oversample increase bandwidth

#include <math.h>
#include <string>
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
 int xn=0, N = 250;
 double x[N],y[3*N];
 string filename,outfilename;
   ifstream instream;
  
   cout<<"file"<<endl;
 cin>>filename;                        
 instream.open(filename.c_str());
 cout <<"successful"<<endl;

while(xn != N)
{
 instream >> x[xn];
 cout << "x"<<xn << " = " << x[xn] << endl;
 xn++;
 }
for(xn=0;xn!=N-1;xn++)
{
 y[xn]=x[xn];
 y[xn+1]=0.3333*x[xn]+0.6666*x[xn+1];
 y[xn+2]=0.6666*x[xn]+0.3333*x[xn+1];
 }

cout<<"outfile"<<endl;
cin>>outfilename;
ofstream outfile(outfilename.c_str());
for(xn=0;xn!=N-1;xn++)
{
 outfile<<0.01*xn<<" "<<y[xn]<<endl;
 outfile<<0.01*(xn+0.33333)<<" "<<y[xn+1]<<endl;
 outfile<<0.01*(xn+0.66666)<<" "<<y[xn+2]<<endl;
 }

cout<<"done"<<endl;
return 0;
 }