How do I structure two for loops which do very different things?

I am modeling a physics problem. I have an initial vector of values for each point in the domain. I want to use explicit Euler method to step in time to model the unsteady problem. How do i manifest the two for loops, one for the time procedure, and one for calling the elements of the vector#?

I have coded the numerical analysis model, however I have come undone at how to keep the loops separate whilst gaining the wanted outcome. I have tried numerous times to no success.

[James Tursa]

Can you post a short example, even if it is just pseudo-code, to show what you have done?

[Physics Guy]

Below is a sample of my code. I have a time dependent problem.

r=[ ri : dr : rf ] ; % values of independent variable
% Boundary conditions in linear system
u(1)= ui;
u(2) = uii;
u(N) = uff;
u(N+1)= uf;
% Create finite difference array
for j = 3:N-1
    A(j,j-2) = -1/(12*(dr^2))+1/(12*dr*r(j));
    A(j,j-1) = 16/(12*(dr^2))-8/(12*dr*r(j));
    A(j,j)   = -30/(12*(dr^2))-1/((r(j))^2);
    A(j,j+1) = 16/(12*(dr^2))+8/(12*dr*r(j));
    A(j,j+2) = -1/(12*(dr^2))-1/(12*dr*r(j));
end
% Boundary Values
b(1) = u(1) ;
b(2) = u(2);
b(N) = u(N);
b(N+1) = u(N+1);
b(3:N-1) = 0;

I can solve this for u no problem.

However i want to invoke a time stepping procedure. I don't know how to structure the two loops.

I need one to implement the time stepping. I believe i need a second one to iterate each element of the array. ( Im unsure if this is right)

Any advice?