--- title: ParameterVectorElement (latest version) description: API reference for qiskit.circuit.ParameterVectorElement in the latest version of qiskit source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit/qiskit.circuit.ParameterVectorElement --- # ParameterVectorElement *class* `qiskit.circuit.ParameterVectorElement(vector, index, uuid=None)` Bases: [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit._accelerate.circuit.Parameter") An element of a [`ParameterVector`](/docs/api/qiskit/qiskit.circuit.ParameterVector "qiskit.circuit.ParameterVector"). > **Note** > > There is very little reason to ever construct this class directly. Objects of this type are automatically constructed efficiently as part of creating a [`ParameterVector`](/docs/api/qiskit/qiskit.circuit.ParameterVector "qiskit.circuit.ParameterVector"). ## Attributes ### index Get the index of this element in the parent vector. ### name Returns the name of the [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter"). ### num\_parameters The number of unbound parameters in the expression. This is equivalent to `len(expr.parameters)` but does not involve the overhead of creating a set and counting its length. ### parameters Get the parameters present in the expression. > **Note** > > Qiskit guarantees equality (via `==`) of parameters retrieved from an expression with the original [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter") objects used to create this expression, but does **not guarantee** `is` comparisons to succeed. ### uuid Returns the [`UUID`](https://docs.python.org/3/library/uuid.html#uuid.UUID) of the [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter"). In advanced use cases, this property can be passed to the [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter") constructor to produce an instance that compares equal to another instance. ### vector Get the parent vector instance. ## Methods ### abs `abs()` Take the absolute value of the expression. ### arccos `arccos()` Arccosine of the expression. ### arcsin `arcsin()` Arcsine of the expression. ### arctan `arctan()` Arctangent of the expression. ### assign `assign(parameter, value)` Assign one parameter to a value, which can either be numeric or another parameter expression. **Parameters** - **parameter** – A parameter in this expression whose value will be updated. - **value** – The new value to bind to. **Returns** A new expression parameterized by any parameters which were not bound by assignment. ### bind `bind(parameter_values, allow_unknown_parameters=False)` Binds the provided set of parameters to their corresponding values. **Parameters** - **parameter\_values** – Mapping of [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter") instances to the numeric value to which they will be bound. - **allow\_unknown\_parameters** – If `False`, raises an error if `parameter_values` contains [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter")s in the keys outside those present in the expression. If `True`, any such parameters are simply ignored. **Raises** - [**CircuitError**](/docs/api/qiskit/circuit#qiskit.circuit.CircuitError "qiskit.circuit.CircuitError") – - If parameter\_values contains parameters outside those in self. - If a non-numeric value is passed in `parameter_values`. - [**ZeroDivisionError**](https://docs.python.org/3/library/exceptions.html#ZeroDivisionError) – - If binding the provided values requires division by zero. **Returns** A new expression parameterized by any parameters which were not bound by `parameter_values`. ### bind\_all `bind_all(values)` Bind all of the parameters in `self` to numeric values in the dictionary, returning a numeric value. This is a special case of [`bind()`](#qiskit.circuit.ParameterVectorElement.bind "qiskit.circuit.ParameterVectorElement.bind") which can reach higher performance. It is no problem for the `values` dictionary to contain parameters that are not used in this expression; the expectation is that the same bindings dictionary will be fed to other expressions as well. It is an error to call this method with a `values` dictionary that does not bind all of the values, or to call this method with non-numeric values, but this is not explicitly checked, since this method is intended for performance-sensitive use. Passing an incorrect dictionary may result in unexpected behavior. Unlike [`bind()`](#qiskit.circuit.ParameterVectorElement.bind "qiskit.circuit.ParameterVectorElement.bind"), this method will not raise an exception if non-finite floating-point values are encountered. **Parameters** **values** – mapping of parameters to numeric values. ### conjugate `conjugate()` Return the complex conjugate of the expression. ### cos `cos()` Cosine of the expression. ### exp `exp()` Exponentiate the expression. ### gradient `gradient(param)` Return derivative of this expression with respect to the input parameter. **Parameters** **param** – The parameter with respect to which the derivative is calculated. **Returns** The derivative as either a constant numeric value or a symbolic [`ParameterExpression`](/docs/api/qiskit/qiskit.circuit.ParameterExpression "qiskit.circuit.ParameterExpression"). ### is\_real `is_real()` Check whether the expression represents a real number. Note that this will return `None` if there are unbound parameters, in which case it cannot be determined whether the expression is real. ### is\_symbol `is_symbol()` Check if the expression corresponds to a plain symbol. **Returns** `True` is this expression corresponds to a symbol, `False` otherwise. ### log `log()` Take the natural logarithm of the expression. ### numeric `numeric(strict=True)` Cast this expression to a numeric value. **Parameters** **strict** – If `True` (default) this function raises an error if there are any unbound symbols in the expression. If `False`, this allows casting if the expression represents a numeric value, regardless of unbound symbols. For example `(0 * Parameter("x"))` is 0 but has the symbol `x` present. ### sign `sign()` Return the sign of the expression. ### sin `sin()` Sine of the expression. ### subs `subs(parameter_map, allow_unknown_parameters=False)` Returns a new expression with replacement parameters. **Parameters** - **parameter\_map** – Mapping from [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter")s in `self` to the [`ParameterExpression`](/docs/api/qiskit/qiskit.circuit.ParameterExpression "qiskit.circuit.ParameterExpression") instances with which they should be replaced. - **allow\_unknown\_parameters** – If `False`, raises an error if `parameter_map` contains [`Parameter`](/docs/api/qiskit/qiskit.circuit.Parameter "qiskit.circuit.Parameter")s in the keys outside those present in the expression. If `True`, any such parameters are simply ignored. **Raises** [**CircuitError**](/docs/api/qiskit/circuit#qiskit.circuit.CircuitError "qiskit.circuit.CircuitError") – - If parameter\_map contains parameters outside those in self. - If the replacement parameters in `parameter_map` would result in a name conflict in the generated expression. **Returns** A new expression with the specified parameters replaced. ### sympify `sympify()` Return a SymPy equivalent of this expression. **Returns** A SymPy equivalent of this expression. ### tan `tan()` Tangent of the expression.