diff --git a/control/statesp.py b/control/statesp.py
index 717fc9a73..56009cc2a 100644
--- a/control/statesp.py
+++ b/control/statesp.py
@@ -2142,6 +2142,113 @@ def _parse_list(signals, signame='input', prefix='u'):
return StateSpace(
ss_sys, inputs=input_names, outputs=output_names, name=name)
+# Reorder states in state-space object
+def xperm(sys, P):
+ r"""Reorder states in a state-space representation.
+
+ ``xperm(sys,P)``
+ Reorder a space system. Always creates a new system.
+
+ Parameters
+ ----------
+ sys : StateSpace or TransferFunction
+ A linear system.
+ A, B, C, D : array_like or string
+ System, control, output, and feed forward matrices.
+ P : permutation vector P.
+ It is a 0:N-1 where N-1 is the number of states in sys.
+
+ Returns
+ -------
+ out: :class:`StateSpace`
+ Linear input/output system.
+
+ Raises
+ ------
+ ValueError
+ ValueError: If matrix sizes are not self-consistent or if the order
+ is invalid.
+
+ Example
+ -------
+
+ Reorder a ss model:
+ >>> G = ct.ss([[-1, -2], [3, -4]], [[5], [7]], [[6, 8]], [[9]])
+ >>> ct.xperm(G,[1, 0, 2])
+
+ """
+ # TODO: transfer the original sys parameters to the new output sys to preserve labels.
+ # TODO: create docstrings for this function - WIP
+
+ # Convert `order` to numpy array if it's a list
+ P = np.array(P)
+
+ # Get order of A
+ n = sys.A.shape[0]
+ _sorted_P = np.sort(P)
+ _sorted_sys_labels = np.sort(sys.state_labels)
+
+ # Check if inputs are state labels from sys
+ if np.array_equal(_sorted_sys_labels, _sorted_P):
+ order_contains_labels = True
+ elif np.array_equal(np.arange(n), _sorted_P):
+ order_contains_labels = False
+ else:
+ raise ValueError('Invalid P array. P must be a list of unique integers from 0 to N-1, or a list of state labels of sys.')
+
+ # Check dimensions of matrices
+ if sys.A.shape[0] != sys.A.shape[1]:
+ raise ValueError("Matrix A must be square.")
+ if sys.B.shape[0] != n:
+ raise ValueError("Matrix B must have the same number of rows as A.")
+ if sys.C.shape[1] != n:
+ raise ValueError("Matrix C must have the same number of columns as A.")
+ if sys.D.shape[0] != sys.C.shape[0] or sys.D.shape[1] != sys.B.shape[1]:
+ raise ValueError("Matrix D dimensions must be consistent with matrices C and B.")
+
+ # if P contains labels, find their order
+ if order_contains_labels:
+ # list1 is the reference, we want indices of list2 elements in list1
+ P = np.searchsorted(sys.state_labels, P)
+
+ # Construct the permutation matrix T
+ T = np.zeros((n, n))
+ for i, j in enumerate(P):
+ T[i, j] = 1
+
+ P_inv = np.linalg.inv(T)
+
+ # Apply the transformation
+ A_perm = T @ sys.A @ P_inv
+ B_perm = T @ sys.B
+ C_perm = sys.C @ P_inv
+ D_perm = sys.D # D remains unchanged
+
+ return ss(A_perm, B_perm, C_perm, D_perm)
+"""
+ Example usage
+ >>> A = np.array([[-10., -24., 0.],
+ [ 1., 0., 0.],
+ [ 0., 1., 0.]])
+
+ >>> B = np.array([[1],[0],[0]])
+ >>> C = np.array([[0, 1, 0]])
+ >>> D = np.array([[0]])
+ >>> sys = ct.ss(A,B,C,D)
+
+ # Desired order of state variables (e.g., swap x1 and x3):
+ >>> P = [2, 1, 0] # permutations: x3->x1, x2->x2, x1->x3.
+
+ try:
+ sys_reordered = xperm(sys, P)
+ print("A' =\n", sys_reordered.A)
+ print("B' =\n", sys_reordered.B)
+ print("C' =\n", sys_reordered.C)
+ print("D' =\n", sys_reordered.D)
+ except ValueError as e:
+ print("Error:", e)
+ """
+
#
# Utility functions
#
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