Hello everyone,

I am interested in the following optimization problem that I would like to solve with JuMP:

\begin{split} \min_{x \in \mathbb{R}^n} & (z^Tx)^2 - \log(b^T x[l:\mbox{end}])\\ \mbox{ subject to }& x[l:\mbox{end}] \geq 0 \end{split}

with k<n, b \in \mathbb{R}^k and z \in \mathbb{R}^n. x[l:\mbox{end}] denotes the last l components of the vector x.

I am not sure of the way to provide parameters to a user-defined function. Also, I got an error saying that I should use only scalar expressions which seems to be the case

Here is my code:

Code for the cost function

```
function _cost(x, wquad, w∂,p)
J = 0.0
nx = length(x)
Ne = size(wquad,2)
for i=1:Ne
J += 0.5*sum(wquad[j,i]*x[j] for j=1:nx)^2
if p==0
J += -log(w∂[1,i]*x[end])
else
J += -log(sum(w∂[j,i]*x[end-(p+2)+j] for j=1:p+2))
end
end
J *= (1/Ne)
return J
end
```

Rest of the code for the optimization

```
p=2
wq = randn(32,200)
w∂ = randn(4,200)
Ne = 200
nx = size(wq,1)
nlog = size(w∂,1)
# Create the model
model = Model(Ipopt.Optimizer)
# Create variable x
@variable(model, x[1:nx], start = 0.001)
@NLparameter(model, p_order[1]== p)
@NLparameter(model, z[i = 1:nx, j = 1:Ne] == max(wq[i,j], 0.001))
@NLparameter(model, b[i = 1:nlog, j = 1:Ne] == max(w∂[i,j], 0.001))
# Define cost function
cost(x...) = _cost(x, z, b, p_order)
register(model, :cost, 1, cost, autodiff=true)
# Set lower bound on x
for i in nx-nlog:nx
set_lower_bound(x[i], 0.0)
end
@NLobjective(model, Min, cost(x))
@time optimize!(model)
```

Code return message

```
Unexpected array VariableRef[x[1], x[2], x[3], x[4], x[5], x[6], x[7], x[8], x[9], x[10], x[11], x[12], x[13], x[14], x[15], x[16], x[17], x[18], x[19], x[20], x[21], x[22], x[23], x[24], x[25], x[26], x[27], x[28], x[29], x[30], x[31], x[32]] in nonlinear expression. Nonlinear expressions may contain only scalar expressions.
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] _parse_NL_expr_runtime(::Model, ::Array{VariableRef,1}, ::Array{JuMP._Derivatives.NodeData,1}, ::Int64, ::Array{Float64,1}) at /Users/me/.julia/packages/JuMP/CZ8vV/src/parse_nlp.jl:207
[3] top-level scope at /Users/me/.julia/packages/JuMP/CZ8vV/src/parse_nlp.jl:82
[4] top-level scope at /Users/me/.julia/packages/JuMP/CZ8vV/src/macros.jl:1274
[5] top-level scope at In[37]:28
```