# Boundaries for @NLparameters

**URL:** <https://discourse.julialang.org/t/boundaries-for-nlparameters/21490>\
**Category:** Optimization (Mathematical)\
**Tags:** jump\
**Created:** [March 5, 2019, 12:01pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490 "2019-03-05T12:01:55Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [March 5, 2019, 12:01pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490/1 "2019-03-05T12:01:55Z")

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I am currently discovering the world of `JuMP.jl` and I have to say, I am amazed, thanks for this package!

**TL;DR:** how do I add constraints or limits to parameters which are not defined by `@variable` but `@NLparameter`?

**Full story:**

For some optimisation problems, I have to rerun the optimisation with different initial values, which is of course a common task. In order to save time (I have to be fast since the code is analysing realtime events), I would like to keep the model as it is and just change the initial values.

Just to have an explicit example (this is not an MWE, it’s just to demonstrate my workflow):

```julia
model = Model(with_optimizer(Ipopt.Optimizer))

register(model, :qfunc, 5, qfunc, autodiff=true)

@variable(model, -1000 <= d_closest <= 1000, start=0.0)
@variable(model, -10000 <= t_closest <= 10000, start=-400)
@variable(model, -1000 <= z_closest <= 1000, start=476)
@variable(model, -1 <= dir_z <= 1, start=0.2)
@variable(model, -1000 <= t₀ <= 1000, start=0)

optimize!(model)

```

Works perfectly fine for some - in the example hardcoded - starting values (they are later obtained by some topology checks beforehand).

I thought I can use `set_value()` to reset the initial values and call `optimize!()` again but there is no method for that type:

```julia
> set_value(d_closest, 10)
MethodError: no method matching set_value(::VariableRef, ::Int64)
Closest candidates are:
  set_value(!Matched::NonlinearParameter, ::Number) at /home/tgal/.julia/packages/JuMP/jnmGG/src/nlp.jl:144

```

Fair enough, the hint pointed me to create a `NonlinearParameter`, which can be constructed using `@NLparameter`.

So far so good:

```julia
@NLparameter(model, d_closest == 0.0)
...
...
set_value(d_closest, 20) # works

```

However, I was not able to figure out how to keep the boundary conditions.

I naively tried this one but it’s obviously not the way to do it:

```julia
@NLparameter(model, d_closest == 0.0)
@NLconstraint(model, cons1, -1000 <= d_closest <= 1000)

```

since later when running `optimize!(model)`, I get

```julia
IPOPT: Failed to construct problem.

```

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<div class="post-metadata">

**Author:** ![leethargo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/leethargo/32/6004_2.png) [@leethargo](https://discourse.julialang.org/u/leethargo)\
**Post date:** [March 5, 2019, 12:04pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490/2 "2019-03-05T12:04:17Z")

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I have not used `@NLParameter` before, so I don’t know about your particular issues.

However, I recently stumbled upon the [Parametron](https://github.com/tkoolen/Parametron.jl) package, that might also be helpful in your project.:

> Parametron makes it easy to set up and efficiently (ideally, with _zero_ allocation) solve instances of a **parameterized family** of optimization problems.

---

<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [March 5, 2019, 12:06pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490/3 "2019-03-05T12:06:03Z")

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Thanks @leethargo I will have a look but I’d rather stick to the current working environment. I switched the frameworks a couple of times already 😉

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**Author:** ![miles.lubin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/miles.lubin/32/279_2.png) [@miles.lubin](https://discourse.julialang.org/u/miles.lubin)\
**Post date:** [March 5, 2019, 1:14pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490/4 "2019-03-05T13:14:05Z")

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> [@tamasgal](#):
>
> Fair enough, the hint pointed me to create a `NonlinearParameter` , which can be constructed using `@NLparameter` .

This was not a good hint at all. You can use `set_start_value` to change the starting value of a variable. Parameters are unrelated to what you’re trying to do. `set_start_value` is easy to miss because it doesn’t appear in the docs ([start\_value and set\_start\_value are not listed in the documentation · Issue #1871 · jump-dev/JuMP.jl · GitHub](https://github.com/JuliaOpt/JuMP.jl/issues/1871)).

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<div class="post-metadata">

**Author:** ![tamasgal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamasgal/32/27946_2.png) [@tamasgal](https://discourse.julialang.org/u/tamasgal)\
**Post date:** [March 5, 2019, 1:47pm UTC](https://discourse.julialang.org/t/boundaries-for-nlparameters/21490/5 "2019-03-05T13:47:43Z")

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Oops, OK thanks 😉