# Error: JuMP model variable not defined
**URL:** https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670
**Category:** Optimization (Mathematical)
**Tags:** question, jump, error
**Created:** [November 24, 2020, 5:15am UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670 "2020-11-24T05:15:07Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@trathi](https://discourse.julialang.org/u/trathi)
#### Post date: [November 24, 2020, 5:15am UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/1 "2020-11-24T05:15:07Z")
I am trying to use the package [Dualization.jl](https://github.com/jump-dev/Dualization.jl), to reformulate a primal problem and solving the dual model separately. However, after solving I am not able to access the solution (the variable values) as they don’t seem to be attached to the JuMP model generated by the function `dualize`.
```julia
model = Model()
@variables(model, begin
x1>=0
x2
x3
x4<=0
end)
@constraint(model, con1, x1 - 2*x2 + 3*x3 + 4*x4 <= 3)
@constraint(model, con2, x2 + 3*x3 <= -5)
@constraint(model, con3, 2*x1 - 3*x2 - 7*x3 - 4*x4 == -2)
@objective(model, Min, 3*x1 + 2*x2 - 3*x3 + 4*x4)
dual_model = dualize(model; dual_names = DualNames("y_", "y_"))
set_optimizer(dual_model, Gurobi.Optimizer)
optimize!(dual_model)
sol = (value.(y_con1_1), value.(y_con2_1), value.(y_con3_1))
println(sol)
```
On running this script, the `dual_model` is solved correctly, but then on trying to access the solution it throws an error: `UndefVarError: y_con1_1 not defined`.
I am probably missing out on something trivial here. Can someone point it out? Thank you.
---
### Author:  [@blegat](https://discourse.julialang.org/u/blegat)
#### Post date: [November 24, 2020, 9:24am UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/2 "2020-11-24T09:24:33Z")
The Julia variable `y_con1_1` is not assigned to anything even if there is a variable with name `y_con1_1` in the model `dual_model`. You can get a variable by its name with [`variable_by_name`](https://jump.dev/JuMP.jl/v0.21.5/variables/#JuMP.variable_by_name), e.g.
```julia
y_con1_1 = variable_by_name(dual_model, "y_con1_1")
```
---
### Author:  [@trathi](https://discourse.julialang.org/u/trathi)
#### Post date: [November 24, 2020, 10:04am UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/3 "2020-11-24T10:04:06Z")
This works! Thank you.
---
### Author:  [@trathi](https://discourse.julialang.org/u/trathi)
#### Post date: [November 24, 2020, 11:48am UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/4 "2020-11-24T11:48:20Z")
@blegat I have a follow up question: In actual, I have a large primal problem with JuMP constraint defined as an array (say `con[1:100])`. Now, the `dual_model` variables form an array. Can I still use `variable_by_name` somehow to assign them all at once to a Julia array after the `dual_model is solved`. Since `variable_by_name` is taking variable name as a string as the parameter, I am not sure how to assign them all at once.
---
### Author:  [@blegat](https://discourse.julialang.org/u/blegat)
#### Post date: [November 24, 2020, 12:13pm UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/5 "2020-11-24T12:13:43Z")
You can assign `y_con` to a vector:
```julia
y_con = [variable_by_name(dual_model, "y_con[$i]") for i in 1:100]
```
---
### Author:  [@trathi](https://discourse.julialang.org/u/trathi)
#### Post date: [November 24, 2020, 12:22pm UTC](https://discourse.julialang.org/t/error-jump-model-variable-not-defined/50670/6 "2020-11-24T12:22:24Z")
Thank you!