---
title: Ordering Functions (latest version)
description: API reference for Ordering Functions in the latest version of qiskit-fermions-c
source: https://quantum.cloud.ibm.com/docs/en/api/qiskit-fermions-c/qf-operators-terms-ordering
---

# Ordering Functions

### qf\_ferm\_op\_canonical\_order

`QfFermionOperator *qf_ferm_op_canonical_order(const QfFermionOperator *op)`

Returns a copy of a fermionic operator with its terms in a canonical order.

The terms are sorted into a canonical order that depends only on each term’s structure (the operator string it represents) and not on its coefficient. The order is therefore deterministic for a given set of terms regardless of how the operator was assembled. The terms themselves are left untouched — this only reorders them, it does not simplify or normal-order the operator.

```c
 1QfFermionOperator *op = qf_ferm_op_zero();
 2bool actions[2] = {true, false};
 3uint32_t modes_a[2] = {1, 0};
 4QkComplex64 coeff_a = {1.0, 0.0};
 5qf_ferm_op_add_term(op, 2, actions, modes_a, &coeff_a);
 6uint32_t modes_b[2] = {0, 1};
 7QkComplex64 coeff_b = {2.0, 0.0};
 8qf_ferm_op_add_term(op, 2, actions, modes_b, &coeff_b);
 9
10QfFermionOperator *ordered = qf_ferm_op_canonical_order(op);
```

> **Note**
>
> Any group indices (see [`qf_ferm_op_get_groups()`](/docs/api/qiskit-fermions-c/qf-ferm-op#qf_ferm_op_get_groups "qf_ferm_op_get_groups")) are preserved: each term carries its group index along as it moves, since a group index is a per-term tag independent of term order. Terms of the same group are simply no longer contiguous afterwards. If the input has no groups, neither does the result.

**Parameters**

- **op** – A pointer to the operator whose terms are to be reordered.

**Returns**

A pointer to the created operator.

### qf\_maj\_op\_canonical\_order

`QfMajoranaOperator *qf_maj_op_canonical_order(const QfMajoranaOperator *op)`

Returns a copy of a Majorana operator with its terms in a canonical order.

The terms are sorted into a canonical order that depends only on each term’s structure (the operator string it represents) and not on its coefficient. The order is therefore deterministic for a given set of terms regardless of how the operator was assembled. The terms themselves are left untouched — this only reorders them, it does not simplify the operator.

```c
1QfMajoranaOperator *op = qf_maj_op_zero();
2uint32_t modes_a[2] = {1, 0};
3QkComplex64 coeff_a = {1.0, 0.0};
4qf_maj_op_add_term(op, 2, modes_a, &coeff_a);
5uint32_t modes_b[2] = {0, 1};
6QkComplex64 coeff_b = {2.0, 0.0};
7qf_maj_op_add_term(op, 2, modes_b, &coeff_b);
8
9QfMajoranaOperator *ordered = qf_maj_op_canonical_order(op);
```

> **Note**
>
> Any group indices (see [`qf_maj_op_get_groups()`](/docs/api/qiskit-fermions-c/qf-maj-op#qf_maj_op_get_groups "qf_maj_op_get_groups")) are preserved: each term carries its group index along as it moves, since a group index is a per-term tag independent of term order. Terms of the same group are simply no longer contiguous afterwards. If the input has no groups, neither does the result.

**Parameters**

- **op** – A pointer to the operator whose terms are to be reordered.

**Returns**

A pointer to the created operator.