why my loop doesn't run or produce an error at all?

for NT = 2:60 %number of time steps

for n = 533:475 %number of grid points

maindiag = 2*(1+r)*ones(n+1,1);%n+1 because including one ghost point below i = 0

lowdiag = -r*ones(n+1,1);

updiag = -r*ones(n+1,1);

S = spdiags([lowdiag maindiag updiag],-1:1,n+1,n+1);

S = full(S);

S(1,1) = 1;

S(1,2) = -0.007875;

S(1,3) = -1;

S(n+1,n-1) = -1;

S(n+1,n) = 0;

S(n+1,n+1) = 1;

b(1,1) = 0;%first row of RHS vector b

%interior rows of RHS vector b

for i = 1:n-1

b(i+1,1) = r*C(i,1)+(2-2*r)*C(i+1,1)+r*C(i+2,1);

end

b(n+1,1) = 0;%final row of RHS vector b

C(1:n+1)= linsolve(S,b);

plot((0:dz:totz)',C)

end

end

I'm trying to use crank nicholson to solve for fick's 2nd law diffusion equation. The above code is a section of it. The fourth line from bottom is how I try to save the calculated result/concentration in each time step. r is a constant. Initial C matrix has been calculated and is a column vector at the first time step (NT = 1).

After run, n = [] and NT = 60. but C matrix didn't update at all and remained as initial at NT = 1.

Answer 1

[Abolfazl Chaman Motlagh]

second line in image (line 31 in your attached code).

for n=533:475

this is an empty loop. here watch this, i run it here:

n=533:475

n =

  1×0 empty double row vector

so what is in this loop doesn't run even one time for whatever value the NT has. hence there is no update for C.

[Xi Luan]

thank you! I should write n = 533:-1:475. You pointed out the direction to me!