# Problem calculating the mean values

Hello, I wrote this code to calculate the emittance

``````Double_t emittance=0.;
Double_t x=0;
Double_t x2=0;
Double_t xp=0;
Double_t xp2=0;
Double_t xxp=0;
Double_t sigmax2=0;
Double_t sigmaxp2=0;
Double_t sigmaxsigmaxp=0;
int jentry;
int nentries = ts->GetEntries();
for(jentry=0; jentry<nentries;jentry++) {
ts->GetEntry(jentry);
x+=x;
x2+=x*x;
xp+=xp;
xp2+=xp*xp;
xxp+=x*xp;
}
x*=1./nentries;
x2*=1./nentries;
xp*=1./nentries;
xp2*=1./nentries;
xxp*=1./nentries;
sigmax2=x2-x*x;
sigmaxp2=xp2-xp*xp;
sigmaxsigmaxp=xxp-x*xp;
std::cout << "N = " << nentries << "\t x = " << x << "\t x2 = " << 2 << " \t xp = " << xp <<  " \t xp2= " << xp2 << " \t xxp= " <<  xxp << std::endl;
emittance=sigmax2*sigmaxp2-sigmaxsigmaxp;
emittance=TMath::Sqrt(emittance);
emittance*=1000000;
``````

You see, that I’ve to calculate some mean values (as , etc) but these values are very different than the values calculated by Root. For example, you see here my =-3,765328*10^-7

and surely the Root value is fine because using my value, I get a very big emittance (impossible)…but I can’t understand what is wrong in my code.

this is the macro emit.cpp (3.0 KB)
and this is the Root file https://we.tl/t-TMHdHaqL5D

Thank you

ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

Hi,
The difference could be caused by the fact that in the ROOT mean value maybe the underflow and overflow;s you are having in y are not used. You should call before filling the histogram `TH1::StatOverflows(true)` and you should get the same value

Lorenzo

Hello @moneta thank you. Unfortunately I tried the code, and the problem was due to the code and not to the underflow and overflow!

Anyway I solvede myself
I had to use the array!

``````Double_t emittance=0.;
Double_t x=0;
Double_t x2=0;
Double_t xp=0;
Double_t xp2=0;
Double_t xxp=0;
Double_t sigmax2=0;
Double_t sigmaxp2=0;
Double_t sigmaxsigmaxp=0;
int jentry;
int nentries = ts->GetEntries();
double xv;
double xpv;
for(jentry=0; jentry<nentries;jentry++) {
ts->GetEntry(jentry);
x+=xv;
x2+=xv*xv;
xp+=xpv;
xp2+=xpv*xpv;
xxp+=xv*xpv;
}
x*=1./nentries;
x2*=1./nentries;
xp*=1./nentries;
xp2*=1./nentries;
xxp*=1./nentries;
sigmax2=x2-x*x;
sigmaxp2=xp2-xp*xp;
sigmaxsigmaxp=xxp-x*xp;
emittance=sigmax2*sigmaxp2-sigmaxsigmaxp;
emittance=TMath::Sqrt(emittance);
emittance*=1000000;
std::cout << "N = " << nentries << "\t x = " << x << "\t x2 = " << x2 << " \t xp = " << xp <<  " \t xp2= " << xp2 << " \t xxp= " <<  xxp << std::endl;
``````

Now it works fine! Here the results and they are equal to the Root results!

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