# Objectives

This page describes macros and functions related to linear and quadratic objective functions only, unless otherwise indicated. For nonlinear objective functions, see Nonlinear Modeling.

## Set a linear objective

Use the @objective macro to set a linear objective function.

Use Min to create a minimization objective:

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x + 1)
2 x + 1

Use Max to create a maximization objective:

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Max, 2x + 1)
2 x + 1

Use the @objective macro to set a quadratic objective function.

Use ^2 to have a variable squared:

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, x^2 + 2x + 1)
x² + 2 x + 1

You can also have bilinear terms between variables:

julia> model = Model();

julia> @variable(model, x)
x

julia> @variable(model, y)
y

julia> @objective(model, Max, x * y + x + y)
x*y + x + y

## Set a nonlinear objective

Use the @objective macro to set a nonlinear objective function:

julia> model = Model();

julia> @variable(model, x <= 1);

julia> @objective(model, Max, log(x))
log(x)

## Query the objective function

Use objective_function to return the current objective function.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x + 1)
2 x + 1

julia> objective_function(model)
2 x + 1

## Evaluate the objective function at a point

Use value to evaluate an objective function at a point specifying values for variables.

julia> model = Model();

julia> @variable(model, x[1:2]);

julia> @objective(model, Min, 2x^2 + x + 0.5*x)
2 x² + x + 0.5 x

julia> f = objective_function(model)
2 x² + x + 0.5 x

julia> point = Dict(x => 2.0, x => 1.0);

julia> value(z -> point[z], f)
10.5

## Query the objective sense

Use objective_sense to return the current objective sense.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x + 1)
2 x + 1

julia> objective_sense(model)
MIN_SENSE::OptimizationSense = 0

## Modify an objective

To modify an objective, call @objective with the new objective function.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x)
2 x

julia> @objective(model, Max, -2x)
-2 x

Alternatively, use set_objective_function.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x)
2 x

julia> new_objective = @expression(model, -2 * x)
-2 x

julia> set_objective_function(model, new_objective)

## Modify an objective coefficient

Use set_objective_coefficient to modify an objective coefficient.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x)
2 x

julia> set_objective_coefficient(model, x, 3)

julia> objective_function(model)
3 x
Info

There is no way to modify the coefficient of a quadratic term. Set a new objective instead.

## Modify the objective sense

Use set_objective_sense to modify the objective sense.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x)
2 x

julia> objective_sense(model)
MIN_SENSE::OptimizationSense = 0

julia> set_objective_sense(model, MAX_SENSE);

julia> objective_sense(model)
MAX_SENSE::OptimizationSense = 1

Alternatively, call @objective and pass the existing objective function.

julia> model = Model();

julia> @variable(model, x);

julia> @objective(model, Min, 2x)
2 x

julia> @objective(model, Max, objective_function(model))
2 x

## Set a vector-valued objective

Define a multi-objective optimization problem by passing a vector of objectives:

julia> model = Model();

julia> @variable(model, x[1:2]);

julia> @objective(model, Min, [1 + x, 2 * x])
2-element Vector{AffExpr}:
x + 1
2 x

julia> f = objective_function(model)
2-element Vector{AffExpr}:
x + 1
2 x
Tip

The Multi-objective knapsack tutorial provides an example of solving a multi-objective integer program.

In most cases, multi-objective optimization solvers will return multiple solutions, corresponding to points on the Pareto frontier. See Multiple solutions for information on how to query and work with multiple solutions.

Note that you must set a single objective sense, that is, you cannot have both minimization and maximization objectives. Work around this limitation by choosing Min and negating any objectives you want to maximize:

julia> model = Model();

julia> @variable(model, x[1:2]);

julia> @expression(model, obj1, 1 + x)
x + 1

julia> @expression(model, obj2, 2 * x)
2 x

julia> @objective(model, Min, [obj1, -obj2])
2-element Vector{AffExpr}:
x + 1
-2 x

Defining your objectives as expressions allows flexibility in how you can solve variations of the same problem, with some objectives removed and constrained to be no worse that a fixed value.

julia> model = Model();

julia> @variable(model, x[1:2]);

julia> @expression(model, obj1, 1 + x)
x + 1

julia> @expression(model, obj2, 2 * x)
2 x

julia> @expression(model, obj3, x + x)
x + x

julia> @objective(model, Min, [obj1, obj2, obj3])  # Three-objective problem
3-element Vector{AffExpr}:
x + 1
2 x
x + x

julia> # optimize!(model), look at the solution, talk to stakeholders, then
# decide you want to solve a new problem where the third objective is
# removed and constrained to be better than 2.0.
nothing

julia> @objective(model, Min, [obj1, obj2])   # Two-objective problem
2-element Vector{AffExpr}:
x + 1
2 x

julia> @constraint(model, obj3 <= 2.0)
x + x ≤ 2