qiskit-documentation/docs/api/qiskit/0.45/qiskit.opflow.state_fns.VectorStateFn.mdx

---
title: VectorStateFn
description: API reference for qiskit.opflow.state_fns.VectorStateFn
in_page_toc_min_heading_level: 1
python_api_type: class
python_api_name: qiskit.opflow.state_fns.VectorStateFn
---

# VectorStateFn

<Class id="qiskit.opflow.state_fns.VectorStateFn" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.45/qiskit/opflow/state_fns/vector_state_fn.py" signature="qiskit.opflow.state_fns.VectorStateFn(*args, **kwargs)" modifiers="class">
  Bases: [`StateFn`](qiskit.opflow.state_fns.StateFn "qiskit.opflow.state_fns.state_fn.StateFn")

  Deprecated: A class for state functions and measurements which are defined in vector representation, and stored using Terra’s `Statevector` class.

  <Admonition title="Deprecated since version 0.24.0" type="danger">
    The class `qiskit.opflow.state_fns.vector_state_fn.VectorStateFn` is deprecated as of qiskit-terra 0.24.0. It will be removed no earlier than 3 months after the release date. For code migration guidelines, visit [https://qisk.it/opflow\_migration](https://qisk.it/opflow_migration).
  </Admonition>

  **Parameters**

  *   **primitive** – The `Statevector`, NumPy array, or list, which defines the behavior of the underlying function.
  *   **coeff** – A coefficient multiplying the state function.
  *   **is\_measurement** – Whether the StateFn is a measurement operator

  ## Attributes

  ### INDENTATION

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.INDENTATION" attributeValue="'  '" />

  ### coeff

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.coeff">
    A coefficient by which the state function is multiplied.
  </Attribute>

  ### instance\_id

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.instance_id">
    Return the unique instance id.
  </Attribute>

  ### is\_measurement

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.is_measurement">
    Whether the StateFn object is a measurement Operator.
  </Attribute>

  ### num\_qubits

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.num_qubits" />

  ### parameters

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.parameters" />

  ### primitive

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.primitive" attributeTypeHint="Statevector">
    The primitive which defines the behavior of the underlying State function.
  </Attribute>

  ### settings

  <Attribute id="qiskit.opflow.state_fns.VectorStateFn.settings">
    Return settings.
  </Attribute>

  ## Methods

  ### add

  <Function id="qiskit.opflow.state_fns.VectorStateFn.add" signature="add(other)">
    Return Operator addition of self and other, overloaded by `+`.

    **Parameters**

    **other** ([*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")) – An `OperatorBase` with the same number of qubits as self, and in the same ‘Operator’, ‘State function’, or ‘Measurement’ category as self (i.e. the same type of underlying function).

    **Returns**

    An `OperatorBase` equivalent to the sum of self and other.

    **Return type**

    [*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")
  </Function>

  ### adjoint

  <Function id="qiskit.opflow.state_fns.VectorStateFn.adjoint" signature="adjoint()">
    Return a new Operator equal to the Operator’s adjoint (conjugate transpose), overloaded by `~`. For StateFns, this also turns the StateFn into a measurement.

    **Returns**

    An `OperatorBase` equivalent to the adjoint of self.

    **Return type**

    [*VectorStateFn*](#qiskit.opflow.state_fns.VectorStateFn "qiskit.opflow.state_fns.vector_state_fn.VectorStateFn")
  </Function>

  ### eval

  <Function id="qiskit.opflow.state_fns.VectorStateFn.eval" signature="eval(front=None)">
    Evaluate the Operator’s underlying function, either on a binary string or another Operator. A square binary Operator can be defined as a function taking a binary function to another binary function. This method returns the value of that function for a given StateFn or binary string. For example, `op.eval('0110').eval('1110')` can be seen as querying the Operator’s matrix representation by row 6 and column 14, and will return the complex value at those “indices.” Similarly for a StateFn, `op.eval('1011')` will return the complex value at row 11 of the vector representation of the StateFn, as all StateFns are defined to be evaluated from Zero implicitly (i.e. it is as if `.eval('0000')` is already called implicitly to always “indexing” from column 0).

    If `front` is None, the matrix-representation of the operator is returned.

    **Parameters**

    **front** ([*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")  *|*[*Dict*](https://docs.python.org/3/library/typing.html#typing.Dict "(in Python v3.12)")*\[*[*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")*,* [*complex*](https://docs.python.org/3/library/functions.html#complex "(in Python v3.12)")*] |* [*ndarray*](https://numpy.org/doc/stable/reference/generated/numpy.ndarray.html#numpy.ndarray "(in NumPy v1.26)")  *|*[*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")  *|*[*Statevector*](qiskit.quantum_info.Statevector "qiskit.quantum_info.states.statevector.Statevector") *| None*) – The bitstring, dict of bitstrings (with values being coefficients), or StateFn to evaluated by the Operator’s underlying function, or None.

    **Returns**

    The output of the Operator’s evaluation function. If self is a `StateFn`, the result is a float or complex. If self is an Operator (`PrimitiveOp, ComposedOp, SummedOp, EvolvedOp,` etc.), the result is a StateFn. If `front` is None, the matrix-representation of the operator is returned, which is a `MatrixOp` for the operators and a `VectorStateFn` for state-functions. If either self or front contain proper `ListOps` (not ListOp subclasses), the result is an n-dimensional list of complex or StateFn results, resulting from the recursive evaluation by each OperatorBase in the ListOps.

    **Return type**

    [*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase") | [complex](https://docs.python.org/3/library/functions.html#complex "(in Python v3.12)")
  </Function>

  ### permute

  <Function id="qiskit.opflow.state_fns.VectorStateFn.permute" signature="permute(permutation)">
    Permute the qubits of the state function.

    **Parameters**

    **permutation** ([*List*](https://docs.python.org/3/library/typing.html#typing.List "(in Python v3.12)")*\[*[*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.12)")*]*) – A list defining where each qubit should be permuted. The qubit at index j of the circuit should be permuted to position permutation\[j].

    **Returns**

    A new StateFn containing the permuted primitive.

    **Return type**

    [*VectorStateFn*](#qiskit.opflow.state_fns.VectorStateFn "qiskit.opflow.state_fns.vector_state_fn.VectorStateFn")
  </Function>

  ### primitive\_strings

  <Function id="qiskit.opflow.state_fns.VectorStateFn.primitive_strings" signature="primitive_strings()">
    Return a set of strings describing the primitives contained in the Operator. For example, `{'QuantumCircuit', 'Pauli'}`. For hierarchical Operators, such as `ListOps`, this can help illuminate the primitives represented in the various recursive levels, and therefore which conversions can be applied.

    **Returns**

    A set of strings describing the primitives contained within the Operator.

    **Return type**

    [*Set*](https://docs.python.org/3/library/typing.html#typing.Set "(in Python v3.12)")\[[str](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")]
  </Function>

  ### sample

  <Function id="qiskit.opflow.state_fns.VectorStateFn.sample" signature="sample(shots=1024, massive=False, reverse_endianness=False)">
    Sample the state function as a normalized probability distribution. Returns dict of bitstrings in order of probability, with values being probability.

    **Parameters**

    *   **shots** ([*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.12)")) – The number of samples to take to approximate the State function.
    *   **massive** ([*bool*](https://docs.python.org/3/library/functions.html#bool "(in Python v3.12)")) – Whether to allow large conversions, e.g. creating a matrix representing over 16 qubits.
    *   **reverse\_endianness** ([*bool*](https://docs.python.org/3/library/functions.html#bool "(in Python v3.12)")) – Whether to reverse the endianness of the bitstrings in the return dict to match Terra’s big-endianness.

    **Returns**

    A dict containing pairs sampled strings from the State function and sampling frequency divided by shots.

    **Return type**

    [dict](https://docs.python.org/3/library/stdtypes.html#dict "(in Python v3.12)")
  </Function>

  ### tensor

  <Function id="qiskit.opflow.state_fns.VectorStateFn.tensor" signature="tensor(other)">
    Return tensor product between self and other, overloaded by `^`. Note: You must be conscious of Qiskit’s big-endian bit printing convention. Meaning, Plus.tensor(Zero) produces a |+⟩ on qubit 0 and a |0⟩ on qubit 1, or |+⟩⨂|0⟩, but would produce a QuantumCircuit like

    > |0⟩– |+⟩–

    Because Terra prints circuits and results with qubit 0 at the end of the string or circuit.

    **Parameters**

    **other** ([*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")) – The `OperatorBase` to tensor product with self.

    **Returns**

    An `OperatorBase` equivalent to the tensor product of self and other.

    **Return type**

    [*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")
  </Function>

  ### to\_circuit\_op

  <Function id="qiskit.opflow.state_fns.VectorStateFn.to_circuit_op" signature="to_circuit_op()">
    Return `StateFnCircuit` corresponding to this StateFn.

    **Return type**

    [*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")
  </Function>

  ### to\_density\_matrix

  <Function id="qiskit.opflow.state_fns.VectorStateFn.to_density_matrix" signature="to_density_matrix(massive=False)">
    Return matrix representing product of StateFn evaluated on pairs of basis states. Overridden by child classes.

    **Parameters**

    **massive** ([*bool*](https://docs.python.org/3/library/functions.html#bool "(in Python v3.12)")) – Whether to allow large conversions, e.g. creating a matrix representing over 16 qubits.

    **Returns**

    The NumPy array representing the density matrix of the State function.

    **Raises**

    [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError "(in Python v3.12)") – If massive is set to False, and exponentially large computation is needed.

    **Return type**

    [*ndarray*](https://numpy.org/doc/stable/reference/generated/numpy.ndarray.html#numpy.ndarray "(in NumPy v1.26)")
  </Function>

  ### to\_dict\_fn

  <Function id="qiskit.opflow.state_fns.VectorStateFn.to_dict_fn" signature="to_dict_fn()">
    Creates the equivalent state function of type DictStateFn.

    **Returns**

    A new DictStateFn equivalent to `self`.

    **Return type**

    [*StateFn*](qiskit.opflow.state_fns.StateFn "qiskit.opflow.state_fns.state_fn.StateFn")
  </Function>

  ### to\_matrix

  <Function id="qiskit.opflow.state_fns.VectorStateFn.to_matrix" signature="to_matrix(massive=False)">
    Return NumPy representation of the Operator. Represents the evaluation of the Operator’s underlying function on every combination of basis binary strings. Warn if more than 16 qubits to force having to set `massive=True` if such a large vector is desired.

    **Returns**

    The NumPy `ndarray` equivalent to this Operator.

    **Return type**

    [*ndarray*](https://numpy.org/doc/stable/reference/generated/numpy.ndarray.html#numpy.ndarray "(in NumPy v1.26)")
  </Function>

  ### to\_matrix\_op

  <Function id="qiskit.opflow.state_fns.VectorStateFn.to_matrix_op" signature="to_matrix_op(massive=False)">
    Return a `VectorStateFn` for this `StateFn`.

    **Parameters**

    **massive** ([*bool*](https://docs.python.org/3/library/functions.html#bool "(in Python v3.12)")) – Whether to allow large conversions, e.g. creating a matrix representing over 16 qubits.

    **Returns**

    A VectorStateFn equivalent to self.

    **Return type**

    [*OperatorBase*](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")
  </Function>
</Class>