Use Matlab to realise iterations in one time step

Dear all,

I would like to use Matlab to realise iterations in one time step.

For example, the Youngs modulus E has a linear relationship with stress S, E=a*S. Before the first iteration, the E=E0, then we calculate the S0, the E0 changes to E1 according to stress; once E1 is obtained, S0 would redistribute to form S1.......until the Sn-S(n-1)<a certain value, then go to next time step.

I don't know how to write the code for this in Matlab, could anybody help me out? Thanks.

Regards,
Albert