# fcn2optimexpr

Convert function to optimization expression

## Syntax

``[out1,out2,...,outN] = fcn2optimexpr(fcn,in1,in2,...,inK)``
``[out1,out2,...,outN] = fcn2optimexpr(fcn,in1,in2,...,inK,Name,Value)``

## Description

example

````[out1,out2,...,outN] = fcn2optimexpr(fcn,in1,in2,...,inK)` converts the function `fcn(in1,in2,...,inK)` to an optimization expression with `N` outputs.```

example

````[out1,out2,...,outN] = fcn2optimexpr(fcn,in1,in2,...,inK,Name,Value)` specifies additional options using one or more name-value pair arguments. For example, you can save a function evaluation by passing `OutputSize`.```

## Examples

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To use a MATLAB™ function in the problem-based approach when it is not composed of supported functions, first convert it to an optimization expression. See Supported Operations on Optimization Variables and Expressions and Convert Nonlinear Function to Optimization Expression.

To use the objective function `gamma` (the mathematical function $\Gamma \left(x\right)$, an extension of the factorial function), create an optimization variable `x` and use it in a converted anonymous function.

```x = optimvar('x'); obj = fcn2optimexpr(@gamma,x); prob = optimproblem('Objective',obj); show(prob)```
``` OptimizationProblem : Solve for: x minimize : gamma(x) ```

To solve the resulting problem, give an initial point structure and call `solve`.

```x0.x = 1/2; sol = solve(prob,x0)```
```Solving problem using fminunc. Local minimum found. Optimization completed because the size of the gradient is less than the value of the optimality tolerance. ```
```sol = struct with fields: x: 1.4616 ```

For more complex functions, convert a function file. The function file `gammabrock.m` computes an objective of two optimization variables.

`type gammabrock`
```function f = gammabrock(x,y) f = (10*(y - gamma(x)))^2 + (1 - x)^2; ```

Include this objective in a problem.

```x = optimvar('x','LowerBound',0); y = optimvar('y'); obj = fcn2optimexpr(@gammabrock,x,y); prob = optimproblem('Objective',obj); show(prob)```
``` OptimizationProblem : Solve for: x, y minimize : gammabrock(x, y) variable bounds: 0 <= x ```

The `gammabrock` function is a sum of squares. You get a more efficient problem formulation by expressing the function as an explicit sum of squares of optimization expressions.

```f = fcn2optimexpr(@(x,y)y - gamma(x),x,y); obj2 = (10*f)^2 + (1-x)^2; prob2 = optimproblem('Objective',obj2);```

To see the difference in efficiency, solve `prob` and `prob2` and examine the differences in number of iterations.

```x0.x = 1/2; x0.y = 1/2; [sol,fval,~,output] = solve(prob,x0);```
```Solving problem using fmincon. Local minimum found that satisfies the constraints. Optimization completed because the objective function is non-decreasing in feasible directions, to within the value of the optimality tolerance, and constraints are satisfied to within the value of the constraint tolerance. ```
`[sol2,fval2,~,output2] = solve(prob2,x0);`
```Solving problem using lsqnonlin. Local minimum found. Optimization completed because the size of the gradient is less than the value of the optimality tolerance. ```
`fprintf('prob took %d iterations, but prob2 took %d iterations\n',output.iterations,output2.iterations)`
```prob took 21 iterations, but prob2 took 2 iterations ```

If your function has several outputs, you can use them as elements of the objective function. In this case, `u` is a 2-by-2 variable, `v` is a 2-by-1 variable, and `expfn3` has three outputs.

`type expfn3`
```function [f,g,mineval] = expfn3(u,v) mineval = min(eig(u)); f = v'*u*v; f = -exp(-f); t = u*v; g = t'*t + sum(t) - 3; ```

Create appropriately sized optimization variables, and create an objective function from the first two outputs.

```u = optimvar('u',2,2); v = optimvar('v',2); [f,g,mineval] = fcn2optimexpr(@expfn3,u,v); prob = optimproblem; prob.Objective = f*g/(1 + f^2); show(prob)```
``` OptimizationProblem : Solve for: u, v minimize : ((arg3 .* arg4) ./ (1 + arg2.^2)) where: [arg2,~,~] = expfn3(u, v); [arg3,~,~] = expfn3(u, v); [~,arg4,~] = expfn3(u, v); ```

You can use the `mineval` output in a subsequent constraint expression.

In problem-based optimization, constraints are two optimization expressions with a comparison operator (`==`, `<=`, or `>=`) between them. You can use `fcn2optimexpr` to create one or both optimization expressions. See Convert Nonlinear Function to Optimization Expression.

Create the nonlinear constraint that `gammafn2` is less than or equal to –1/2. This function of two variables is in the `gammafn2.m` file.

`type gammafn2`
```function f = gammafn2(x,y) f = -gamma(x)*(y/(1+y^2)); ```

Create optimization variables, convert the function file to an optimization expression, and then express the constraint as `confn`.

```x = optimvar('x','LowerBound',0); y = optimvar('y','LowerBound',0); expr1 = fcn2optimexpr(@gammafn2,x,y); confn = expr1 <= -1/2; show(confn)```
``` gammafn2(x, y) <= -0.5 ```

Create another constraint that `gammafn2` is greater than or equal to `x + y`.

`confn2 = expr1 >= x + y;`

Create an optimization problem and place the constraints in the problem.

```prob = optimproblem; prob.Constraints.confn = confn; prob.Constraints.confn2 = confn2; show(prob)```
``` OptimizationProblem : Solve for: x, y minimize : subject to confn: gammafn2(x, y) <= -0.5 subject to confn2: gammafn2(x, y) >= (x + y) variable bounds: 0 <= x 0 <= y ```

If your problem involves a common, time-consuming function to compute the objective and nonlinear constraint, you can save time by using the `'ReuseEvaluation'` name-value pair argument. The `rosenbrocknorm` function computes both the Rosenbrock objective function and the norm of the argument for use in the constraint $‖x{‖}^{2}\le 4$.

`type rosenbrocknorm`
```function [f,c] = rosenbrocknorm(x) pause(1) % Simulates time-consuming function c = dot(x,x); f = 100*(x(2) - x(1)^2)^2 + (1 - x(1))^2; ```

Create a 2-D optimization variable `x`. Then convert `rosenbrocknorm` to an optimization expression by using `fcn2optimexpr` and specifying `'ReuseEvaluation'`.

```x = optimvar('x',2); [f,c] = fcn2optimexpr(@rosenbrocknorm,x,'ReuseEvaluation',true);```

Create objective and constraint expressions from the returned expressions. Include the objective and constraint expressions in an optimization problem. Review the problem using `show`.

```prob = optimproblem('Objective',f); prob.Constraints.cineq = c <= 4; show(prob)```
``` OptimizationProblem : Solve for: x minimize : [argout,~] = rosenbrocknorm(x) subject to cineq: arg_LHS <= 4 where: [~,arg_LHS] = rosenbrocknorm(x); ```

Solve the problem starting from the initial point `x0.x = [-1;1]`, timing the result.

```x0.x = [-1;1]; tic [sol,fval,exitflag,output] = solve(prob,x0)```
```Solving problem using fmincon. Local minimum found that satisfies the constraints. Optimization completed because the objective function is non-decreasing in feasible directions, to within the value of the optimality tolerance, and constraints are satisfied to within the value of the constraint tolerance. <stopping criteria details> ```
```sol = struct with fields: x: [2×1 double] ```
```fval = 4.5793e-11 ```
```exitflag = OptimalSolution ```
```output = struct with fields: iterations: 44 funcCount: 164 constrviolation: 0 stepsize: 4.3124e-08 algorithm: 'interior-point' firstorderopt: 5.1691e-07 cgiterations: 10 message: '↵Local minimum found that satisfies the constraints.↵↵Optimization completed because the objective function is non-decreasing in ↵feasible directions, to within the value of the optimality tolerance,↵and constraints are satisfied to within the value of the constraint tolerance.↵↵<stopping criteria details>↵↵Optimization completed: The relative first-order optimality measure, 5.169074e-07,↵is less than options.OptimalityTolerance = 1.000000e-06, and the relative maximum constraint↵violation, 0.000000e+00, is less than options.ConstraintTolerance = 1.000000e-06.↵↵' bestfeasible: [1×1 struct] solver: 'fmincon' ```
`toc`
```Elapsed time is 164.410724 seconds. ```

The solution time in seconds is nearly the same as the number of function evaluations. This result indicates that the solver reused function values, and did not waste time by reevaluating the same point twice.

For a more extensive example, see Objective and Constraints Having a Common Function in Serial or Parallel, Problem-Based. For more information on using `fcn2optimexpr`, see Convert Nonlinear Function to Optimization Expression.

## Input Arguments

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Function to convert, specified as a function handle.

Example: `@sin` specifies the sine function.

Data Types: `function_handle`

Input argument, specified as a MATLAB variable. The input can have any data type and any size. You can include any problem variables or data in the input argument `in`; see Pass Extra Parameters in Problem-Based Approach.

Data Types: `single` | `double` | `int8` | `int16` | `int32` | `int64` | `uint8` | `uint16` | `uint32` | `uint64` | `logical` | `char` | `string` | `struct` | `table` | `cell` | `function_handle` | `categorical` | `datetime` | `duration` | `calendarDuration` | `fi`
Complex Number Support: Yes

### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside quotes. You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

Example: ```[out1,out2] = fcn2optimexpr(@fun,x,y,'OutputSize',[1,1],'ReuseEvaluation',true)``` specifies that `out1` and `out2` are scalars that will be reused between objective and constraint functions without recalculation.

Size of the output expressions, specified as:

• An integer vector — If the function has one output `out`1, `OutputSize` specifies the size of `out`1. If the function has multiple outputs `out`1,…,`out`N, `OutputSize` specifies that all outputs have the same size.

• A cell array of integer vectors — The size of output `out`j is the jth element of `OutputSize`.

Note

A scalar has size `[1,1]`.

If you do not specify the `'OutputSize'` name-value pair argument, then `fcn2optimexpr` passes data to `fcn` in order to determine the size of the outputs (see Algorithms). By specifying `'OutputSize'`, you enable `fcn2optimexpr` to skip this step, which saves time. Also, if you do not specify `'OutputSize'` and the evaluation of `fcn` fails for any reason, then `fcn2optimexpr` fails as well.

Example: ```[out1,out2,out3] = fcn2optimexpr(@fun,x,'OutputSize',[1,1])``` specifies that the three outputs `[out1,out2,out3]` are scalars.

Example: ```[out1,out2] = fcn2optimexpr(@fun,x,'OutputSize',{[4,4],[3,5]})``` specifies that `out1` has size 4-by-4 and `out2` has size 3-by-5.

Data Types: `double` | `cell`

Indicator to reuse values, specified as `false` (do not reuse) or `true` (reuse).

`'ReuseEvaluation'` can make your problem run faster when, for example, the objective and some nonlinear constraints rely on a common calculation. In this case, the solver stores the value for reuse wherever needed and avoids recalculating the value.

Reusable values involve some overhead, so it is best to enable reusable values only for expressions that share a value.

Example: ```[out1,out2,out3] = fcn2optimexpr(@fun,x,'ReuseEvaluation',true)``` allows `out1`, `out2`, and `out3` to be used in multiple computations, with the outputs being calculated only once per evaluation point.

Data Types: `logical`

## Output Arguments

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Output argument, returned as an `OptimizationExpression`. The size of the expression depends on the input function.

## Tips

• When possible, create your objective or nonlinear constraint functions by using supported operations on optimization variables and expressions instead of `fcn2optimexpr`. Doing so has these advantages:

• `solve` includes gradients calculated by automatic differentiation. See Effect of Automatic Differentiation in Problem-Based Optimization.

• `solve` has a wider choice of available solvers. When using `fcn2optimexpr`, `solve` uses only `fmincon` or `fminunc`.

## Algorithms

To find the output size of each returned expression when you do not specify `OutputSize`, `fcn2optimexpr` evaluates the function at the following point for each element of the problem variables.

Variable CharacteristicsEvaluation Point
Finite upper bound `ub` and finite lower bound `lb``(lb + ub)/2 + ((ub - lb)/2)*eps`
Finite lower bound and no upper bound`lb + max(1,abs(lb))*eps`
Finite upper bound and no lower bound`ub - max(1,abs(ub))*eps`
No bounds`1 + eps`
Variable is specified as an integer`floor` of the point given previously

An evaluation point might lead to an error in function evaluation. To avoid this error, specify '`OutputSize`'.