How to write 1D matlab code to solve poission's equation by multigrid method

Hello Friends, I am developing a code to solve 1D Poisson's equation in matlab by multigrid method. I am getting the answer but not accurately. Please, help me to overcome with this difficulties. Here is my code for two grid method....

%%clear command and workspace window
    clc;
    clear; close all;
    tic;
    %%parameters
    Nx               = 256;                    % no of grid on x axis
    Lx               = pi/2 ;                % length of the system along x direction
    tol1             = 1.e-12;               % tolerence
    tol2             = 1.e-8;               % tolerence
    error1           = 1; 
    error2           = error1;                         % error
    dx               = Lx/(Nx);
    x                = (0:dx:Lx)' ;             % length of x axis
    %%boundary condition
    u               = ones(Nx+1,1);
    u(1)            = 1;                    % top
    u(end)          = 1;                    % bottom
    f               = -u;
    uold            = u;

      %%exact solution
      u_exact         = sin(x)+cos(x);
      p               = 0;
      residual         = zeros(Nx+1,1);
      nc2              = length(residual(1:2:end));
      nc4              = length(residual(1:4:end));

      cor_res2         =   zeros(nc2,1);
      cor_res4         =   zeros(nc4,1);

      ec2               = zeros(nc2,1);
      ec4               = zeros(nc4,1);

      %%iterate till GS converges
      itrc = 6;
  %     while  max(error2(:))> tol2
  for p = 1:5
  %     p = p+1;
          f               = -u;
          uold        = u;

      for k           = 1 : itrc
          uold        = u;
          for i       = 2:Nx
              u(i)    = 0.5.*( uold(i+1) + u(i-1)- dx^2.*f(i) );             
          end 
      end

      %%residual on fine grid
      for k =1:itrc
          for i           = 2:Nx
              residual(i) = f(i)-( u(i-1) - 2.*u(i) + u(i+1) )./dx^2;
          end 
      end

      %%coarse resdual calculation 
      cor_res2(1)      = 0.5.*residual(1)+0.25.*residual(2);
      for i            = 2:nc2-1
          cor_res2(i)  =  0.25.*residual(2.*i-1)+ 0.5.*residual(2*i)+ 0.25.*residual(2*i+1);
      end
      cor_res2(end)    = 0.25.*residual(end-1)+0.5.*residual(end);

      %%coarse grid error calculation
      z               = 0;
      while  max(error1(:))> tol1 
          z           = z+1;
          ecold        = ec2;
          for i       = 2:nc2-1
             ec2(i)     = 0.5.*( ec2(i-1) + ec2(i+1) + dx^2 .* cor_res2(i) );

             error1 = abs((ecold-ec2)./ec2);
          end 
          if (error1<tol1)
          break
          end
          z           = z+1;
      end
      nc4 = length(1:2:cor_res2);
      cor_res4 = zeros(nc4,1);
      cor_res4(1)      = 0.5.*cor_res2(1)+0.25.*cor_res2(2);
      for i            = 2:nc4-1
          cor_res4(i)  =  0.25.*cor_res2(2.*i-1)+ 0.5.*cor_res2(2*i)+ 0.25.*cor_res2(2*i+1);
      end
      cor_res4(end)    = 0.25.*cor_res2(end-1)+0.5.*cor_res2(end);

        %%coarse grid error calculation
      q               = 0;
      while  max(error1(:))> tol1 
          q           = q+1;
          e4cold        = ec4;
          for i       = 2:nc4-1
             ec4(i)     = 0.5.*( ec4(i-1) + ec4(i+1) + dx^2 .* cor_res4(i) );

             error2 = abs((e4cold-ec4)./ec4);
          end 
          if (error1<tol1)
          break
          end
          q           = q+1;
      end

      err_true        = u_exact-u;
      %%fine grid error by interpolation
      err4             = interp1( 1:4:Nx+1, ec4, 1:Nx+1,'linear' ); % error on fine grid

      err2             = interp1( 1:2:Nx+1, ec2, 1:Nx+1,'linear' ); % error on fine grid
      u               = u - err2'-err4'; % correction

      for k              = 1:itrc
          uold2          = u;
          for i          = 2:Nx
              u(i)       = 0.5.*( uold2(i+1) + u(i-1)- dx^2.*f(i) ); 
              error2 = abs((uold2-u)./u);
          end 
      end
      if (error2<tol2)
          break
      end
  end
      plot(x,u,x,u_exact,x,error2);
      % plot(residual);
      % plot(err);
      title({'Two point GS';['Number of iterations: ',num2str(p)]});

      toc;