How do you loop code for all gridsizes in order to obtain iterations for each of them

I am trying to run everything within the first for loop for each ndiv so that at the end I can obtain 8 different iterations that line up with the different values within the ndiv array. How do I set up my code so that it loops through all of the ndiv (Gridsizes) values and gives me 8 different values of k that can be set up into an array format

Code:

a=25;
b=a;
ndiv=[4,8,16,32,128,256,512];
for nx=ndiv(1)
    nz=nx;
x = linspace(0, a, nx);
z = linspace(0, b, nz);
[X, Z] = meshgrid(x,z);

Tnp1 = zeros(nx, nz);

Tnp1(:,1) = 20;   % Left boundary
Tnp1(:,end) = 20; % Right boundary
Tnp1(1,:) = 20+380*sin((x*pi)/25)+205*sin((x*5*pi)/25);   % Bottom boundary
Tnp1(end,:) = 20; % Top boundary

err = 1;
tol = 1e-8;
k=0;

while  err > tol
   
    Tn = Tnp1;
    k=k+1;
 
    for i = 2:nx-1
        for j = 2:nz-1
            Tnp1(i,j) = (1/4)*(Tn(i+1,j)+Tn(i-1,j)+Tn(i,j+1)+Tn(i,j-1));           
        end
    end
    
    err = max(abs(Tnp1(:) - Tn(:)));
end
    
T = Tnp1;

end

Answer 1

[Walter Roberson]

ndiv=[4,8,16,32,128,256,512];
for nx=ndiv(1)

replace that with something like

ndiv=[4,8,16,32,128,256,512];
for nidx = 1:length(ndiv)
   nx = ndiv(nidx)
   stuff goes here
   all_k(nidx) = k;
end