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

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Xi Luan on 16 Sep 2022
Commented: Xi Luan on 19 Sep 2022
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.

Abolfazl Chaman Motlagh on 16 Sep 2022
Edited: Abolfazl Chaman Motlagh on 16 Sep 2022
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 on 19 Sep 2022
thank you! I should write n = 533:-1:475. You pointed out the direction to me!