A really weired issue in (c++) code - c++

Question

I have defined a variable in my code:
double R0;
When I set the variable less than 0.9, the code doesn’t run without no error! I have also written cout<<2; exactly after main(){ but the program doesn’t even show that! I am very confused :( what is the problem? When I change R0 to 0.9 or bigger than it, the code runs. This is the most minimal example I could provide:
#include <iostream>
#include <math.h>
#include <vector>
#include <functional>
#include <string>
#include <sstream>
using namespace std;
vector<double> getBoxCoords(int boxID, double L, double R0, int nbox)
{
vector<double> coords(4);
int yID = ceil((double)(boxID+1)/nbox);
int xID = fmod((boxID+1-(yID-1)*nbox), nbox);
if(xID==0)
xID = nbox;
coords[0] = (xID-1) * R0; // minX
coords[1] = (yID-1) * R0; // minY
coords[2] = min(xID * R0, L); // maxX
coords[3] = min(yID * R0, L); // maxY
return coords;
}
double PBC(double pos, double L)
{
if(fabs(pos) > L / 2.0)
return L-fabs(pos);
else
return fabs(pos);
}
int main()
{
std::cout << 2;
int N=100;
double rho=4.0, v0=0.03, eta=0.2, R0=0.03;
double L = pow(N/rho,0.5);
int nbox = (int)ceil(L/R0);
vector<vector<int>> box_neighbors;
vector<int> indices;
for(int i = 0; i < nbox * nbox; i++) //NUMBER OF BOX
{
vector<double> ci = getBoxCoords(i, L, R0, nbox);
indices.clear();
for(int j = 0; j < nbox * nbox; j++)
{
vector<double> cj = getBoxCoords(j, L, R0, nbox);
bool xflag=false,
yflag=false;
if (PBC(ci[0]-cj[0],L)<R0 || PBC(ci[0]-cj[2],L)<R0 || PBC(ci[2]-cj[0],L)<R0 || PBC(ci[2]-cj[2],L)<R0)
xflag=true;
if (PBC(ci[1]-cj[1],L)<R0 || PBC(ci[1]-cj[3],L)<R0 || PBC(ci[3]-cj[1],L)<R0 || PBC(ci[3]-cj[3],L)<R0)
yflag=true;
if(xflag && yflag)
indices.push_back(j);
}
box_neighbors.push_back(indices);
}
return 0;
}
How can I remove the problem? Could anyone provide a runnable answer?

Answer 1

First thing first, the debug std::cout << 2; is now shown because you do not end the stream, proper way of doing it is
std::cout << 2 << std::endl;
then you will be able to see the debugging message.
Secondly, Your program runs, but takes too much time to finish, when R0 is small. For the given value, that is, 0.03, both layers of the loop will execute nbox * nbox times which is 27889, thus, 777796321 in total.