I have written the program below to calculate Standard deviation and mean of a given file of data. If I run it gives the wrong answers. Can someone help me go along?

```
#include <cstdlib>
#include <iostream>
#include <cmath>
#include <fstream>
using namespace std;
class Statistics{
double x;
int n;
double sum;
double sum2;
public:
Statistics(double _x, int n);
~Statistics();
void Accumulate(double data);
double mean()const;
double sigma()const;
void Print() const;
};
int main(){
double data;
Statistics a(data, 200);
a.Print();
system("PAUSE");
return EXIT_SUCCESS;
}
Statistics::Statistics(double _x, int n):x(_x),n(n){}
Statistics::~Statistics(){
cout<<"Statistics destructor called"<<endl;
}
void Statistics::Accumulate(double data){//This should add a data item to the accumulated statistics
ifstream in("file.txt");
for(int i=0; i < n ; i++){
in >> data;
sum+=data;
sum2+=data*data;
}
in.close();
}
double Statistics::mean()const{
return sum/n;
}
double Statistics::sigma()const{
return sqrt( (sum2 - (sum*mean()))/(n-1));
}
void Statistics::Print()const{
cout.precision(8);
cout << " N = " << n <<endl;
cout << " Mean = " << mean() <<endl;
cout << " Sigma = " << sigma()<<endl ;
}
```