Reference

# API Reference

[Some introduction to API. List basic standalone methods.]

## Attributes

List of attribute categories.

AbstractOptimizerAttribute

Abstract supertype for attribute objects that can be used to set or get attributes (properties) of the optimizer.

Note

The difference between AbstractOptimizerAttribute and AbstractModelAttribute lies in the behavior of is_empty, empty! and copy_to. Typically optimizer attributes only affect how the model is solved.

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AbstractModelAttribute

Abstract supertype for attribute objects that can be used to set or get attributes (properties) of the model.

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AbstractVariableAttribute

Abstract supertype for attribute objects that can be used to set or get attributes (properties) of variables in the model.

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AbstractConstraintAttribute

Abstract supertype for attribute objects that can be used to set or get attributes (properties) of constraints in the model.

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Attributes can be set in different ways:

The following functions allow to distinguish between some of these different categories:

is_set_by_optimize(::AnyAttribute)

Return a Bool indicating whether the value of the attribute is modified during an optimize! call, that is, the attribute is used to query the result of the optimization.

Important note when defining new attributes

This function returns false by default so it should be implemented for attributes that are modified by optimize!.

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is_copyable(::AnyAttribute)

Return a Bool indicating whether the value of the attribute may be copied during copy_to using set.

Important note when defining new attributes

By default is_copyable(attr) returns !is_set_by_optimize(attr). A specific method should be defined for attibutes which are copied indirectly during copy_to. For instance, both is_copyable and is_set_by_optimize return false for the following attributes:

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Functions for getting and setting attributes.

get(optimizer::AbstractOptimizer, attr::AbstractOptimizerAttribute)

Return an attribute attr of the optimizer optimizer.

get(model::ModelLike, attr::AbstractModelAttribute)

Return an attribute attr of the model model.

get(model::ModelLike, attr::AbstractVariableAttribute, v::VariableIndex)

Return an attribute attr of the variable v in model model.

get(model::ModelLike, attr::AbstractVariableAttribute, v::Vector{VariableIndex})

Return a vector of attributes corresponding to each variable in the collection v in the model model.

get(model::ModelLike, attr::AbstractConstraintAttribute, c::ConstraintIndex)

Return an attribute attr of the constraint c in model model.

get(model::ModelLike, attr::AbstractConstraintAttribute, c::Vector{ConstraintIndex{F,S}})

Return a vector of attributes corresponding to each constraint in the collection c in the model model.

get(model::ModelLike, ::Type{VariableIndex}, name::String)

If a variable with name name exists in the model model, return the corresponding index, otherwise return nothing. Errors if two variables have the same name and the model implementation does not check for duplicates when the names are set.

get(model::ModelLike, ::Type{ConstraintIndex{F,S}}, name::String) where {F<:AbstractFunction,S<:AbstractSet}

If an F-in-S constraint with name name exists in the model model, return the corresponding index, otherwise return nothing. Errors if two constraints have the same name and the model implementation does not check for duplicates when the names are set.

get(model::ModelLike, ::Type{ConstraintIndex}, name::String)

If any constraint with name name exists in the model model, return the corresponding index, otherwise return nothing. This version is available for convenience but may incur a performance penalty because it is not type stable. Errors if two constraints have the same name and the model implementation does not check for duplicates when the names are set.

Examples

get(model, ObjectiveValue())
get(model, VariablePrimal(), ref)
get(model, VariablePrimal(5), [ref1, ref2])
get(model, OtherAttribute("something specific to cplex"))
get(model, VariableIndex, "var1")
get(model, ConstraintIndex{ScalarAffineFunction{Float64},LessThan{Float64}}, "con1")
get(model, ConstraintIndex, "con1")
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get!(output, model::ModelLike, args...)

An in-place version of get. The signature matches that of get except that the the result is placed in the vector output.

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set(optimizer::AbstractOptimizer, attr::AbstractOptimizerAttribute, value)

Assign value to the attribute attr of the optimizer optimizer.

set(model::ModelLike, attr::AbstractModelAttribute, value)

Assign value to the attribute attr of the model model.

set(model::ModelLike, attr::AbstractVariableAttribute, v::VariableIndex, value)

Assign value to the attribute attr of variable v in model model.

set(model::ModelLike, attr::AbstractVariableAttribute, v::Vector{VariableIndex}, vector_of_values)

Assign a value respectively to the attribute attr of each variable in the collection v in model model.

set(model::ModelLike, attr::AbstractConstraintAttribute, c::ConstraintIndex, value)

Assign a value to the attribute attr of constraint c in model model.

set(model::ModelLike, attr::AbstractConstraintAttribute, c::Vector{ConstraintIndex{F,S}}, vector_of_values)

Assign a value respectively to the attribute attr of each constraint in the collection c in model model.

An UnsupportedAttribute error is thrown if model does not support the attribute attr (see supports) and a SetAttributeNotAllowed error is thrown if it supports the attribute attr but it cannot be set.

Replace set in a constraint

set(model::ModelLike, ::ConstraintSet, c::ConstraintIndex{F,S}, set::S)

Change the set of constraint c to the new set set which should be of the same type as the original set.

Examples

If c is a ConstraintIndex{F,Interval}

set(model, ConstraintSet(), c, Interval(0, 5))
set(model, ConstraintSet(), c, GreaterThan(0.0))  # Error

Replace function in a constraint

set(model::ModelLike, ::ConstraintFunction, c::ConstraintIndex{F,S}, func::F)

Replace the function in constraint c with func. F must match the original function type used to define the constraint.

Examples

If c is a ConstraintIndex{ScalarAffineFunction,S} and v1 and v2 are VariableIndex objects,

set(model, ConstraintFunction(), c, ScalarAffineFunction([v1,v2],[1.0,2.0],5.0))
set(model, ConstraintFunction(), c, SingleVariable(v1)) # Error
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supports(model::ModelLike, attr::AbstractOptimizerAttribute)::Bool

Return a Bool indicating whether model supports the optimizer attribute attr. That is, it returns false if copy_to(model, src) shows a warning in case attr is in the ListOfOptimizerAttributesSet of src; see copy_to for more details on how unsupported optimizer attributes are handled in copy.

supports(model::ModelLike, attr::AbstractModelAttribute)::Bool

Return a Bool indicating whether model supports the model attribute attr. That is, it returns false if copy_to(model, src) cannot be performed in case attr is in the ListOfModelAttributesSet of src.

supports(model::ModelLike, attr::AbstractVariableAttribute, ::Type{VariableIndex})::Bool

Return a Bool indicating whether model supports the variable attribute attr. That is, it returns false if copy_to(model, src) cannot be performed in case attr is in the ListOfVariableAttributesSet of src.

supports(model::ModelLike, attr::AbstractConstraintAttribute, ::Type{ConstraintIndex{F,S}})::Bool where {F,S}

Return a Bool indicating whether model supports the constraint attribute attr applied to an F-in-S constraint. That is, it returns false if copy_to(model, src) cannot be performed in case attr is in the ListOfConstraintAttributesSet of src.

For all four methods, if the attribute is only not supported in specific circumstances, it should still return true.

Note that supports is only defined for attributes for which is_copyable returns true as other attributes do not appear in the list of attributes set obtained by ListOf...AttributesSet.

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## Model Interface

ModelLike

Abstract supertype for objects that implement the "Model" interface for defining an optimization problem.

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isempty(collection) -> Bool

Determine whether a collection is empty (has no elements).

Examples

julia> isempty([])
true

julia> isempty([1 2 3])
false
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empty!(model::ModelLike)

Empty the model, that is, remove all variables, constraints and model attributes but not optimizer attributes.

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write_to_file(model::ModelLike, filename::String)

Writes the current model data to the given file. Supported file types depend on the model type.

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read_from_file(model::ModelLike, filename::String)

Read the file filename into the model model. If model is non-empty, this may throw an error.

Supported file types depend on the model type.

Note

Once the contents of the file are loaded into the model, users can query the variables via get(model, ListOfVariableIndices()). However, some filetypes, such as LP files, do not maintain an explicit ordering of the variables. Therefore, the returned list may be in an arbitrary order. To avoid depending on the order of the indices, users should look up each variable index by name: get(model, VariableIndex, "name").

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Copying

copy_to(dest::ModelLike, src::ModelLike; copy_names=true, warn_attributes=true)

Copy the model from src into dest. The target dest is emptied, and all previous indices to variables or constraints in dest are invalidated. Returns a dictionary-like object that translates variable and constraint indices from the src model to the corresponding indices in the dest model.

If copy_names is false, the Name, VariableName and ConstraintName attributes are not copied even if they are set in src. If a constraint that is copied from src is not supported by dest then an UnsupportedConstraint error is thrown. Similarly, if a model, variable or constraint attribute that is copied from src is not supported by dest then an UnsupportedAttribute error is thrown. Unsupported optimizer attributes are treated differently:

• If warn_attributes is true, a warning is displayed, otherwise,

• the attribute is silently ignored.

Example

# Given empty ModelLike objects src and dest.

is_valid(src, x)   # true
is_valid(dest, x)  # false (dest has no variables)

index_map = copy_to(dest, src)
is_valid(dest, x) # false (unless index_map[x] == x)
is_valid(dest, index_map[x]) # true
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List of model attributes

Name()

A model attribute for the string identifying the model.

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ObjectiveSense()

A model attribute for the OptimizationSense of the objective function, which can be MinSense, MaxSense, or FeasiblitySense.

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NumberOfVariables()

A model attribute for the number of variables in the model.

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ListOfVariableIndices()

A model attribute for the Vector{VariableIndex} of all variable indices present in the model (i.e., of length equal to the value of NumberOfVariables()) in the order in which they were added.

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ListOfConstraints()

A model attribute for the list of tuples of the form (F,S), where F is a function type and S is a set type indicating that the attribute NumberOfConstraints{F,S}() has value greater than zero.

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NumberOfConstraints{F,S}()

A model attribute for the number of constraints of the type F-in-S present in the model.

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ListOfConstraintIndices{F,S}()

A model attribute for the Vector{ConstraintIndex{F,S}} of all constraint indices of type F-in-S in the model (i.e., of length equal to the value of NumberOfConstraints{F,S}()) in the order in which they were added.

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ListOfOptimizerAttributesSet()

An optimizer attribute for the Vector{AbstractOptimizerAttribute} of all optimizer attributes that were set.

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ListOfModelAttributesSet()

A model attribute for the Vector{AbstractModelAttribute} of all model attributes attr such that 1) is_copyable(attr) returns true and 2) the attribute was set to the model.

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ListOfVariableAttributesSet()

A model attribute for the Vector{AbstractVariableAttribute} of all variable attributes attr such that 1) is_copyable(attr) returns true and 2) the attribute was set to variables.

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ListOfConstraintAttributesSet{F, S}()

A model attribute for the Vector{AbstractConstraintAttribute} of all constraint attributes attr such that 1) is_copyable(attr) returns true and

1. the attribute was set to F-in-S constraints.

Note

The attributes ConstraintFunction and ConstraintSet should not be included in the list even if then have been set with set.

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## Optimizers

AbstractOptimizer

Abstract supertype for objects representing an instance of an optimization problem tied to a particular solver. This is typically a solver's in-memory representation. In addition to ModelLike, AbstractOptimizer objects let you solve the model and query the solution.

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optimize!(optimizer::AbstractOptimizer)

Start the solution procedure.

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List of attributes optimizers attributes

SolverName()

An optimizer attribute for the string identifying the solver/optimizer.

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List of attributes useful for optimizers

RawSolver()

A model attribute for the object that may be used to access a solver-specific API for this optimizer.

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ResultCount()

A model attribute for the number of results available.

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ObjectiveFunction{F<:AbstractScalarFunction}()

A model attribute for the objective function which has a type F<:AbstractScalarFunction. F should be guaranteed to be equivalent but not necessarily identical to the function type provided by the user. Throws an InexactError if the objective function cannot be converted to F, e.g. the objective function is quadratic and F is ScalarAffineFunction{Float64} or it has non-integer coefficient and F is ScalarAffineFunction{Int}.

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ObjectiveFunctionType()

A model attribute for the type F of the objective function set using the ObjectiveFunction{F} attribute.

Examples

In the following code, attr should be equal to MOI.SingleVariable:

x = MOI.add_variable(model)
MOI.set(model, MOI.ObjectiveFunction{MOI.SingleVariable}(),
MOI.SingleVariable(x))
attr = MOI.get(model, MOI.ObjectiveFunctionType())
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ObjectiveValue(resultidx::Int=1)

A model attribute for the objective value of the resultindexth primal result.

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ObjectiveBound()

A model attribute for the best known bound on the optimal objective value.

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RelativeGap()

A model attribute for the final relative optimality gap, defined as $\frac{|b-f|}{|f|}$, where $b$ is the best bound and $f$ is the best feasible objective value.

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SolveTime()

A model attribute for the total elapsed solution time (in seconds) as reported by the optimizer.

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SimplexIterations()

A model attribute for the cumulative number of simplex iterations during the optimization process. In particular, for a mixed-integer program (MIP), the total simplex iterations for all nodes.

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BarrierIterations()

A model attribute for the cumulative number of barrier iterations while solving a problem.

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NodeCount()

A model attribute for the total number of branch-and-bound nodes explored while solving a mixed-integer program (MIP).

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TerminationStatus()

A model attribute for the TerminationStatusCode explaining why the optimizer stopped.

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PrimalStatus(N)
PrimalStatus()

A model attribute for the ResultStatusCode of the primal result N. If N is omitted, it defaults to 1.

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DualStatus(N)
DualStatus()

A model attribute for the ResultStatusCode of the dual result N. If N is omitted, it defaults to 1.

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### Termination Status

The TerminationStatus attribute indicates why the optimizer stopped executing. The value of the attribute is of type TerminationStatusCode.

TerminationStatusCode

An Enum of possible values for the TerminationStatus attribute. This attribute is meant to explain the reason why the optimizer stopped executing.

OK

These are generally OK statuses.

• Success: the algorithm ran successfully and has a result; this includes cases where the algorithm converges to an infeasible point (NLP) or converges to a solution of a homogeneous self-dual problem and has a certificate of primal/dual infeasibility

• InfeasibleNoResult: the algorithm stopped because it decided that the problem is infeasible but does not have a result to return

• UnboundedNoResult: the algorithm stopped because it decided that the problem is unbounded but does not have a result to return

• InfeasibleOrUnbounded: the algorithm stopped because it decided that the problem is infeasible or unbounded (no result is available); this occasionally happens during MIP presolve

Limits

The optimizer stopped because of some user-defined limit. To be documented: IterationLimit, TimeLimit, NodeLimit, SolutionLimit, MemoryLimit, ObjectiveLimit, NormLimit, OtherLimit.

Problematic

This group of statuses means that something unexpected or problematic happened.

• SlowProgress: the algorithm stopped because it was unable to continue making progress towards the solution

• AlmostSuccess should be used if there is additional information that relaxed convergence tolerances are satisfied

To be documented: NumericalError, InvalidModel, InvalidOption, Interrupted, OtherError.

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### Result Status

The PrimalStatus and DualStatus attributes indicate how to interpret the result returned by the solver. The value of the attribute is of type ResultStatusCode.

ResultStatusCode

An Enum of possible values for the PrimalStatus and DualStatus attributes. The values indicate how to interpret the result vector.

• NoSolution

• FeasiblePoint

• NearlyFeasiblePoint

• InfeasiblePoint

• InfeasibilityCertificate

• NearlyInfeasibilityCertificate

• ReductionCertificate

• NearlyReductionCertificate

• UnknownResultStatus

• OtherResultStatus

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## Variables and Constraints

### Basis Status

The BasisStatus attribute of a variable or constraint describes its status with respect to a basis, if one is known. The value of the attribute is of type BasisStatusCode.

BasisStatusCode

An Enum of possible values for the VariableBasisStatus and ConstraintBasisStatus attributes. This explains the status of a given element with respect to an optimal solution basis. Possible values are:

• Basic: element is in the basis

• Nonbasic: element is not in the basis

• NonbasicAtLower: element is not in the basis and is at its lower bound

• NonbasicAtUpper: element is not in the basis and is at its upper bound

• SuperBasic: element is not in the basis but is also not at one of its bounds

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### Index types

VariableIndex

A type-safe wrapper for Int64 for use in referencing variables in a model. To allow for deletion, indices need not be consecutive.

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ConstraintIndex{F,S}

A type-safe wrapper for Int64 for use in referencing F-in-S constraints in a model. The parameter F is the type of the function in the constraint, and the parameter S is the type of set in the constraint. To allow for deletion, indices need not be consecutive. Indices within a constraint type (i.e. F-in-S) must be unique, but non-unique indices across different constraint types are allowed.

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is_valid(model::ModelLike, index::Index)::Bool

Return a Bool indicating whether this index refers to a valid object in the model model.

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delete(model::ModelLike, index::Index)

Delete the referenced object from the model.

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### Variables

Functions for adding variables. For deleting, see index types section.

add_variables(model::ModelLike, n::Int)::Vector{VariableIndex}

Add n scalar variables to the model, returning a vector of variable indices.

A AddVariableNotAllowed error is thrown if adding variables cannot be done in the current state of the model model.

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add_variable(model::ModelLike)::VariableIndex

Add a scalar variable to the model, returning a variable index.

A AddVariableNotAllowed error is thrown if adding variables cannot be done in the current state of the model model.

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List of attributes associated with variables. [category AbstractVariableAttribute] Calls to get and set should include as an argument a single VariableIndex or a vector of VariableIndex objects.

VariableName()

A variable attribute for the string identifying the variable. It is invalid for two variables to have the same name.

Note

An implementation may but is not required to check for duplicate names when the VariableName attribute is set. If this check is not performed when the name is set, then looking up a variable by name must throw an error when more than one variable has the same name.

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VariablePrimalStart()

A variable attribute for the initial assignment to some primal variable's value that the optimizer may use to warm-start the solve.

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VariablePrimal(N)
VariablePrimal()

A variable attribute for the assignment to some primal variable's value in result N. If N is omitted, it is 1 by default.

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VariableBasisStatus()

A variable attribute for the BasisStatusCode of some variable, with respect to an available optimal solution basis.

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### Constraints

Functions for adding and modifying constraints.

is_valid(model::ModelLike, index::Index)::Bool

Return a Bool indicating whether this index refers to a valid object in the model model.

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add_constraint(model::ModelLike, func::F, set::S)::ConstraintIndex{F,S} where {F,S}

Add the constraint $f(x) \in \mathcal{S}$ where $f$ is defined by func, and $\mathcal{S}$ is defined by set.

add_constraint(model::ModelLike, v::VariableIndex, set::S)::ConstraintIndex{SingleVariable,S} where {S}
add_constraint(model::ModelLike, vec::Vector{VariableIndex}, set::S)::ConstraintIndex{VectorOfVariables,S} where {S}

Add the constraint $v \in \mathcal{S}$ where $v$ is the variable (or vector of variables) referenced by v and $\mathcal{S}$ is defined by set.

An UnsupportedConstraint error is thrown if model does not support F-in-S constraints and a AddConstraintNotAllowed error is thrown if it supports F-in-S constraints but it cannot add the constraint(s) in its current state.

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add_constraints(model::ModelLike, funcs::Vector{F}, sets::Vector{S})::Vector{ConstraintIndex{F,S}} where {F,S}

Add the set of constraints specified by each function-set pair in funcs and sets. F and S should be concrete types. This call is equivalent to add_constraint.(model, funcs, sets) but may be more efficient.

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Transform Constraint Set

transform(model::ModelLike, c::ConstraintIndex{F,S1}, newset::S2)::ConstraintIndex{F,S2}

Replace the set in constraint c with newset. The constraint index c will no longer be valid, and the function returns a new constraint index with the correct type.

Solvers may only support a subset of constraint transforms that they perform efficiently (for example, changing from a LessThan to GreaterThan set). In addition, set modification (where S1 = S2) should be performed via the modify function.

Typically, the user should delete the constraint and add a new one.

Examples

If c is a ConstraintIndex{ScalarAffineFunction{Float64},LessThan{Float64}},

c2 = transform(model, c, GreaterThan(0.0))
transform(model, c, LessThan(0.0)) # errors
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MOI.supports_constraint(BT::Type{<:AbstractBridge}, F::Type{<:MOI.AbstractFunction}, S::Type{<:MOI.AbstractSet})::Bool

Return a Bool indicating whether the bridges of type BT support bridging F-in-S constraints.

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supports_constraint(model::ModelLike, ::Type{F}, ::Type{S})::Bool where {F<:AbstractFunction,S<:AbstractSet}

Return a Bool indicating whether model supports F-in-S constraints, that is, copy_to(model, src) does not return CopyUnsupportedConstraint when src contains F-in-S constraints. If F-in-S constraints are only not supported in specific circumstances, e.g. F-in-S constraints cannot be combined with another type of constraint, it should still return true.

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List of attributes associated with constraints. [category AbstractConstraintAttribute] Calls to get and set should include as an argument a single ConstraintIndex or a vector of ConstraintIndex{F,S} objects.

ConstraintName()

A constraint attribute for the string identifying the constraint. It is invalid for two constraints of any kind to have the same name.

Note

An implementation may but is not required to check for duplicate names when the ConstraintName attribute is set. If this check is not performed when the name is set, then looking up a constraint by name must throw an error when more than one constraint (of any type) has the same name.

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ConstraintPrimalStart()

A constraint attribute for the initial assignment to some constraint's primal value(s) that the optimizer may use to warm-start the solve.

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ConstraintDualStart()

A constraint attribute for the initial assignment to some constraint's dual value(s) that the optimizer may use to warm-start the solve.

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ConstraintPrimal(N)
ConstraintPrimal()

A constraint attribute for the assignment to some constraint's primal value(s) in result N. If N is omitted, it is 1 by default.

Given a constraint function-in-set, the ConstraintPrimal is the value of the function evaluated at the primal solution of the variables. For example, given the constraint ScalarAffineFunction([x,y], [1, 2], 3)-in-Interval(0, 20) and a primal solution of (x,y) = (4,5), the ConstraintPrimal solution of the constraint is 1 * 4 + 2 * 5 + 3 = 17.

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ConstraintDual(N)
ConstraintDual()

A constraint attribute for the assignment to some constraint's dual value(s) in result N. If N is omitted, it is 1 by default.

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ConstraintBasisStatus()

A constraint attribute for the BasisStatusCode of some constraint, with respect to an available optimal solution basis.

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ConstraintFunction()

A constraint attribute for the AbstractFunction object used to define the constraint. It is guaranteed to be equivalent but not necessarily identical to the function provided by the user.

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ConstraintSet()

A constraint attribute for the AbstractSet object used to define the constraint.

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## Functions and function modifications

List of recognized functions.

AbstractFunction

Abstract supertype for function objects.

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SingleVariable(variable)

The function that extracts the scalar variable referenced by variable, a VariableIndex. This function is naturally be used for single variable bounds or integrality constraints.

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VectorOfVariables(variables)

The function that extracts the vector of variables referenced by variables, a Vector{VariableIndex}. This function is naturally be used for constraints that apply to groups of variables, such as an "all different" constraint, an indicator constraint, or a complementarity constraint.

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struct ScalarAffineTerm{T}
coefficient::T
variable_index::VariableIndex
end

Represents $c x_i$ where $c$ is coefficient and $x_i$ is the variable identified by variable_index.

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ScalarAffineFunction{T}(terms, constant)

The scalar-valued affine function $a^T x + b$, where:

Duplicate variable indices in terms are accepted, and the corresponding coefficients are summed together.

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struct VectorAffineTerm{T}
output_index::Int64
scalar_term::ScalarAffineTerm{T}
end

A ScalarAffineTerm plus its index of the output component of a VectorAffineFunction or VectorQuadraticFunction. output_index can also be interpreted as a row index into a sparse matrix, where the scalar_term contains the column index and coefficient.

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VectorAffineFunction{T}(terms, constants)

The vector-valued affine function $A x + b$, where:

• $A$ is a sparse matrix specified by a list of VectorAffineTerm objects.

• $b$ is a vector specified by constants

Duplicate indices in the $A$ are accepted, and the corresponding coefficients are summed together.

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struct ScalarQuadraticTerm{T}
coefficient::T
variable_index_1::VariableIndex
variable_index_2::VariableIndex
end

Represents $c x_i x_j$ where $c$ is coefficient, $x_i$ is the variable identified by variable_index_1 and $x_j$ is the variable identified by variable_index_2.

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ScalarQuadraticFunction{T}(affine_terms, quadratic_terms, constant)

The scalar-valued quadratic function $\frac{1}{2}x^TQx + a^T x + b$, where:

• $a$ is a sparse vector specified by a list of ScalarAffineTerm structs.

• $b$ is a scalar specified by constant.

• $Q$ is a symmetric matrix specified by a list of ScalarQuadraticTerm structs.

Duplicate indices in $a$ or $Q$ are accepted, and the corresponding coefficients are summed together. "Mirrored" indices (q,r) and (r,q) (where r and q are VariableIndexes) are considered duplicates; only one need be specified.

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struct VectorQuadraticTerm{T}
output_index::Int64
end

A ScalarQuadraticTerm plus its index of the output component of a VectorQuadraticFunction. Each output component corresponds to a distinct sparse matrix $Q_i$.

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VectorQuadraticFunction{T}(affine_terms, quadratic_terms, constant)

The vector-valued quadratic function with ith component ("output index") defined as $\frac{1}{2}x^TQ_ix + a_i^T x + b_i$, where:

• $a_i$ is a sparse vector specified by the VectorAffineTerms with output_index == i.

• $b_i$ is a scalar specified by constants[i]

• $Q_i$ is a symmetric matrix specified by the VectorQuadraticTerm with output_index == i.

Duplicate indices in $a_i$ or $Q_i$ are accepted, and the corresponding coefficients are summed together. "Mirrored" indices (q,r) and (r,q) (where r and q are VariableIndexes) are considered duplicates; only one need be specified.

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Functions for getting and setting properties of sets.

output_dimension(f::AbstractFunction)

Return 1 if f has a scalar output and the number of output components if f has a vector output.

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## Sets

List of recognized sets.

AbstractSet

Abstract supertype for set objects used to encode constraints.

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Reals(dimension)

The set $\mathbb{R}^{dimension}$ (containing all points) of dimension dimension.

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Zeros(dimension)

The set $\{ 0 \}^{dimension}$ (containing only the origin) of dimension dimension.

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Nonnegatives(dimension)

The nonnegative orthant $\{ x \in \mathbb{R}^{dimension} : x \ge 0 \}$ of dimension dimension.

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Nonpositives(dimension)

The nonpositive orthant $\{ x \in \mathbb{R}^{dimension} : x \le 0 \}$ of dimension dimension.

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GreaterThan{T <: Real}(lower::T)

The set $[lower,\infty) \subseteq \mathbb{R}$.

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LessThan{T <: Real}(upper::T)

The set $(-\infty,upper] \subseteq \mathbb{R}$.

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EqualTo{T <: Number}(value::T)

The set containing the single point $x \in \mathbb{R}$ where $x$ is given by value.

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Interval{T <: Real}(lower::T,upper::T)

The interval $[lower, upper] \subseteq \mathbb{R}$. If lower or upper is -Inf or Inf, respectively, the set is interpreted as a one-sided interval.

Interval(s::GreaterThan{<:AbstractFloat})

Construct a (right-unbounded) Interval equivalent to the given GreaterThan set.

Interval(s::LessThan{<:AbstractFloat})

Construct a (left-unbounded) Interval equivalent to the given LessThan set.

Interval(s::EqualTo{<:Real})

Construct a (degenerate) Interval equivalent to the given EqualTo set.

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SecondOrderCone(dimension)

The second-order cone (or Lorenz cone) $\{ (t,x) \in \mathbb{R}^{dimension} : t \ge || x ||_2 \}$ of dimension dimension.

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RotatedSecondOrderCone(dimension)

The rotated second-order cone $\{ (t,u,x) \in \mathbb{R}^{dimension} : 2tu \ge || x ||_2^2, t,u \ge 0 \}$ of dimension dimension.

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GeometricMeanCone(dimension)

The geometric mean cone $\{ (t,x) \in \mathbb{R}^{n+1} : x \ge 0, t \le \sqrt[n]{x_1 x_2 \cdots x_n} \}$ of dimension dimension${}=n+1$.

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ExponentialCone()

The 3-dimensional exponential cone $\{ (x,y,z) \in \mathbb{R}^3 : y \exp (x/y) \le z, y > 0 \}$.

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DualExponentialCone()

The 3-dimensional dual exponential cone $\{ (u,v,w) \in \mathbb{R}^3 : -u \exp (v/u) \le \exp(1) w, u < 0 \}$.

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PowerCone{T <: Real}(exponent::T)

The 3-dimensional power cone $\{ (x,y,z) \in \mathbb{R}^3 : x^{exponent} y^{1-exponent} >= |z|, x \ge 0, y \ge 0 \}$ with parameter exponent.

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DualPowerCone{T <: Real}(exponent::T)

The 3-dimensional power cone $\{ (u,v,w) \in \mathbb{R}^3 : (\frac{u}{exponent})^{exponent} (\frac{v}{1-exponent})^{1-exponent} \ge |w|, u \ge 0, v \ge 0 \}$ with parameter exponent.

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PositiveSemidefiniteConeTriangle(side_dimension)

The (vectorized) cone of symmetric positive semidefinite matrices, with side_dimension rows and columns. The entries of the upper-right triangular part of the matrix are given column by column (or equivalently, the entries of the lower-left triangular part are given row by row). A vectorized cone of dimension $n$ corresponds to a square matrix with side dimension $\sqrt{1/4 + 2 n} - 1/2$. (Because a $d \times d$ matrix has $d(d+1)/2$ elements in the upper or lower triangle.)

Examples

The matrix

$\begin{bmatrix} 1 & 2 & 4\\ 2 & 3 & 5\\ 4 & 5 & 6 \end{bmatrix}$

corresponds to $(1, 2, 3, 4, 5, 6)$ for PositiveSemidefiniteConeTriangle(3)

Note

Two packed storage formats exist for symmetric matrices, the respective orders of the entries are:

• upper triangular column by column (or lower triangular row by row);

• lower triangular column by column (or upper triangular row by row).

The advantage of the first format is the mapping between the (i, j) matrix indices and the k index of the vectorized form. It is simpler and does not depend on the side dimension of the matrix. Indeed,

• the entry of matrix indices (i, j) has vectorized index k = div((j-1)*j, 2) + i if $i \leq j$ and k = div((i-1)*i, 2) + j if $j \leq i$;

• and the entry with vectorized index k has matrix indices i = isqrt(2k) and j = k - div((i-1)*i, 2) or j = isqrt(2k) and i = k - div((j-1)*j, 2).

Duality note

The scalar product for the symmetric matrix in its vectorized form is the sum of the pairwise product of the diagonal entries plus twice the sum of the pairwise product of the upper diagonal entries; see [p. 634, 1]. This has important consequence for duality. Consider for example the following problem

\begin{align*} & \max_{x \in \mathbb{R}} & x \\ & \;\;\text{s.t.} & (1, -x, 1) & \in \text{PositiveSemidefiniteConeTriangle}(2). \end{align*}

The dual is the following problem

\begin{align*} & \min_{x \in \mathbb{R}^3} & y_1 + y_3 \\ & \;\;\text{s.t.} & 2y_2 & = 1\\ & & y & \in \text{PositiveSemidefiniteConeTriangle}(2). \end{align*}

Why do we use $2y_2$ in the dual constraint instead of $y_2$ ? The reason is that $2y_2$ is the scalar product between $y$ and the symmetric matrix whose vectorized form is $(0, 1, 0)$. Indeed, with our modified scalar products we have

$\langle (0, 1, 0), (y_1, y_2, y_3) \rangle = \mathrm{trace} \begin{pmatrix} 0 & 1\\ 1 & 0 \end{pmatrix} \begin{pmatrix} y_1 & y_2\\ y_2 & y_3 \end{pmatrix} = 2y_2.$

References

[1] Boyd, S. and Vandenberghe, L.. Convex optimization. Cambridge university press, 2004.

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PositiveSemidefiniteConeSquare(side_dimension)

The cone of symmetric positive semidefinite matrices, with side length side_dimension. The entries of the matrix are given column by column (or equivalently, row by row). The matrix is both constrained to be symmetric and to be positive semidefinite. That is, if the functions in entries $(i, j)$ and $(j, i)$ are different, then a constraint will be added to make sure that the entries are equal.

Examples

Constraining the matrix

$\begin{bmatrix} 1 & -y\\ -z & 0\\ \end{bmatrix}$

to be symmetric positive semidefinite can be achieved by constraining the vector $(1, -z, -y, 0)$ (or $(1, -y, -z, 0)$) to belong to the PositiveSemidefiniteConeSquare(2). It both constrains $y = z$ and $(1, -y, 0)$ (or $(1, -z, 0)$) to be in PositiveSemidefiniteConeTriangle(2).

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LogDetConeTriangle(side_dimension)

The Log-Determinant cone $\{ (t, X) \in \mathbb{R}^{1 + d(d+1)/2} : t \le \log(\det(X)) \}$ where the matrix X is represented in the same symmetric packed format as in the PositiveSemidefiniteConeTriangle. The argument side_dimension is the side dimension of the matrix X, i.e., its number of rows or columns.

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LogDetConeSquare(side_dimension)

The Log-Determinant cone $\{ (t, X) \in \mathbb{R}^{1 + d^2} : t \le \log(\det(X)), X \text{ symmetric} \}$ where the matrix X is represented in the same format as in the PositiveSemidefiniteConeSquare. Similarly to PositiveSemidefiniteConeSquare, constraints are added to ensures that X is symmetric. The argument side_dimension is the side dimension of the matrix X, i.e., its number of rows or columns.

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RootDetConeTriangle(side_dimension)

The Root-Determinant cone $\{ (t, X) \in \mathbb{R}^{1 + d(d+1)/2} : t \le \det(X)^{1/d} \}$ where the matrix X is represented in the same symmetric packed format as in the PositiveSemidefiniteConeTriangle. The argument side_dimension is the side dimension of the matrix X, i.e., its number of rows or columns.

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RootDetConeSquare(side_dimension)

The Root-Determinant cone $\{ (t, X) \in \mathbb{R}^{1 + d^2} : t \le \det(X)^{1/d}, X \text{ symmetric} \}$ where the matrix X is represented in the same format as in the PositiveSemidefiniteConeSquare. Similarly to PositiveSemidefiniteConeSquare, constraints are added to ensure that X is symmetric. The argument side_dimension is the side dimension of the matrix X, i.e., its number of rows or columns.

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Integer()

The set of integers $\mathbb{Z}$.

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ZeroOne()

The set $\{ 0, 1 \}$.

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Semicontinuous{T <: Real}(lower::T,upper::T)

The set $\{0\} \cup [lower,upper]$.

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Semiinteger{T <: Real}(lower::T,upper::T)

The set $\{0\} \cup \{lower,lower+1,\ldots,upper-1,upper\}$.

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SOS1{T <: Real}(weights::Vector{T})

The set corresponding to the special ordered set (SOS) constraint of type 1. Of the variables in the set, at most one can be nonzero. The weights induce an ordering of the variables; as such, they should be unique values. The kth element in the set corresponds to the kth weight in weights. See here for a description of SOS constraints and their potential uses.

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SOS2{T <: Real}(weights::Vector{T})

The set corresponding to the special ordered set (SOS) constraint of type 2. Of the variables in the set, at most two can be nonzero, and if two are nonzero, they must be adjacent in the ordering of the set. The weights induce an ordering of the variables; as such, they should be unique values. The kth element in the set corresponds to the kth weight in weights. See here for a description of SOS constraints and their potential uses.

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Functions for getting and setting properties of sets.

dimension(s::AbstractSet)

Return the output_dimension that an AbstractFunction should have to be used with the set s.

Examples

julia> dimension(Reals(4))
4

julia> dimension(LessThan(3.0))
1

julia> dimension(PositiveSemidefiniteConeTriangle(2))
3
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## Modifications

Functions for modifying objective and constraint functions.

Constraint Function

modify(model::ModelLike, ci::ConstraintIndex, change::AbstractFunctionModification)

Apply the modification specified by change to the function of constraint ci.

An ModifyConstraintNotAllowed error is thrown if modifying constraints is not supported by the model model.

Examples

modify(model, ci, ScalarConstantChange(10.0))

Objective Function

modify(model::ModelLike, ::ObjectiveFunction, change::AbstractFunctionModification)

Apply the modification specified by change to the objective function of model. To change the function completely, call set instead.

An ModifyObjectiveNotAllowed error is thrown if modifying objectives is not supported by the model model.

Examples

modify(model, ObjectiveFunction{ScalarAffineFunction{Float64}}(), ScalarConstantChange(10.0))
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AbstractFunctionModification

An abstract supertype for structs which specify partial modifications to functions, to be used for making small modifications instead of replacing the functions entirely.

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ScalarConstantChange{T}(new_constant::T)

A struct used to request a change in the constant term of a scalar-valued function. Applicable to ScalarAffineFunction and ScalarQuadraticFunction.

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VectorConstantChange{T}(new_constant::Vector{T})

A struct used to request a change in the constant vector of a vector-valued function. Applicable to VectorAffineFunction and VectorQuadraticFunction.

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ScalarCoefficientChange{T}(variable::VariableIndex, new_coefficient::T)

A struct used to request a change in the linear coefficient of a single variable in a scalar-valued function. Applicable to ScalarAffineFunction and ScalarQuadraticFunction.

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MultirowChange{T}(variable::VariableIndex, new_coefficients::Vector{Tuple{Int64, T}})

A struct used to request a change in the linear coefficients of a single variable in a vector-valued function. New coefficients are specified by (output_index, coefficient) tuples. Applicable to VectorAffineFunction and VectorQuadraticFunction.

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## Nonlinear programming (NLP)

### Attributes

NLPBlock()

Holds the NLPBlockData that represents a set of nonlinear constraints, and optionally a nonlinear objective.

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NLPBoundsPair(lower,upper)

A struct holding a pair of lower and upper bounds. -Inf and Inf can be used to indicate no lower or upper bound, respectively.

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struct NLPBlockData
constraint_bounds::Vector{NLPBoundsPair}
evaluator::AbstractNLPEvaluator
has_objective::Bool
end

A struct encoding a set of nonlinear constraints of the form $lb \le g(x) \le ub$ and, if has_objective == true, a nonlinear objective function $f(x)$. constraint_bounds holds the pairs of $lb$ and $ub$ elements. It is an error to set both a nonlinear objective function and another objective function using an ObjectiveFunction attribute. The evaluator is a callback object that is used to query function values, derivatives, and expression graphs. If has_objective == false, then it is an error to query properties of the objective function, and in Hessian-of-the-Lagrangian queries, σ must be set to zero. Throughout the evaluator, all variables are ordered according to ListOfVariableIndices().

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NLPBlockDual(N)
NLPBlockDual()

The Lagrange multipliers on the constraints from the NLPBlock in result N. If N is omitted, it is 1 by default.

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NLPBlockDualStart()

An initial assignment of the Lagrange multipliers on the constraints from the NLPBlock that the solver may use to warm-start the solve.

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### NLP evaluator methods

AbstractNLPEvaluator

Abstract supertype for the callback object used in NLPBlock.

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initialize(d::AbstractNLPEvaluator, requested_features::Vector{Symbol})

Must be called before any other methods. The vector requested_features lists features requested by the solver. These may include :Grad for gradients of $f$, :Jac for explicit Jacobians of $g$, :JacVec for Jacobian-vector products, :HessVec for Hessian-vector and Hessian-of-Lagrangian-vector products, :Hess for explicit Hessians and Hessian-of-Lagrangians, and :ExprGraph for expression graphs.

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features_available(d::AbstractNLPEvaluator)

Returns the subset of features available for this problem instance, as a list of symbols in the same format as in initialize.

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eval_objective(d::AbstractNLPEvaluator, x)

Evaluate the objective $f(x)$, returning a scalar value.

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eval_constraint(d::AbstractNLPEvaluator, g, x)

Evaluate the constraint function $g(x)$, storing the result in the vector g which must be of the appropriate size.

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eval_objective_gradient(d::AbstractNLPEvaluator, g, x)

Evaluate $\nabla f(x)$ as a dense vector, storing the result in the vector g which must be of the appropriate size.

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jacobian_structure(d::AbstractNLPEvaluator)::Vector{Tuple{Int64,Int64}}

Returns the sparsity structure of the Jacobian matrix $J_g(x) = \left[ \begin{array}{c} \nabla g_1(x) \\ \nabla g_2(x) \\ \vdots \\ \nabla g_m(x) \end{array}\right]$ where $g_i$ is the $i\text{th}$ component of $g$. The sparsity structure is assumed to be independent of the point $x$. Returns a vector of tuples, (row, column), where each indicates the position of a structurally nonzero element. These indices are not required to be sorted and can contain duplicates, in which case the solver should combine the corresponding elements by adding them together.

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hessian_lagrangian_structure(d::AbstractNLPEvaluator)::Vector{Tuple{Int64,Int64}}

Returns the sparsity structure of the Hessian-of-the-Lagrangian matrix $\nabla^2 f + \sum_{i=1}^m \nabla^2 g_i$ as a vector of tuples, where each indicates the position of a structurally nonzero element. These indices are not required to be sorted and can contain duplicates, in which case the solver should combine the corresponding elements by adding them together. Any mix of lower and upper-triangular indices is valid. Elements (i,j) and (j,i), if both present, should be treated as duplicates.

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eval_constraint_jacobian(d::AbstractNLPEvaluator, J, x)

Evaluates the sparse Jacobian matrix $J_g(x) = \left[ \begin{array}{c} \nabla g_1(x) \\ \nabla g_2(x) \\ \vdots \\ \nabla g_m(x) \end{array}\right]$. The result is stored in the vector J in the same order as the indices returned by jacobian_structure.

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eval_constraint_jacobian_product(d::AbstractNLPEvaluator, y, x, w)

Computes the Jacobian-vector product $J_g(x)w$, storing the result in the vector y.

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eval_constraint_jacobian_transpose_product(d::AbstractNLPEvaluator, y, x, w)

Computes the Jacobian-transpose-vector product $J_g(x)^Tw$, storing the result in the vector y.

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eval_hessian_lagrangian(d::AbstractNLPEvaluator, H, x, σ, μ)

Given scalar weight σ and vector of constraint weights μ, computes the sparse Hessian-of-the-Lagrangian matrix $\sigma\nabla^2 f(x) + \sum_{i=1}^m \mu_i \nabla^2 g_i(x)$, storing the result in the vector H in the same order as the indices returned by hessian_lagrangian_structure.

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eval_hessian_lagrangian_prod(d::AbstractNLPEvaluator, h, x, v, σ, μ)

Given scalar weight σ and vector of constraint weights μ, computes the Hessian-of-the-Lagrangian-vector product $\left(\sigma\nabla^2 f(x) + \sum_{i=1}^m \mu_i \nabla^2 g_i(x)\right)v$, storing the result in the vector h.

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objective_expr(d::AbstractNLPEvaluator)

Returns an expression graph for the objective function as a standard Julia Expr object. All sums and products are flattened out as simple Expr(:+,...) and Expr(:*,...) objects. The symbol x is used as a placeholder for the vector of decision variables. No other undefined symbols are permitted; coefficients are embedded as explicit values. For example, the expression $x_1+\sin(x_2/\exp(x_3))$ would be represented as the Julia object :(x[1] + sin(x[2]/exp(x[3]))). See the Julia manual for more information on the structure of Expr objects. There are currently no restrictions on recognized functions; typically these will be built-in Julia functions like ^, exp, log, cos, tan, sqrt, etc., but modeling interfaces may choose to extend these basic functions.

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constraint_expr(d::AbstractNLPEvaluator, i)

Returns an expression graph for the $i\text{th}$ constraint in the same format as described above, with an additional comparison operator indicating the sense of and bounds on the constraint. The right-hand side of the comparison must be a constant; that is, :(x[1]^3 <= 1) is allowed, while :(1 <= x[1]^3) is not valid. Double-sided constraints are allowed, in which case both the lower bound and upper bounds should be constants; for example, :(-1 <= cos(x[1]) + sin(x[2]) <= 1) is valid.

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## Errors

When an MOI call fails on a model, precise errors should be thrown when possible instead of simply calling error with a message. The docstrings for the respective methods describe the errors that the implementation should thrown in certain situations. This error-reporting system allows code to distinguish between internal errors (that should be shown to the user) and unsupported operations which may have automatic workarounds.

When an invalid index is used in an MOI call, an InvalidIndex should be thrown:

struct InvalidIndex{IndexType<:Index} <: Exception
index::IndexType
end

An error indicating that the index index is invalid.

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The rest of the errors defined in MOI fall in two categories represented by the following two abstract types:

UnsupportedError <: Exception

Abstract type for error thrown when an element is not supported by the model.

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NotAllowedError <: Exception

Abstract type for error thrown when an operation is supported but cannot be applied in the current state of the model.

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The different UnsupportedError and NotAllowedError are the following errors:

struct UnsupportedAttribute{AttrType} <: UnsupportedError
attr::AttrType
message::String
end

An error indicating that the attribute attr is not supported by the model, i.e. that supports returns false.

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struct SetAttributeNotAllowed{AttrType} <: NotAllowedError
attr::AttrType
message::String # Human-friendly explanation why the attribute cannot be set
end

An error indicating that the attribute attr is supported (see supports) but cannot be set for some reason (see the error string).

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struct AddVariableNotAllowed <: NotAllowedError
message::String # Human-friendly explanation why the attribute cannot be set
end

An error indicating that variables cannot be added to the model.

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struct UnsupportedConstraint{F<:AbstractFunction, S<:AbstractSet} <: UnsupportedError
message::String # Human-friendly explanation why the attribute cannot be set
end

An error indicating that constraints of type F-in-S are not supported by the model, i.e. that supports_constraint returns false.

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struct AddConstraintNotAllowed{F<:AbstractFunction, S<:AbstractSet} <: NotAllowedError
message::String # Human-friendly explanation why the attribute cannot be set
end

An error indicating that constraints of type F-in-S are supported (see supports_constraint) but cannot be added.

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struct ModifyConstraintNotAllowed{F<:AbstractFunction, S<:AbstractSet,
C<:AbstractFunctionModification} <: NotAllowedError
constraint_index::ConstraintIndex{F, S}
change::C
message::String
end

An error indicating that the constraint modification change cannot be applied to the constraint of index ci.

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struct ModifyObjectiveNotAllowed{C<:AbstractFunctionModification} <: NotAllowedError
change::C
message::String
end

An error indicating that the objective modification change cannot be applied to the objective.

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struct DeleteNotAllowed{IndexType <: Index} <: NotAllowedError
index::IndexType
message::String
end

An error indicating that the index index cannot be deleted.

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## Bridges

Bridges can be used for automatic reformulation of a certain constraint type into equivalent constraints.

AbstractBridge

A bridge represents a bridged constraint in an AbstractBridgeOptimizer. It contains the indices of the constraints that it has created in the model. These can be obtained using MOI.NumberOfConstraints and MOI.ListOfConstraintIndices and using the bridge in place of a ModelLike. Attributes of the bridged model such as MOI.ConstraintDual and MOI.ConstraintPrimal, can be obtained using the bridge in place of the constraint index. These calls are used by the AbstractBridgeOptimizer to communicate with the bridge so they should be implemented by the bridge.

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AbstractBridgeOptimizer

A bridge optimizer applies given constraint bridges to a given optimizer thus extending the types of supported constraints. The attributes of the inner optimizer are automatically transformed to make the bridges transparent, e.g. the variables and constraints created by the bridges are hidden.

By convention, the inner optimizer should be stored in a model field and the dictionary mapping constraint indices to bridges should be stored in a bridges field. If a bridge optimizer deviates from these conventions, it should implement the functions MOI.optimize! and bridge respectively.

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SingleBridgeOptimizer{BT<:AbstractBridge, MT<:MOI.ModelLike, OT<:MOI.ModelLike} <: AbstractBridgeOptimizer

The SingleBridgeOptimizer bridges any constraint supported by the bridge BT. This is in contrast with the LazyBridgeOptimizer which only bridges the constraints that are unsupported by the internal model, even if they are supported by one of its bridges.

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LazyBridgeOptimizer{OT<:MOI.ModelLike, MT<:MOI.ModelLike} <: AbstractBridgeOptimizer

The LazyBridgeOptimizer combines several bridges, which are added using the add_bridge function. Whenever a constraint is added, it only attempts to bridge it if it is not supported by the internal model (hence its name Lazy). When bridging a constraint, it selects the minimal number of bridges needed. For instance, a constraint F-in-S can be bridged into a constraint F1-in-S1 (supported by the internal model) using bridge 1 or bridged into a constraint F2-in-S2 (unsupported by the internal model) using bridge 2 which can then be bridged into a constraint F3-in-S3 (supported by the internal model) using bridge 3, it will choose bridge 1 as it allows to bridge F-in-S using only one bridge instead of two if it uses bridge 2 and 3.

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add_bridge(b::LazyBridgeOptimizer, BT::Type{<:AbstractBridge})

Enable the use of the bridges of type BT by b.

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Below is the list of bridges implemented in this package.

SplitIntervalBridge{T}

The SplitIntervalBridge splits a constraint $l ≤ ⟨a, x⟩ + α ≤ u$ into the constraints $⟨a, x⟩ + α ≥ l$ and $⟨a, x⟩ + α ≤ u$.

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RSOCBridge{T}

The RotatedSecondOrderCone is SecondOrderCone representable; see [1, p. 104]. Indeed, we have $2tu = (t/√2 + u/√2)^2 - (t/√2 - u/√2)^2$ hence

$2tu \ge || x ||_2^2$

is equivalent to

$(t/√2 + u/√2)^2 \ge || x ||_2^2 + (t/√2 - u/√2)^2.$

We can therefore use the transformation $(t, u, x) \mapsto (t/√2+u/√2, t/√2-u/√2, x)$. Note that the linear transformation is a symmetric involution (i.e. it is its own transpose and its own inverse). That means in particular that the norm is of constraint primal and duals are preserved by the tranformation.

[1] Ben-Tal, Aharon, and Arkadi Nemirovski. Lectures on modern convex optimization: analysis, algorithms, and engineering applications. Society for Industrial and Applied Mathematics, 2001.

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GeoMeanBridge{T}

The GeometricMeanCone is SecondOrderCone representable; see [1, p. 105]. The reformulation is best described in an example. Consider the cone of dimension 4

$t \le \sqrt[3]{x_1 x_2 x_3}$

This can be rewritten as $\exists x_{21} \ge 0$ such that

\begin{align*} t & \le x_{21},\\ x_{21}^4 & \le x_1 x_2 x_3 x_{21}. \end{align*}

Note that we need to create $x_{21}$ and not use $t^4$ directly as $t$ is allowed to be negative. Now, this is equivalent to

\begin{align*} t & \le x_{21}/\sqrt{4},\\ x_{21}^2 & \le 2x_{11} x_{12},\\ x_{11}^2 & \le 2x_1 x_2, & x_{21}^2 & \le 2x_3(x_{21}/\sqrt{4}). \end{align*}

[1] Ben-Tal, Aharon, and Arkadi Nemirovski. Lectures on modern convex optimization: analysis, algorithms, and engineering applications. Society for Industrial and Applied Mathematics, 2001.

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SquarePSDBridge{T}

The SquarePSDBridge reformulates the constraint of a square matrix to be PSD and symmetric, i.e. belongs to the MOI.PositiveSemidefiniteConeSquare, to a list of equality constraints for pair or off-diagonal entries with different expressions and a PSD constraint the upper triangular part of the matrix.

For instance, the constraint for the matrix

$\begin{pmatrix} 1 & 1 + x & 2 - 3x\ 1 + x & 2 + x & 3 - x\ 2 - 3x & 2 + x & 2x \end{pmatrix}$

to be PSD can be broken down to the constraint of the symmetric matrix

$\begin{pmatrix} 1 & 1 + x & 2 - 3x\ \cdot & 2 + x & 3 - x\ \cdot & \cdot & 2x \end{pmatrix}$

and the equality constraint between the off-diagonal entries (2, 3) and (3, 2) $2x == 1$. Note that now symmetrization constraint need to be added between the off-diagonal entries (1, 2) and (2, 1) or between (1, 3) and (3, 1) since the expressions are the same.

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RootDetBridge{T}

The RootDetConeTriangle is representable by a PositiveSemidefiniteConeTriangle and an GeometricMeanCone constraints; see [1, p. 149]. Indeed, $t \le \det(X)^(1/n)$ if and only if there exists a lower triangular matrix $Δ$ such that

\begin{align*} \begin{pmatrix} X & Δ\\ Δ^\top & \mathrm{Diag}(Δ) \end{pmatrix} & \succeq 0\\ t & \le (Δ_{11} Δ_{22} \cdots Δ_{nn})^{1/n} \end{align*}

[1] Ben-Tal, Aharon, and Arkadi Nemirovski. Lectures on modern convex optimization: analysis, algorithms, and engineering applications. Society for Industrial and Applied Mathematics, 2001.

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LogDetBridge{T}

The LogDetConeTriangle is representable by a PositiveSemidefiniteConeTriangle and ExponentialCone constraints. Indeed, $\log\det(X) = \log(\delta_1) + \cdots + \log(\delta_n)$ where $\delta_1$, ..., $\delta_n$ are the eigenvalues of $X$. Adapting, the method from [1, p. 149], we see that $t \le \log(\det(X))$ if and only if there exists a lower triangular matrix $Δ$ such that

\begin{align*} \begin{pmatrix} X & Δ\\ Δ^\top & \mathrm{Diag}(Δ) \end{pmatrix} & \succeq 0\\ t & \le \log(Δ_{11}) + \log(Δ_{22}) + \cdots + \log(Δ_{nn}) \end{align*}

[1] Ben-Tal, Aharon, and Arkadi Nemirovski. Lectures on modern convex optimization: analysis, algorithms, and engineering applications. Society for Industrial and Applied Mathematics, 2001. 

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The SOCtoPSDBridge transforms the second order cone constraint $\lVert x \rVert \le t$ into the semidefinite cone constraints

$\begin{pmatrix} t & x^\top\\ x & tI \end{pmatrix} \succeq 0$

Indeed by the Schur Complement, it is positive definite iff

\begin{align*} tI & \succ 0\\ t - x^\top (tI)^{-1} x & \succ 0 \end{align*}

which is equivalent to

\begin{align*} t & > 0\\ t^2 & > x^\top x \end{align*}
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The RSOCtoPSDBridge transforms the second order cone constraint $\lVert x \rVert \le 2tu$ with $u \ge 0$ into the semidefinite cone constraints

$\begin{pmatrix} t & x^\top\\ x & 2uI \end{pmatrix} \succeq 0$

Indeed by the Schur Complement, it is positive definite iff

\begin{align*} uI & \succ 0\\ t - x^\top (2uI)^{-1} x & \succ 0 \end{align*}

which is equivalent to

\begin{align*} u & > 0\\ 2tu & > x^\top x \end{align*}
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For each bridge defined in this package, a corresponding bridge optimizer is available with the same name without the "Bridge" suffix, e.g., SplitInterval is an SingleBridgeOptimizer for the SplitIntervalBridge`.