MATLAB: Index exceeds array bounds in BVP problem

index exceeds array bounds

I'm trying to make FEM for my boundary value problem. matlab gives an error "Index exceeds array bounds.
Error in Gas_system_4>fun (line 28)
Y_gHMX= Y_gHMX_(i);
"
I assume its because i use element of vector Y_gHMX_ outside of the loop. But how to fix it
function Gas_system_4
clc
close all
n = 100;
  x0 = ones(2*n,1);
  sol = fsolve(@(x)fun(x,n),x0);
  norm(fun(sol,n))
  x = ((1:n)-1)/(n-1);
  plot(x,sol(1:n))
end
function res = fun(Yg,n)
%  some initial and boundary conditions and constanta
    % ...

    % ...
  x = ((1:n)-1)/(n-1);
  dx = 1/(n-1);
  T_g_ = Yg(1:n);
  Y_gHMX_ = Yg(n+1:2*n);
  for i=1:n
T_g= T_g_(i);
  end
  for i=n+1:2*n
Y_gHMX= Y_gHMX_(i);
  end
  %  some functions of T_g and  Y_gHMX
 Pe_g=... 
Pe_dk = ..
  w_gHMX=...
  Q_react_g=...
  
  
  res_T_g_ = zeros(n,1);
  res_Y_gHMX_ = zeros(n,1);
  
  res_T_g_(1) = T_g_(1)-T_fout;
  for i=2:n-1
    res_T_g_(i) = (1./Pe_g).*(T_g_(i+1)-2*T_g_(i)+T_g_(i-1))/dx^2 - (T_g_(i+1)-T_g_(i-1))/(2*dx) + Q_react_g;
  end
  res_T_g(n) = T_g_(n);
  res_Y_gHMX_(1) = Y_gHMXout(1);
  for i = 2:n-1
    res_Y_gHMX_(i) = (1./Pe_dk)*(Y_gHMX_(i+1)-2*Y_gHMX_(i)+Y_gHMX_(i-1))/dx^2-(Y_gHMX_(i+1)-Y_gHMX_(i-1))/(2*dx) +w_gHMX;
  end
  res_Y_gHMX(n) = Y_gHMX_(n)-0;
  res = [res_T_g;res_Y_gHMX]; 
end

Best Answer

Because i exceeds the length of Y_gHMX.
I see your index relates to the values you extracted from Y_gHMX_. However, once extracted, the index of Y_gHMX starts at 1, not n+1, and ends at (2*n-(n+1)+1). You could just use length(Y_gHMX) instead.
Related Question