ADNLPModel is simple to use and is useful for classrooms. It only needs the objective function $$f$$ and a starting point $$x^0$$ to be well-defined. For constrained problems, you'll also need the constraints function $$c$$, and the constraints vectors $$c_L$$ and $$c_U$$, such that $$c_L \leq c(x) \leq c_U$$. Equality constraints will be automatically identified as those indices $$i$$ for which $$c_{L_i} = c_{U_i}$$.

Let's define the famous Rosenbrock function

$f(x) = (x_1 - 1)^2 + 100(x_2 - x_1^2)^2,$

with starting point $$x^0 = (-1.2,1.0)$$.

using ADNLPModels

nlp = ADNLPModel(x->(x[1] - 1.0)^2 + 100*(x[2] - x[1]^2)^2 , [-1.2; 1.0])
ADNLPModel - Model with automatic differentiation backend ADModelBackend{
}
Problem name: Generic
All variables: ████████████████████ 2      All constraints: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
free: ████████████████████ 2                 free: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
lower: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                lower: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
upper: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                upper: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
low/upp: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0              low/upp: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
fixed: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                fixed: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
infeas: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               infeas: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nnzh: (  0.00% sparsity)   3               linear: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nonlinear: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nnzj: (------% sparsity)

Counters:
obj: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 grad: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 cons: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
cons_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0             cons_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 jcon: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jgrad: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                  jac: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0              jac_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jac_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                jprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0            jprod_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jprod_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               jtprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0           jtprod_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jtprod_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 hess: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                hprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jhess: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               jhprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0

This is enough to define the model. Let's get the objective function value at $$x^0$$, using only nlp.

using NLPModels # To access the API

fx = obj(nlp, nlp.meta.x0)
println("fx = $fx") fx = 24.199999999999996 Done. Let's try the gradient and Hessian. gx = grad(nlp, nlp.meta.x0) Hx = hess(nlp, nlp.meta.x0) println("gx =$gx")
println("Hx = $Hx") gx = [-215.59999999999997, -87.99999999999999] Hx = [1330.0 480.0; 480.0 200.0] Notice that the Hessian is dense. This is a current limitation of this model. It doesn't return sparse matrices, so use it with care. Let's do something a little more complex here, defining a function to try to solve this problem through steepest descent method with Armijo search. Namely, the method 1. Given $$x^0$$, $$\varepsilon > 0$$, and $$\eta \in (0,1)$$. Set $$k = 0$$; 2. If $$\Vert \nabla f(x^k) \Vert < \varepsilon$$ STOP with $$x^* = x^k$$; 3. Compute $$d^k = -\nabla f(x^k)$$; 4. Compute $$\alpha_k \in (0,1]$$ such that $$f(x^k + \alpha_kd^k) < f(x^k) + \alpha_k\eta \nabla f(x^k)^Td^k$$ 5. Define $$x^{k+1} = x^k + \alpha_kx^k$$ 6. Update $$k = k + 1$$ and go to step 2. using LinearAlgebra function steepest(nlp; itmax=100000, eta=1e-4, eps=1e-6, sigma=0.66) x = nlp.meta.x0 fx = obj(nlp, x) ∇fx = grad(nlp, x) slope = dot(∇fx, ∇fx) ∇f_norm = sqrt(slope) iter = 0 while ∇f_norm > eps && iter < itmax t = 1.0 x_trial = x - t * ∇fx f_trial = obj(nlp, x_trial) while f_trial > fx - eta * t * slope t *= sigma x_trial = x - t * ∇fx f_trial = obj(nlp, x_trial) end x = x_trial fx = f_trial ∇fx = grad(nlp, x) slope = dot(∇fx, ∇fx) ∇f_norm = sqrt(slope) iter += 1 end optimal = ∇f_norm <= eps return x, fx, ∇f_norm, optimal, iter end x, fx, ngx, optimal, iter = steepest(nlp) println("x =$x")
println("fx = $fx") println("ngx =$ngx")
println("optimal = $optimal") println("iter =$iter")
x = [1.0000006499501406, 1.0000013043156974]
fx = 4.2438440239813445e-13
ngx = 9.984661274466946e-7
optimal = true
iter = 17962

Maybe this code is too complicated? If you're in a class you just want to show a Newton step.

g(x) = grad(nlp, x)
H(x) = hess(nlp, x)
x = nlp.meta.x0
d = -H(x)\g(x)
2-element Vector{Float64}:
0.0247191011235955
0.38067415730337073

or a few

for i = 1:5
global x
x = x - H(x)\g(x)
println("x = $x") end x = [-1.1752808988764045, 1.3806741573033707] x = [0.763114871176314, -3.1750338547478294] x = [0.7634296788839187, 0.5828247754969103] x = [0.9999953110849887, 0.9440273238532714] x = [0.9999956956536669, 0.9999913913257132] Also, notice how we can reuse the method. f(x) = (x[1]^2 + x[2]^2 - 5)^2 + (x[1]*x[2] - 2)^2 x0 = [3.0; 2.0] nlp = ADNLPModel(f, x0) x, fx, ngx, optimal, iter = steepest(nlp) ([1.9999999068493834, 1.000000113517522], 3.911490500207018e-14, 8.979870927068038e-7, true, 153) External models can be tested with steepest as well, as long as they implement obj and grad. For constrained minimization, you need the constraints vector and bounds too. Bounds on the variables can be passed through a new vector. f(x) = (x[1] - 1.0)^2 + 100*(x[2] - x[1]^2)^2 x0 = [-1.2; 1.0] lvar = [-Inf; 0.1] uvar = [0.5; 0.5] c(x) = [x[1] + x[2] - 2; x[1]^2 + x[2]^2] lcon = [0.0; -Inf] ucon = [Inf; 1.0] nlp = ADNLPModel(f, x0, lvar, uvar, c, lcon, ucon) println("cx =$(cons(nlp, nlp.meta.x0))")
println("Jx = \$(jac(nlp, nlp.meta.x0))")
cx = [-2.2, 2.44]
Jx = sparse([1, 2, 1, 2], [1, 1, 2, 2], [1.0, -2.4, 1.0, 2.0], 2, 2)

In addition to the general nonlinear model, we can define the residual function for a nonlinear least-squares problem. In other words, the objective function of the problem is of the form $$f(x) = \tfrac{1}{2}\|F(x)\|^2$$, and we can define the function $$F$$ and its derivatives.

A simple way to define an NLS problem is with ADNLSModel, which uses automatic differentiation.

F(x) = [x[1] - 1.0; 10 * (x[2] - x[1]^2)]
x0 = [-1.2; 1.0]
nls = ADNLSModel(F, x0, 2) # 2 nonlinear equations
ADNLSModel - Nonlinear least-squares model with automatic differentiation backend ADModelBackend{
}
Problem name: Generic
All variables: ████████████████████ 2      All constraints: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0        All residuals: ████████████████████ 2
free: ████████████████████ 2                 free: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               linear: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
lower: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                lower: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0            nonlinear: ████████████████████ 2
upper: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                upper: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 nnzj: (  0.00% sparsity)   4
low/upp: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0              low/upp: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 nnzh: (  0.00% sparsity)   3
fixed: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                fixed: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
infeas: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               infeas: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nnzh: (  0.00% sparsity)   3               linear: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nonlinear: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
nnzj: (------% sparsity)

Counters:
obj: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 grad: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 cons: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
cons_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0             cons_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 jcon: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jgrad: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                  jac: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0              jac_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jac_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                jprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0            jprod_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jprod_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               jtprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0           jtprod_lin: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jtprod_nln: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                 hess: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0                hprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jhess: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0               jhprod: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0             residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
jac_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0       jprod_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0      jtprod_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
hess_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0       jhess_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0       hprod_residual: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0
residual(nls, x0)
2-element Vector{Float64}:
-2.2
-4.3999999999999995
jac_residual(nls, x0)
2×2 Matrix{Float64}:
1.0   0.0
24.0  10.0