing parameters with the same Fitter object and each time prints the FitResult, at some point all parameters are printed as "fixed", even though they are free. This did not happen in ROOT 5, because the result object was recreated on each fitting run, so maps holding the information were recreated. In ROOT 6 the object is updated, but the fFixedParams, fBoundParams are not updated in the process. Furthermore, fFixedParams and fBoundParams are maps, while it seems it would be simpler and more correct if these were std::vector<bool> with the size of the number of parameters, with values updated in FitResult::FillResult() and FitResult::Update() (not sure when one or the other function is called).

Reproducer

I guess Description is sufficient in this case.

ROOT version

at least 6.28 to 6.38

Installation method

irrelevant

Operating system

irrelevant

Additional context

No response

[guitargeek]

Hi! Based on your post, I have created this reproducer. Can you confirm that it reflects the problem you described?

double gauss(double *x, double *p) {
    double A = p[0];
    double mu = p[1];
    double sigma = p[2];
    double xx = x[0];
    return A * exp(-(xx - mu) * (xx - mu) / (2 * sigma * sigma));
}

void repro() {
    // --- Create data ---
    const int n = 50;
    double x[n], y[n];
    for (int i = 0; i < n; ++i) {
        x[i] = i * 0.1;
        y[i] = gauss(&x[i], (double[]){1.0, 2.0, 1.0});
    }

    // --- Model functor ---
    ROOT::Math::Functor f([&](const double *p){
        double chi2 = 0.0;
        for (int i = 0; i < n; ++i) {
            double yi = gauss(&x[i], (double*)p);
            double diff = y[i] - yi;
            chi2 += diff * diff;
        }
        return chi2;
    }, 3);

    ROOT::Fit::Fitter fitter;

    std::vector<double> initVals{1.0, 2.0, 1.0};
    fitter.Config().SetParamsSettings(initVals.size(), initVals.data());

    fitter.Config().ParSettings(0).SetName("A");
    fitter.Config().ParSettings(1).SetName("mu");
    fitter.Config().ParSettings(2).SetName("sigma");

    // Repeatedly run fits using the SAME fitter
    for (int iter = 0; iter < 6; ++iter) {
        bool fix = (iter % 2 == 0);
        int iparam = (iter / 2) % initVals.size();

        if (fix) {
            std::cout << "\n=== Iteration " << iter << " : FIX param " << iparam << " ===\n";
            fitter.Config().ParSettings(iparam).Fix();
        } else {
            std::cout << "\n=== Iteration " << iter << " : RELEASE param " << iparam << " ===\n";
            fitter.Config().ParSettings(iparam).Release();
        }

        fitter.FitFCN(f, nullptr, 3);
        const ROOT::Fit::FitResult &res = fitter.Result();

        std::cout << "FitResult says:\n";
        for (unsigned i = 0; i < 3; ++i) {
            std::cout << "  Param " << i
                      << " fixed? " << res.IsParameterFixed(i)
                      << "   value = " << res.Value(i)
                      << "\n";
        }
    }
}

Output:

=== Iteration 0 : FIX param 0 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 0   value = 2
  Param 2 fixed? 0   value = 1

=== Iteration 1 : RELEASE param 0 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 0   value = 2
  Param 2 fixed? 0   value = 1

=== Iteration 2 : FIX param 1 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 1   value = 2
  Param 2 fixed? 0   value = 1

=== Iteration 3 : RELEASE param 1 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 1   value = 2
  Param 2 fixed? 0   value = 1

=== Iteration 4 : FIX param 2 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 1   value = 2
  Param 2 fixed? 1   value = 1

=== Iteration 5 : RELEASE param 2 ===
FitResult says:
  Param 0 fixed? 1   value = 1
  Param 1 fixed? 1   value = 2
  Param 2 fixed? 1   value = 1

[AntoniMarcinek]

@guitargeek Yes, it does. Sorry for not creating the reproducer myself.

[guitargeek]

Don't worry, it was generatet using ChatGPT with only small fixups :)