How to reverse column order in a matrix with Python?
Last Updated :
08 Sep, 2021
In this article, we will see how to reverse the column order of a matrix in Python.
Examples:
Input:
arr = [[10,20,30],
[40,50,60],
[70,80,90]]
Output:
30 20 10
60 50 40
90 80 70
Input:
arr = [[15,30],
[45,60],
[75,90],
[105,120]]
Output:
30 15
60 45
90 75
120 105
Matrices are created in python by using nested lists/arrays. However, a more efficient way to handle arrays in python is the NumPy library. To create arrays using NumPy use this or matrix in python once go through this.
Method 1:
- Iterate through each row
- For every row, use list comprehension to reverse the row (i.e. a[::-1])
- Append the reversed rows into a new matrix
- Print the matrix
Example:
Python3
# creating a 3X4 matrix using nested lists
matrix_1 = [['c1', 'c2', 'c3'],
[10, 20, 30],
[40, 50, 60],
[70, 80, 90]]
# creating an empty array to store the reversed column matrix
matrix_2 = []
# looping through matrix_1 and appending matrix_2
for i in range(len(matrix_1)):
matrix_2.append(matrix_1[i][::-1])
print('Matrix before changing column order:\n')
for rows in matrix_1:
print(rows)
print('\nMatrix after changing column order:\n')
for rows in matrix_2:
print(rows)
Output:
Method 2:
An array object in NumPy is called ndarray, which is created using the array() function. To reverse column order in a matrix, we make use of the numpy.fliplr() method. The method flips the entries in each row in the left/right direction. Column data is preserved but appears in a different order than before.
Syntax: numpy.fliplr(m)
Parameters: m (array_like) - Input array must be at least 2-D.
Returned Value: ndarray - A view of m is returned with the columns reversed, and this operation's time complexity is O(1).
Example:
Python3
import numpy as np
# creating a numpy array(matrix) with 3-columns and 4-rows
arr = np.array([
['c1', 'c2', 'c3'],
[10, 20, 30],
[40, 50, 60],
[70, 80, 90]])
# reversing column order in matrix
flipped_arr = np.fliplr(arr)
print('Array before changing column order:\n', arr)
print('\nArray after changing column order:\n', flipped_arr)
Output:
Flipped_arr contains a reversed column order matrix where the column order has changed from c1,c2,c3 to c3,c2,c1, and the elements of each column remain intact under their respective headers (c1,c2,c3).