Example of fitting in ROOT

Makefile

ifndef ROOTSYS
$(error ROOTSYS is not defined)
endif

all: minuit_example

minuit_example:
        g++ -o minuit_example minuit_example.cpp `root-config --libs --cflags` -lMinuit

clean:
        rm minuit_example

minuit_example.cpp

/**
 * Example of fitting in ROOT
 *
 * Uses ROOT's TMinuit (via TFitter) to find the minimum of a 1D parabola,
 * starting from a uniform random seed. The fit function uses some externally-
 * defined global state.
 */

#include <iostream>
#include <TMath.h>
#include <TFitter.h>
#include <TRandom.h>

static float two = 2.0;

void likelihood(int& npar, double* g, double& result, double* par, int flag) {
    result = TMath::Power(par[0], two);  // function using some external global
}

int main(int argc, char* argv[]) {
    gRandom->SetSeed(0);

    TFitter minuit(1);  // 1 parameter
    minuit.SetFCN(likelihood);

    float guess = gRandom->Uniform(-5, 5);
    minuit.SetParameter(0, "x", guess, 0.5, 0, 0);

    minuit.ExecuteCommand("MIGRAD", 0, 0);

    float best_fit = minuit.GetParameter(0);

    std::cout << "Best fit: " << best_fit << std::endl;

    return 0;
}