`UndefVarError: x not defined` but x is in the model

Specific Domains Optimization (Mathematical)
1

Hi, I’m having the next problem:

I want to solve a given optimization problem using a JuMP model. In the algorithm that I am using I need to pass the model itself to another function having just the variables and I will construct the model (objective function and constraints) in this function. When I do that in the function I have the model but when I try to access to any variable I get the next Error:

>>@show m
m = A JuMP Model
Feasibility problem with:
Variables: 4
`VariableRef`-in-`MathOptInterface.EqualTo{Float64}`: 1 constraint
Model mode: AUTOMATIC
CachingOptimizer state: NO_OPTIMIZER
Solver name: No optimizer attached.
Names registered in the model: l, x, y

>>@show x
UndefVarError: x not defined

Can you help me?

2

You can do

x = m[:x]

See the doc for more details.

3

Thank you for your help, but I noticed right now that my problems isn’t that, but when I use Expressions and I want to use the eval() of this expressions as my objective function or constraints, like this:

m = Model()
@variable(m, x[1:2])
f = :(x[1])
@objective(m, Min, eval(f))

That works, but if I do exactly the same inside a function I get the next Error:

function fun()
    m = Model()
    @variable(m, x[1:2])
    f = :(x[1])
    @objective(m, Min, eval(f))
end

fun()
ERROR: VariableNotOwned{VariableRef}(x[1])

I want to keep using the expressions, it’s an important part of my algorithm. Please do you know why is this happening?

4

Using eval is generally a sign that you shouldn’t be doing what you are trying to do. In this case, eval is evaling things in the global-scope, not function scope.

What are you trying to do? And why/how are you building up f as an expression?

5

Ok, first at all thank you for your answer.

I’m trying to create a function that solve a class of problem doing an specific algorithm. This function must solve any problem of this class and isn’t just build the model and solve, so the idea is that I must pass to this function all the elements to build the model inside. My idea was passing the objective function and the constraints as expressions. Do you have any suggestion?

6

My idea was passing the objective function and the constraints as expressions. Do you have any suggestion?

I would pass a function that takes as argument the model and variables and calls the macro inside, e.g. instead of

function fun(f)
    m = Model()
    @variable(m, x[1:2])
    @objective(m, Min, eval(f))
end
fun(:(x[1]))

do

function fun(add_obj)
    m = Model()
    @variable(m, x[1:2])
    add_obj(m, x)
end
fun((m, x) -> @objective(m, Min, x[1]))