Runtime error in C ++ with native library, any suggestions?

I am new to C ++ and my own library. I am trying to code some simple things but dont know where i am going wrong. Here is my code:

#include<iostream>
#include<Eigen\Dense>
#include<cmath>
#include<fstream>

  using namespace std;
  using namespace Eigen;

  int main (int argc, char* argv[])
  {
      int n = 200;

      Matrix<double, Dynamic,1> u_n;
      u_n.setZero(n,1);

      Matrix<double, Dynamic,1> u_n_minus_one;
      u_n_minus_one.setZero(n,1);

      Matrix<double, Dynamic,1> u_n_plus_one;
      u_n_plus_one.setZero(n,1);

      std::ofstream fileWriter ("Values.txt");
      assert(fileWriter.is_open());

      float r=2;
      float F=100;

      for (int t=0;t<=5;t=t+1)
      {


                u_n_plus_one (0,0) =0;
//source of error
                u_n_plus_one.block(1,0,n-1,0) = pow(r,2)*( u_n.block(1,0,n-2,0)+ u_n.block(3,0,n,0)) + 2*(1-pow(r,2))*u_n.block(1,0,n-1,0)-u_n_minus_one.block(1,0,n-1,0);
//source of error
                u_n_plus_one (floor(n/2),0)=F;
                u_n_plus_one (n-1,0) =0 ;    //corrected from (n,0) to (n-1,0)


    u_n_minus_one = u_n ;
    u_n = u_n_plus_one ;

    //writing values to file
    if (remainder(t, 10) == 0)
    {
        fileWriter<<u_n.transpose()<<std::endl;
    }

   }

      fileWriter.close();
  }

      

        
        
        
      

    

I am trying to declare multiple matrices (although they are vectors). Then I do operations on the matrix blocks and finally write the results to a file. I didn't have any compile-time error, but during the run, the program worked.

I tried to debug the code and the error seems to lie in the // source of the error statements. Can anyone help me with this?