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I would like to define a difference equation in pyomo for a temperature variable. The equation (simplified) looks like: $Temperature(t) = Temperature (t-1) + x(t)$ , for t>0

$Temperature (0) = 20 $

So this is what I tried in Pyomo

model.set_timeslots = pyo.RangeSet(0,95)  

model.variable_x = pyo.Var(model.set_timeslots, within=pyo.Binary)  
model.variable_temperature[0].fix(20)

def temperatureConstraintRule(model):
    model.variable_temperature(t) = model.variable_temperature(t-1) + model.variable_x(t) 
model.constraint_temperature = pyo.Constraint (model.set_timeslots, rule=temperatureConstraintRule)

However, I get the error message "SyntaxError: can't assign to function call"

Can anybody tell me how I can implement this difference equation in Pyomo?

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  • $\begingroup$ I was wondering whether this could be solved using CPLEX or what kind of solver did you use? $\endgroup$
    – Snowflake
    Apr 22, 2021 at 15:10
  • $\begingroup$ Hi Snowflake. Yes you could use CPLEX or GUROBI. I used both for this problem. Basically it is still a mixed-integer linear optimization problem which can be solved by any solver which supports these types of problems $\endgroup$
    – PeterBe
    Apr 22, 2021 at 15:14
  • $\begingroup$ If you do not have any integer variables then it would of course be a linear optimization problem. $\endgroup$
    – PeterBe
    Apr 22, 2021 at 15:16
  • $\begingroup$ Hey PeterBe, thank you for your response, however I have an error using Pyomo using CPLEX regarding nonlinear item problem (see: or.stackexchange.com/questions/6134/…). Could you perhaps give some pointers how one could solve this? $\endgroup$
    – Snowflake
    Apr 22, 2021 at 15:18
  • $\begingroup$ Well, CPLEX can't solve non-linear problems (if they are non-quadratic). So I guess you have to use another solver for that (I have no experience whatsoever with non-linear solvers). Or maybe you can try to formulate as a linear problem if that is possible. $\endgroup$
    – PeterBe
    Apr 22, 2021 at 15:27

1 Answer 1

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try:

model.set_timeslots = pyo.RangeSet(0,95)  

model.variable_x = pyo.Var(model.set_timeslots, within=pyo.Binary)  
i0 = 20

def temperatureConstraintRule(model, t):
    if t == model.set_timeslots.first():
         return model.variable_x[t] == i0 + model.variable_x[t]
    return model.variable_x[t] == model.variable_x[t-1] + model.variable_x[t]
model.constraint_temperature = Constraint(model.set_timeslots, rule=temperatureConstraintRule)
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    $\begingroup$ Cool. Thank you Steven. Your suggested code seems to work. I upvoted and accepted your answer. $\endgroup$
    – PeterBe
    Feb 22, 2021 at 17:37
  • $\begingroup$ Glad it helped @PeterBe $\endgroup$
    – Steven01123581321
    Feb 22, 2021 at 17:50

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