# Quadratic Models: Constrained¶

Many real-world problems include constraints. For example, a routing problem might limit the number of airplanes on the ground at an airport and a scheduling problem might require a minimum interval between shifts.

When using unconstrained samplers to handle problems with constraints, you
typically formulate the constraints as penalties. Constrained models such as
`ConstrainedQuadraticModel`

can support constraints by encoding both an
objective and its set of constraints, as models or in symbolic form.

## Supported Models¶

dimod provides a constrained quadratic model (CQM) class that encodes a quadratic objective and possibly one or more quadratic equality and inequality constraints.

For an introduction to CQMs, see Constrained Quadratic Models.

For descriptions of the CQM class and its methods, see Quadratic Models: Constrained.

## Model Construction¶

dimod provides a variety of model generators. These are especially useful for testing code and learning.

### Example: dimod CQM Generator¶

This example creates a CQM representing a knapsack problem of ten items.

```
>>> cqm = dimod.generators.knapsack(10)
```

Typically you construct a model when reformulating your problem, using such techniques as those presented in D-Wave’s system documentation’s oceandocs:doc_handbook.

### Example: Formulating a CQM¶

This example constructs a CQM from symbolic math, which is especially useful for learning and testing with small CQMs.

```
>>> x = dimod.Binary('x')
>>> y = dimod.Integer('y')
>>> cqm = dimod.CQM()
>>> objective = cqm.set_objective(x+y)
>>> cqm.add_constraint(y <= 3)
```

For very large models, you might read the data from a file or construct from a NumPy array.