# NLopt:FORCED\_STOP for vector-valued constraint

**URL:** <https://discourse.julialang.org/t/nlopt-forced-stop-for-vector-valued-constraint/72117>\
**Category:** General Usage\
**Tags:** optimization, nlopt\
**Created:** [November 26, 2021, 2:53pm UTC](https://discourse.julialang.org/t/nlopt-forced-stop-for-vector-valued-constraint/72117 "2021-11-26T14:53:27Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![DanielVandH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielvandh/32/31134_2.png) [@DanielVandH](https://discourse.julialang.org/u/DanielVandH)\
**Post date:** [November 26, 2021, 2:53pm UTC](https://discourse.julialang.org/t/nlopt-forced-stop-for-vector-valued-constraint/72117/1 "2021-11-26T14:53:27Z")

</div>

The following code tries to solve Ax = b subject to x \>= 0 using the [NLopt](https://github.com/JuliaOpt/NLopt.jl) package.

```julia
using NLopt
A = [0.0372 0.2869; 0.6861 0.7071; 0.6233 0.6245; 0.6344 0.6170]
b = [0.8587; 0.1781; 0.0747; 0.8405]
function myfunc(x::Vector, grad::Matrix) 
    return sum((A*x-b).^2)
end
function myconstraint(result::Vector, x::Vector, grad::Matrix)
    result .= -x
end
opt = Opt(:LN_COBYLA, 2)
opt.min_objective = myfunc
inequality_constraint!(opt, (r, x, g) -> myconstraint(r, x, g), [1e-8, 1e-8])
minf, minx, ret = optimize!(opt, [0.5, 0.5])

```

The function `myfunc` computes ||Ax-b||\_2^2 and the `myconstraint` function imposes the vector-valued constraint [-x\_1, x\_2] \<= 0, modifying `result` in place. When this runs, I get a `:FORCED_STOP` error:

```julia
julia> minf, minx, ret = optimize!(opt, [0.5, 0.5])
(7.59933607e-315, [0.5, 0.5], :FORCED_STOP)

```

Why might this be? If I were to instead create scalar-valued constraints for x\_1 and x\_2 individually:

```julia
function myconstraint1(x::Vector, grad::Vector) 
    -x[1]
end
function myconstraint2(x::Vector, grad::Vector) 
    -x[2]
end
opt = Opt(:LN_COBYLA, 2)
opt.min_objective = myfunc
inequality_constraint!(opt, (x, g) -> myconstraint1(x, g), 1e-8)
inequality_constraint!(opt, (x, g) -> myconstraint2(x, g), 1e-8)
minf, minx, ret = optimize!(opt, [0.5, 0.5])

```

I get the correct result:

```julia
julia> minf, minx, ret = optimize!(opt, [0.5, 0.5])
(0.831455951263312, [1.4907648850642344e-17, 0.6929343970280805], :ROUNDOFF_LIMITED)

```

But I would like to implement more complicated constraints later on (not bound constraints and for many variables using matrix-vector products) so this is not feasible.

What is wrong with the first code that I am unable to obtain any results from `optimize!`, surely these should be the same?

---

<div class="post-metadata">

**Author:** ![DanielVandH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielvandh/32/31134_2.png) [@DanielVandH](https://discourse.julialang.org/u/DanielVandH)\
**Post date:** [November 27, 2021, 11:00pm UTC](https://discourse.julialang.org/t/nlopt-forced-stop-for-vector-valued-constraint/72117/2 "2021-11-27T23:00:22Z")

</div>

One other thing I notice is that, after putting println inside the functions, it doesn’t seem to actually evaluate the constraint at all, which seems even more confusing.