# How do I write @variable(z=x\[1\]+x\[2\])?
**URL:** https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149
**Category:** Optimization (Mathematical)
**Tags:** question, jump
**Created:** [April 7, 2022, 8:30am UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149 "2022-04-07T08:30:45Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@castlemas](https://discourse.julialang.org/u/castlemas)
#### Post date: [April 7, 2022, 8:30am UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/1 "2022-04-07T08:30:45Z")
Hi,
I am trying to solve a min-max optimisation problem that requires me to add in a constraint
How do I assign a constraint such that a variable equals to another variable?
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [April 7, 2022, 8:53am UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/2 "2022-04-07T08:53:04Z")
Hi there,
Since this is your first post, please read: [Please read: make it easier to help you](https://discourse.julialang.org/t/please-read-make-it-easier-to-help-you/14757). It has some advice on how to write a good question.
I’m not entirely sure what you mean by an equality in the objective function, but here’s how I would write your model:
```julia
model = Model()
@variable(model, x[1:2] >= 0, Int)
@variable(model, z)
@constraint(model, z == x[1]) # Note sure which one of these you meant
@constraint(model, z == 8x[1] + 5x[2]) #
@constraint(model, 8x[1] + 1.5x[2] <= z)
@constraint(model, x[1] + x[2] <= 6)
@objective(model, Max, z)
```
If that isn’t helpful, perhaps you could explain in a bit more detail what you’re trying to achieve?
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### Author:  [@castlemas](https://discourse.julialang.org/u/castlemas)
#### Post date: [April 7, 2022, 9:47am UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/3 "2022-04-07T09:47:19Z")
Hi, Thank you for your reply!
Here is the equation i was trying to solve:
What I have done so far on Julia-Jump
```julia
m = Model(Mosek.Optimizer)
@objective(m, Min, Z);
@variable(m, Z);
@variable(m, F[1:14]>=0);
#I have only included a portion of the constriant
@constraint(m,8*F[1]+1.5*F[2]+10*F[3] <= Z);
print(m)
```
**However, I face the error**
```julia
UndefVarError: Z not defined
Stacktrace:
[1] macro expansion
@ C:\Users\Admin\.julia\packages\MutableArithmetics\UtY4H\src\rewrite.jl:281 [inlined]
[2] macro expansion
@ C:\Users\Admin\.julia\packages\JuMP\lnUbA\src\macros.jl:1403 [inlined]
[3] top-level scope
@ In[34]:2
[4] eval
@ .\boot.jl:373 [inlined]
[5] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
@ Base .\loading.jl:1196
```
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [April 7, 2022, 9:52am UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/4 "2022-04-07T09:52:21Z")
Swapping the order of statements
```julia
using JuMP
m = Model()
@variable(m, Z);
@variable(m, F[1:14]>=0);
#I have only included a portion of the constriant
@constraint(m,8*F[1]+1.5*F[2]+10*F[3] <= Z);
@objective(m, Min, Z);
print(m)
```
seems to help.One tip: if you want to know what the macros (`@variable` etc.) are doing you can use `@macroexpand`.
---
### Author:  [@castlemas](https://discourse.julialang.org/u/castlemas)
#### Post date: [April 7, 2022, 9:27pm UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/5 "2022-04-07T21:27:07Z")
thanks sosososososososo much!!
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [April 7, 2022, 9:32pm UTC](https://discourse.julialang.org/t/how-do-i-write-variable-z-x-1-x-2/79149/6 "2022-04-07T21:32:38Z")
I don’t really understand the problem. Are these coupled as a bilevel program? Or do you want to solve two separate problems? First to find an upper bound on `Z`, and then to find a lower bound?