# Line search

## Description

The line search functionality has been moved to LineSearches.jl.

Line search is used to decide the step length along the direction computed by an optimization algorithm.

The following Optim algorithms use line search:

• Accelerated Gradient Descent
• (L-)BFGS
• Momentum Gradient Descent
• Newton

By default Optim calls the line search algorithm HagerZhang() provided by LineSearches. Different line search algorithms can be assigned with the linesearch keyword argument to the given algorithm.

LineSearches also allows the user to decide how the initial step length for the line search algorithm is chosen. This is set with the alphaguess keyword argument for the Optim algorithm. The default procedure varies.

## Example

This example compares two different line search algorithms on the Rosenbrock problem.

First, run Newton with the default line search algorithm:

using Optim, LineSearches
prob = Optim.UnconstrainedProblems.examples["Rosenbrock"]

algo_hz = Newton(;alphaguess = LineSearches.InitialStatic(), linesearch = LineSearches.HagerZhang())
res_hz = Optim.optimize(prob.f, prob.g!, prob.h!, prob.initial_x, method=algo_hz)


This gives the result

 * Algorithm: Newton's Method
* Starting Point: [0.0,0.0]
* Minimizer: [0.9999999999999994,0.9999999999999989]
* Minimum: 3.081488e-31
* Iterations: 14
* Convergence: true
* |x - x'| ≤ 0.0e+00: false
|x - x'| = 3.06e-09
* |f(x) - f(x')| ≤ 0.0e+00 |f(x)|: false
|f(x) - f(x')| = 2.94e+13 |f(x)|
* |g(x)| ≤ 1.0e-08: true
|g(x)| = 1.11e-15
* stopped by an increasing objective: false
* Reached Maximum Number of Iterations: false
* Objective Calls: 44
* Gradient Calls: 44
* Hessian Calls: 14


Now we can try Newton with the More-Thuente line search:

algo_mt = Newton(;alphaguess = LineSearches.InitialStatic(), linesearch = LineSearches.MoreThuente())
res_mt = Optim.optimize(prob.f, prob.g!, prob.h!, prob.initial_x, method=algo_mt)


This gives the following result, reducing the number of function and gradient calls:

Results of Optimization Algorithm
* Algorithm: Newton's Method
* Starting Point: [0.0,0.0]
* Minimizer: [0.9999999999999992,0.999999999999998]
* Minimum: 2.032549e-29
* Iterations: 14
* Convergence: true
* |x - x'| ≤ 0.0e+00: false
|x - x'| = 3.67e-08
* |f(x) - f(x')| ≤ 0.0e00 |f(x)|: false
|f(x) - f(x')| = 1.66e+13 |f(x)|
* |g(x)| ≤ 1.0e-08: true
|g(x)| = 1.76e-13
* stopped by an increasing objective: false
* Reached Maximum Number of Iterations: false
* Objective Calls: 17
* Gradient Calls: 17
* Hessian Calls: 14