I tried implementing a function to swap the rows of a numpy array, but it does not seem to work

This is the code:

def SwapRows(M: np.ndarray, row_num_1: int, row_num_2: int) -> np.ndarray:
    M_copy = M.copy()
    # exchange row 1 and row 2
    temp = M_copy[row_num_1]
    M_copy[row_num_1] = M_copy[row_num_2]
    M_copy[row_num_2] = temp # I dont know why this method does not seem to work
    
    return M_copy

any ideas as to why?

I tried implementing a function to swap the rows of a numpy array, but it does not seem to work

This is the code:

def SwapRows(M: np.ndarray, row_num_1: int, row_num_2: int) -> np.ndarray:
    M_copy = M.copy()
    # exchange row 1 and row 2
    temp = M_copy[row_num_1]
    M_copy[row_num_1] = M_copy[row_num_2]
    M_copy[row_num_2] = temp # I dont know why this method does not seem to work
    
    return M_copy

any ideas as to why?

• python
• numpy

• 1 Try M[[row1,row2]] = M[[row2,row1]] – Mark Setchell Commented Mar 4 at 3:08
• a[[0,3]]=a[[3,0]] swap 0 and 3 rows in matrix a – Alex Alex Commented Mar 4 at 3:08
• temp is a whole row, and thus a view. Check its value after the next line. – hpaulj Commented Mar 4 at 4:43

1 Answer 1

You need to add .copy() to ensure no unintended references.

import numpy as np

def SwapRowsYours(M: np.ndarray, row_num_1: int, row_num_2: int) -> np.ndarray:
    M_copy = M.copy()
    # exchange row 1 and row 2.
    temp = M_copy[row_num_1]
    M_copy[row_num_1] = M_copy[row_num_2]
    M_copy[row_num_2] = temp # I dont know why this method does not seem to work.
    
    return M_copy

def SwapRowsWorking(M: np.ndarray, row_num_1: int, row_num_2: int) -> np.ndarray:
    # Create a copy of the input array to avoid modifying the original array.
    M_copy = M.copy()
    
    # Swap the specified rows.
    temp = M_copy[row_num_1].copy()  # Use .copy() to ensure no unintended references.
    M_copy[row_num_1] = M_copy[row_num_2]
    M_copy[row_num_2] = temp
    
    return M_copy
    
# Define a sample 2D NumPy array
M = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

# Swap rows 0 and 2
result = SwapRowsYours(M, 0, 2)

print("Original Array:")
print(M)

print("\nArray After Swapping Rows (Your Function):")
print(result)

# Swap rows 0 and 2
result = SwapRowsWorking(M, 0, 2)

print("\nArray After Swapping Rows (Working Function):")
print(result)

The output should be as follows:

Original Array:
[[1 2 3]
 [4 5 6]
 [7 8 9]]

Array After Swapping Rows (Your Function):
[[7 8 9]
 [4 5 6]
 [7 8 9]]

Array After Swapping Rows (Working Function):
[[7 8 9]
 [4 5 6]
 [1 2 3]]