# Automatic generation user-defined functions and NLexpression

**URL:** <https://discourse.julialang.org/t/automatic-generation-user-defined-functions-and-nlexpression/57216>\
**Category:** Optimization (Mathematical)\
**Tags:** jump\
**Created:** [March 15, 2021, 5:22pm UTC](https://discourse.julialang.org/t/automatic-generation-user-defined-functions-and-nlexpression/57216 "2021-03-15T17:22:33Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![GiuliaM](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giuliam/32/209287_2.png) [@GiuliaM](https://discourse.julialang.org/u/GiuliaM)\
**Post date:** [March 15, 2021, 5:22pm UTC](https://discourse.julialang.org/t/automatic-generation-user-defined-functions-and-nlexpression/57216/1 "2021-03-15T17:22:33Z")

</div>

Hi everyone!  
I’m trying to create a non-linear expression for a non-linear problem.  
I need to automatically register some user-defined functions (in the MWE func1 and func2) and use them in the NLexpression.  
In particular, I would like to obtain a 4x2 non-linear expression. The first column should contain the first function (func1) and the second column the second function(func2).  
I get the following error: ‘UndefVarError: i not defined’, but I don’t understand why.

I thank you and apologize if I have not been clear enough.

```julia
using Ipopt
using JuMP

model = Model(optimizer_with_attributes(Ipopt.Optimizer))
@variable(model,x[1:6])

#create 2 functions func1=(tanh(z-1)+1)/2 and func2=(tanh(z-1)+2)/2
for j in 1:2
    @eval ($(Symbol("func$j")))(z) = (tanh(z-j)+1)/2
    register(model, (Symbol("func$j")), 1, @eval ($(Symbol("func$j"))); autodiff = true)
end

#create a non linear expression (4 rows and 2 columns)
#[1,1] (func1(x[1]))*x[1] #[1,2] (func2(x[2]))*x[2]
#[2,1] (func1(x[2]))*x[2] #[2,2] (func2(x[3]))*x[3]
#[3,1] (func1(x[3]))*x[3] #[3,2] (func2(x[4]))*x[4]
#[4,1] (func1(x[4]))*x[4] #[4,2] (func2(x[5]))*x[4]
#[5,1] (func1(x[5]))*x[5] #[5,2] (func2(x[6]))*x[6]

#more in detail
#[1,1] ((tanh(x[1]-1)+1)/2)*x[1] #[1,2] ((tanh(x[2]-1)+2)/2)*x[2]
#[2,1] ((tanh(x[2]-1)+1)/2)*x[2] #[2,2] ((tanh(x[3]-1)+2)/2)*x[3]
#[3,1] ((tanh(x[3]-1)+1)/2)*x[3] #[3,2] ((tanh(x[4]-1)+2)/2)*x[4]
#[4,1] ((tanh(x[4]-1)+1)/2)*x[4] #[4,2] ((tanh(x[5]-1)+2)/2)*x[5]
#[5,1] ((tanh(x[5]-1)+1)/2)*x[5] #[5,2] ((tanh(x[6]-1)+2)/2)*x[6]

@NLexpression(model,con[i = 1:5, j=1:2], (@eval ($(Symbol("func$j")))(x[(i-1)+j]))*(x[(i-1)+j]))

#UndefVarError: i not defined

```

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [March 15, 2021, 7:42pm UTC](https://discourse.julialang.org/t/automatic-generation-user-defined-functions-and-nlexpression/57216/2 "2021-03-15T19:42:25Z")

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I’ve recently re-written the documentation for this: [Nonlinear Modeling · JuMP](https://jump.dev/JuMP.jl/dev/manual/nlp/#Raw-expression-input) (It isn’t in the released docs yet, still in `dev`.) Take a read, and post back if things are still unclear.

Unfortunately, you can’t create expressions using the raw-expression syntax, so you’ll have to create the objectives or constraints instead: [https://github.com/jump-dev/JuMP.jl/issues/2506](https://github.com/jump-dev/JuMP.jl/issues/2506)

As a general rule, using `@eval` is warning sign that things are incorrect. `@eval` runs thing in the global scope, which is why it doesn’t have access to the variable `I`.