How to Reverse a 1D & 2D numpy array using np.flip() and [] operator in Python

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Reversing a 1D and 2D numpy array using np.flip() and [] operator in Python.

Reverse 1D Numpy array using ‘[]’ operator :

By not passing any start or end parameter, so by default complete array is picked. And as step size is -1, so elements selected from last to first.

# Program :

import numpy as sc

# Creating a numpy array
num_arr = sc.array([11,22,33,44,55,66])

print('Original Array: ',num_arr)

# To get reverse of numpy array
rev_arr = num_arr[::-1]

print('Reversed Array : ', rev_arr)
Output :
Original Array:  [11 22 33 44 55 66]                                
Reversed Array :  [66 55 44 33 22 11]

Reverse Array is View Only :

Here if we do any modification in reversed array, it will also be reflected in original array.

# Program :

import numpy as sc

# Creatin a numpy array
num_arr = sc.array([11,22,33,44,55,66])

# To get reverse of numpy array
rev_arr = num_arr[::-1]
rev_arr[4]=63

print('Modified reversed Array : ', rev_arr)
print('Original array is: ',num_arr)
Output :
Modified reversed Array :  [66 55 44 33 63 11]                      
Original array is:  [11 63 33 44 55 66]

Reverse Numpy array using np.flip() :

flip() function provided by Python’s numpy module helps to flip or reverse the content of numpy array.

Syntax : numpy.flip(arr, axis=None)

where,

  • arr :  A numpy array
  • axis : Axis along which contents need to be flipped. If None, contents will be flipped along axis of array.

Reverse 1D Numpy array using np.flip() :

Here as it is 1 -D Numpy array, there is no need of axis parameter.

# Program :

import numpy as sc

# To create a Numpy array
num_arr = sc.array([11,22,33,44,55])

print('Original array: ',num_arr)

# To get reverse of numpy array
rev_arr = sc.flip(num_arr)

print('Reversed Array is : ', rev_arr)
Output :
Original array:  [11 22 33 44 55]
Reversed Array is :  [55 44 33 22 11]

Reverse 2D Numpy Array using np.flip() :

Reverse contents in all rows and all columns of 2D Numpy Array :

Here we don’t provide parameter in np.flip() function, then contents will be reversed along the axes of 2-D Numpy array.

# Program :

import numpy as sc

# to create a 2D Numpy array
twoD_Arr = sc.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

print('Original Array is: ',twoD_Arr)

# to reverse 2D numpy array
rev_Arr = sc.flip(twoD_Arr)

print('Reversed Array : ')
print(rev_Arr)
Output :
Original Array is:
[[1 2 3]
[4 5 6]
[7 8 9]]
Reversed Array :
[[9 8 7]
[6 5 4]
[3 2 1]]

Reverse contents of all rows only in 2D Numpy Array :

If we provide axix parameter i.e. axis=1, then rows of array will be reversed.

# Program :

import numpy as sc

# to create a 2D Numpy array
twoD_Arr = sc.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

print('Original Array is: ',twoD_Arr)

# to reverse the content of each row in array
rev_Arr = sc.flip(twoD_Arr, axis=1)

print('Reversed Array : ')
print(rev_Arr)
Output :
Original Array is:  [[1 2 3]
[4 5 6]
[7 8 9]]
Reversed Array :
[[3 2 1]
[6 5 4]
[9 8 7]]

Reverse contents of all columns only in 2D Numpy Array :

If we provide axix parameter i.e. axis=0, then rows of array will be reversed.

# Program :

import numpy as sc

# to create a 2D Numpy array
twoD_Arr = sc.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

print('Original Array is: ',twoD_Arr)

# to reverse the content of each column in array
rev_Arr = sc.flip(twoD_Arr, axis=0)

print('Reversed Array : ')
print(rev_Arr)
Output :
Original Array is:
[[1 2 3]
[4 5 6]
[7 8 9]]
Reversed Array :
[[7 8 9]
[4 5 6]
[1 2 3]]

Reverse contents of only one row in 2D Numpy Array :

Let, we want to reverse only 1st row of a Numpy array.

# Program :

import numpy as sc

# to create a 2D Numpy array
twoD_Arr = sc.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

print('Original Array is: ',twoD_Arr)

# to reverse only 1st row
twoD_Arr[0] = sc.flip(twoD_Arr[0])

print('Reversed Array : ')
print(twoD_Arr)
Output :
Original Array is:
[[1 2 3]
[4 5 6]
[7 8 9]]
Reversed Array :
[[3 2 1]
[4 5 6]
[7 8 9]]

Reverse contents of only one column in 2D Numpy Array :

Let, we want to reverse only 3rd column of a Numpy array.

# Program :

import numpy as sc

# to create a 2D Numpy array
twoD_Arr = sc.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

print('Original Array is: ',twoD_Arr)

# to reverse the content of 3rd column in array
twoD_Arr[:,2] = sc.flip(twoD_Arr[:,2])

print('Reversed Array : ')
print(twoD_Arr)
Output :
Original Array is:
[[1 2 3]
[4 5 6]
[7 8 9]]
Reversed Array :
[[1 2 9]
[4 5 6]
[7 8 3]]