I'm having problems formulating a constraint in Pyomo where I want to determine a dependency between two variables. In my model I have 3 binary variables in total:

model.x = pyo.Var(model.M, domain=pyo.Binary)
model.y = pyo.Var(model.M, domain=pyo.Binary)
model.z = pyo.Var(model.M, domain=pyo.Binary)

My objective function with the parameter model.l would be:

obj_expr = sum(model.l[i] * model.x[i]   model.l[i] * model.y[i] for i in model.M)
model.obj = pyo.Objective(sense=pyo.maximize, expr=obj_expr)

I would like model.z to be the opposite of model.x, so that a 1 in model.x[i] would be a 0 in model.z[i].

I tried to do this with the following constraint:

    def dependent_var_con(model, i):
        lhs = model.z[i]
        rhs = model.x[i]
        return lhs != rhs 
    model.con = pyo.Constraint(model.M, expr=dependent_var_con)

Unfortunately this doesn't work and I'm getting the following error:

Cannot convert non-constant Pyomo expression (z[0] == x[0]) to bool. This error is usually caused by using a Var, unit, or mutable Param in a Boolean context such as an "if" statement, or when checking container membership or equality.

Does anyone have an idea how to deal with this issue? Thank you!

CodePudding user response：

You cannot use the "not equal" logical operator in a linear program. All you get are inequalities and equal. :)

However, if you just want to invert a binary variable, it is pretty straightforward with a little algebra. In your case:

z[i] == 1 - x[i]

pop that in a constraint for all i