Circular references in modelling or Variable bounds for constraints

[Prateek Gupta]

HI Guys,

I am new to Pyomo and trying to find my way creating a complex model that has circular dependencies. 

To explain it better - (The model is for manufacturing plant which uses return items of last process as input for other process)

ProcessA has an element X which is used to define total value of this process.

ProcessB - uses total value of ProcessA and generates bounds for element X. 

When I am creating this using different methods I face different problem - 

1> When using expressions to create constraints - Creating expressions is not possible and the expression it depends on hasn't been constructed yet.  - Order of defining expressions is a problem as both depends on each other.

2> When using Var for first Var and then using two sided constraint on it - error creating constraints: found a double-sided inequality expression (lower <= expression <= upper) but the lower bound was non-constant.

I am sure I haven't explained the problem in best possible way, but any insight would be great.

Thanks.

[Gabriel Hackebeil]

Can you try expressing your model as a small example using a ConcreteModel. This way you don’t have to worry about things not being constructed. E.g.,

model.x = Var()

model.y = Var([1,2,3])

model.o = Objective(expr=model.x + model.y[1])

model.c = Constraint(expr=model.x >= summation(model.y))

When using Var for first Var and then using two sided constraint on it - error creating constraints

When Pyomo encounters a double-sided inequality constraint, it must transform it into a body expression with lower and upper bounds that are simple numeric constants. If you write an equality constraint such as

x <= y <= z

where x, y, and z are variables, then Pyomo cannot do this. You must split the constraint into two separate constraints:

x <= y

and

y <= z

Gabe

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[Prateek Gupta]

Hi Gabriel,

Thanks, that's good to know. 

Although my problem was pretty simple and I guess I was just exhausted to make it work. 

Constraint I was trying to build was something like this

g(x) - a <= f(x) <- g(x) + a  

All I had to do was 

-a <= f(x) - g(x) <= a 

Cheers!

Prateek.