# Debugging NLopt :FORCED\_STOP

**URL:** <https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840>\
**Category:** Optimization (Mathematical)\
**Tags:** diffeq, optimization, nonlinear\
**Created:** [August 4, 2021, 9:12pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840 "2021-08-04T21:12:30Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![cricci](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cricci/32/25160_2.png) [@cricci](https://discourse.julialang.org/u/cricci)\
**Post date:** [August 4, 2021, 9:12pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/1 "2021-08-04T21:12:30Z")

</div>

Hello,  
I have some issue when using NLopt.jl to perform constrained minimization. I have a function that I am trying to maximize, whose value depend from the solution of a system of ODE. However the optimizer suddenly terminate with return code `:FORCED_STOP`. By this I don’t mean that it stops immediately after the first attempt/step as in [here](https://discourse.julialang.org/t/nlopt-forced-stop/47747), but it perform a certain amount of function calls (~ 100) and then stops.  
I have to specify also that

1. I am using a derivative-free method, in particular `:GN_AGS` or `:LN_COBYLA` to refine locally the solution
2. I added a check on the function I am trying to maximize in the form of `if sol.retcode != :Success do other stuff` in order to avoid problems with the solution
3. Weirdly the optimization fully works when changing some other parameters (unoptimized) of the system
4. I am using a wrapper function `nlfunc` to have it in the form that NLopt.jl likes, and possibly compute the gradient with Autodiff.

Here is a sketch of the code

```julia
u0 = init_profile()
prob = ODEProblem(df!,u0,(0.0,T_horizon))

N = 4 # or something in the same order
p_start = rand(N) # or some more accurate choice 
opt = Opt(:GN_AGS, N)
opt.max_objective = nlfunc
opt.initial_step = 0.01*ones(N)
opt.maxtime = MAX_TIME

#constraints
opt.lower_bounds = zeros(N)
opt.upper_bounds = ones(N)
inequality_constraint!(opt,example_constraint,1e-4)

V,p_next,ret_code = optimize(opt, p_start)

```

with the auxiliary functions

```julia
function df!(du,u,p,t)
     du .= your_ode(p,some_other_parameters)
end
function maximize_me(p)
    sol_p = DifferentialEquations.solve(
        prob,someAlg(),p=p,saveat=save_times)

    if sol_p.retcode != :Success
        return 0.0
    else
        return value_depending_on_sol(sol_p,p)
    end
end

function nlfunc(p,grad) #wrapper for NLopt with AutoDiff for grandient
    if length(grad) != 0
        println("---> computed gradient")
        ForwardDiff.gradient!(grad,maximize_me, p)
    end
    ret = maximize_me(p)
    return ret
end

```

Now the question is: How do you usually debug such problems with NLopt.jl? From the documentation of NLopt it looks like the error `:FORCED_STOP` arise when there is an error or an exception in the function one tries to minimize/maximize. However julia doesn’t throws any warning or such, hence I don’t know how to dig deeper.

Thanks in advance

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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:** [August 4, 2021, 9:36pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/2 "2021-08-04T21:36:08Z")

</div>

Log the points at which it is evaluating the function by adding a `println(p)` to your `nlfunc`. That may help.

It’s hard to offer more advice without a minimal working example (what is `value_depending_on_sol`, for example).

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**Author:** ![cgeoga](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cgeoga/32/216186_2.png) [@cgeoga](https://discourse.julialang.org/u/cgeoga)\
**Post date:** [August 4, 2021, 9:40pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/3 "2021-08-04T21:40:41Z")

</div>

You might also consider a try/catch block in your function that catches the error and prints it:

```julia
function maximize_me(p)
  try 
    return sum(log, p)
  catch er
    println("An error happened! It was $er")
    throw(er) # NLopt will probably swallow this one, which is why the printing is helpful.
  end
end

```

or something of that sort. I’m sure there’s a more elegant way to show the error with the whole stacktrace and stuff, but something like that is typically good enough for print debugging in the settings where I’ve had to use it.

---

<div class="post-metadata">

**Author:** ![cricci](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cricci/32/25160_2.png) [@cricci](https://discourse.julialang.org/u/cricci)\
**Post date:** [August 4, 2021, 11:22pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/4 "2021-08-04T23:22:20Z")

</div>

Partial answer to both @cgeoga and @odow  
I already tried adding `println` here and there to debug everything.  
However @cgeoga advice showed something quite surprising: I did exactly as you suggested and nothing happened. However I tried also to do the same thing inside the function `nlfunc` (wrapper for NLopt) and what happen is that:

- the optimization terminates immediately, at the first step (while usually kept running for a while)
- nothing gets printed!!! (of course with the `println` before the `throw`

That is quite suprising to me.  
I don’t know how NLopt.jl works internally, when the C++ library gets called, but maybe it is related to how/when the julia objects are passed to it.

I tried also something else, like initializing an empty array and pushing the error message into it but no luck. Actually the optimization terminates even without the `throw(er)` command. It seems the try catch statement already breaks it.

This doesn’t solve the problem, but at least gives some advice on where to look at

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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:** [August 5, 2021, 9:10am UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/5 "2021-08-05T09:10:04Z")

</div>

> I already tried adding `println` here and there to debug everything.

So what did it show? What was `p` and what was `ret`?

---

<div class="post-metadata">

**Author:** ![cricci](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cricci/32/25160_2.png) [@cricci](https://discourse.julialang.org/u/cricci)\
**Post date:** [August 5, 2021, 3:23pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/6 "2021-08-05T15:23:20Z")

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reasonable values,  
In my example `p` is a Vector of length 4 with values between 0 and 1. And that was what I saw by printing it in various stages.

moving on I worked some more time on it and find the solution to my problem, but nothing to the general usage of NLopt.  
In my case there was an error in

```julia
function df!(du,u,p,t)
     du .= your_ode(p,some_other_parameters)
end

```

As I anticipated, there was an error somewhere, I just needed to find where. Anyhow the error was not in the `DifferentialEquations.solve` part, meaning that for some choices of the parameter `p` the function ` your_ode(p,some_other_parameters)` actually produced an error (in Julia). So there was no solution to come out and hence no way for me to detect it with `if sol_p.retcode != :Success`.

What makes this a bit difficult is that, even if there was a Julia error (trying to select `x[nothing]` where `x` is a Vector), there was no way to know this, since NLopt swallows everything inside.

Do you know if using a Julia logger, a verbose mode, or some other shenanigans on NLopt, could makes it a bit more descriptive on the problem behind `:FORCED_STOP`? It’s a bit tricky otherwise

(btw thanks for help, in particular for the usage of `try/catch` that eventually lead me to the answer)

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<div class="post-metadata">

**Author:** ![hendri54](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hendri54/32/9621_2.png) [@hendri54](https://discourse.julialang.org/u/hendri54)\
**Post date:** [August 5, 2021, 5:39pm UTC](https://discourse.julialang.org/t/debugging-nlopt-forced-stop/65840/7 "2021-08-05T17:39:06Z")

</div>

Something like:

```julia
scalarDev = try
       f(x)
catch e
    	bt = catch_backtrace()
    	showerror(stdout, e, bt)
        @info "Guess vector: $(round.(x, digits = 3))"
        rethrow(e)
end        

```