Hi,
I would like to integrate a member multi function func(const double* x) over 0-coordinate only (x[0] is integration variable the rest coordinates correspond to some parameters) using ROOT::Math::IntegratorOneDim (see also attachement):
[code]#include “Math/Integrator.h”
#include “Math/WrappedFunction.h”
class MyClass{
public:
MyClass(){};
~MyClass(){};
double func(const double* x){
for(UInt_t i=0; i < 3; i++){
std::cout << " x[" << i << "] = " << x[i];
}
std::cout << std::endl;
return x[0]+x[1]+x[2];
}
typedef double (MyClass::*ForWrapper)(const double *x) ;
};
void integration(){
MyClass* myclass = new MyClass();
// double dim=3;
ROOT::Math::WrappedMemMultiFunction<MyClass,MyClass::ForWrapper> func(*myclass, &MyClass::func, 3);
//ROOT::Math::IntegratorOneDim integrator(ROOT::Math::IntegrationOneDim::kADAPTIVE,1.E-6,1E-4,1000, 0, Integration::GKRule::kGAUS21);
ROOT::Math::IntegratorOneDim integrator(ROOT::Math::IntegrationOneDim::kADAPTIVE);
double par1 = 2.;
double par2 = -1.;
double params[3] = {0.,par1,par2};
integrator.SetFunction(func,0,params); // integrate over 0-coordinate??
Double_t value = integrator.Integral(-1,1.);
/// integral should be equal to 2
std::cout << "Integral=" << value << " (should be equal to 2); par1=" << par1 << " par2=" << par2 << std::endl;
};[/code]
but the result of the integration is not 1 as I would expect but 0 instead:
[code]root [0] .x integration.C++
Info in TUnixSystem::ACLiC: creating shared library /home/jiri/Desktop/rootProblem/integration/./integration_C.so
x[0] = 0 x[1] = 0 x[2] = 0
x[0] = -0.987993 x[1] = 0 x[2] = 0
…
x[0] = -0.101142 x[1] = 0 x[2] = 0
x[0] = 0.101142 x[1] = 0 x[2] = 0
Integral=0 (should be equal to 2); par1=2 par2=-1[/code]
Two parameters par1 (x[1]) and par2 (x[2]) are zero inside MyClass::func. Am I passing the params vector somehow wrongly to the integrator? Or is there any other reason for obtaining the wrong result? I’ve tried several possibilities but without any success… 
Many thanks for any useful hint how to fix it!
Cheers,
Jiri
integration.C (1.12 KB)