# WeightedAdder

*class *`qiskit.circuit.library.WeightedAdder(num_state_qubits=None, weights=None, name='adder')`

Bases: `BlueprintCircuit`

A circuit to compute the weighted sum of qubit registers.

Given $n$ qubit basis states $q_0, \ldots, q_{n-1} \in \{0, 1\}$ and non-negative integer weights $\lambda_0, \ldots, \lambda_{n-1}$, this circuit performs the operation

$|q_0 \ldots q_{n-1}\rangle |0\rangle_s \mapsto |q_0 \ldots q_{n-1}\rangle |\sum_{j=0}^{n-1} \lambda_j q_j\rangle_s$where $s$ is the number of sum qubits required. This can be computed as

$s = 1 + \left\lfloor \log_2\left( \sum_{j=0}^{n-1} \lambda_j \right) \right\rfloor$or $s = 1$ if the sum of the weights is 0 (then the expression in the logarithm is invalid).

For qubits in a circuit diagram, the first weight applies to the upper-most qubit. For an example where the state of 4 qubits is added into a sum register, the circuit can be schematically drawn as

```
┌────────┐
state_0: ┤0 ├ | state_0 * weights[0]
│ │ |
state_1: ┤1 ├ | + state_1 * weights[1]
│ │ |
state_2: ┤2 ├ | + state_2 * weights[2]
│ │ |
state_3: ┤3 ├ | + state_3 * weights[3]
│ │
sum_0: ┤4 ├ |
│ Adder │ |
sum_1: ┤5 ├ | = sum_0 * 2^0 + sum_1 * 2^1 + sum_2 * 2^2
│ │ |
sum_2: ┤6 ├ |
│ │
carry_0: ┤7 ├
│ │
carry_1: ┤8 ├
│ │
control_0: ┤9 ├
└────────┘
```

Computes the weighted sum controlled by state qubits.

**Parameters**

**num_state_qubits**(*int*(opens in a new tab)*| None*) – The number of state qubits.**weights**(*List*(opens in a new tab)*[**int*(opens in a new tab)*] | None*) – List of weights, one for each state qubit. If none are provided they default to 1 for every qubit.**name**(*str*(opens in a new tab)) – The name of the circuit.

## Attributes

### ancillas

A list of `AncillaQubit`

s in the order that they were added. You should not mutate this.

### calibrations

Return calibration dictionary.

The custom pulse definition of a given gate is of the form `{'gate_name': {(qubits, params): schedule}}`

### clbits

A list of `Clbit`

s in the order that they were added. You should not mutate this.

### data

### global_phase

The global phase of the current circuit scope in radians.

### instances

Default value: `160`

### layout

Return any associated layout information about the circuit

This attribute contains an optional `TranspileLayout`

object. This is typically set on the output from `transpile()`

or `PassManager.run()`

to retain information about the permutations caused on the input circuit by transpilation.

There are two types of permutations caused by the `transpile()`

function, an initial layout which permutes the qubits based on the selected physical qubits on the `Target`

, and a final layout which is an output permutation caused by `SwapGate`

s inserted during routing.

### metadata

Arbitrary user-defined metadata for the circuit.

Qiskit will not examine the content of this mapping, but it will pass it through the transpiler and reattach it to the output, so you can track your own metadata.

### num_ancillas

Return the number of ancilla qubits.

### num_captured_vars

The number of real-time classical variables in the circuit marked as captured from an enclosing scope.

This is the length of the `iter_captured_vars()`

iterable. If this is non-zero, `num_input_vars`

must be zero.

### num_carry_qubits

The number of carry qubits required to compute the sum.

Note that this is not necessarily equal to the number of ancilla qubits, these can be queried using `num_ancilla_qubits`

.

**Returns**

The number of carry qubits required to compute the sum.

### num_clbits

Return number of classical bits.

### num_control_qubits

The number of additional control qubits required.

Note that the total number of ancilla qubits can be obtained by calling the method `num_ancilla_qubits`

.

**Returns**

The number of additional control qubits required (0 or 1).

### num_declared_vars

The number of real-time classical variables in the circuit that are declared by this circuit scope, excluding inputs or captures.

This is the length of the `iter_declared_vars()`

iterable.

### num_input_vars

The number of real-time classical variables in the circuit marked as circuit inputs.

This is the length of the `iter_input_vars()`

iterable. If this is non-zero, `num_captured_vars`

must be zero.

### num_parameters

### num_qubits

Return number of qubits.

### num_state_qubits

The number of qubits to be summed.

**Returns**

The number of state qubits.

### num_sum_qubits

The number of sum qubits in the circuit.

**Returns**

The number of qubits needed to represent the weighted sum of the qubits.

### num_vars

The number of real-time classical variables in the circuit.

This is the length of the `iter_vars()`

iterable.

### op_start_times

Return a list of operation start times.

This attribute is enabled once one of scheduling analysis passes runs on the quantum circuit.

**Returns**

List of integers representing instruction start times. The index corresponds to the index of instruction in `QuantumCircuit.data`

.

**Raises**

**AttributeError**(opens in a new tab) – When circuit is not scheduled.

### parameters

### prefix

Default value: `'circuit'`

### qregs

Type: `list[QuantumRegister]`

A list of the `QuantumRegister`

s in this circuit. You should not mutate this.

### qubits

A list of `Qubit`

s in the order that they were added. You should not mutate this.

### weights

The weights for the qubit states.

**Returns**

The weight for the qubit states.

### name

Type: `str`

A human-readable name for the circuit.

### cregs

Type: `list[ClassicalRegister]`

A list of the `ClassicalRegister`

s in this circuit. You should not mutate this.

### duration

Type: `int | float | None`

The total duration of the circuit, set by a scheduling transpiler pass. Its unit is specified by `unit`

.

### unit

The unit that `duration`

is specified in.