How to determine some calculations in UDS function

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I would like to know how to calculate some cases as follows: 1. In unsteady state, we can calculate the previous time step value by using C_UDSI_M1(c,t,i). But for steady state, if I want to calculate the new value of UDS by using the old value (it means the value in previous iteration). How can I determine the old value? For example, I want to have: C_UDSI(c,t,i)_new = anpha*C_UDSI(c,t,i)new+(1-alpha)*C_UDSI_M1(c,t,i)old.

2. If I have two values of UDS like C_UDSI(c,t,1) and C_UDSI(c,t,2). I want to compare two value by using a subtract, for example e = C_UDSI(c,t,1) - C_UDSI(c,t,2) If e is smaller than a given value, then the program will stop (condition statement). How can I implement the condition? (Actually, I want to know where I write some statements for this).

I highly appreciate your help. Thank,