Random Shuffle Python

The random package of Python has a built-in function shuffle(). A sequence can be shuffled using it (like a list or a tuple) in Python; shuffling means changing the indices of the elements of a collection.

Syntax of random.shuffle()

We use the shuffle() function to change the indices of the elements of any iterable randomly. This function does not return a new list. It alters the original list.

Syntax of the shuffle() function

Parameters of the function:

sequence: This can take any Python iterable object.

function: This parameter is optional. The default value of this function is the random() function. If we pass a user-defined function, then the function must return a value in the range (0, 1].

Returns: This function does not return anything.

Example of the shuffle() Function

Shuffling a List

Code

# Python program to shuffle a list using random.shuffle method

# importing the random module
import random

# Creating a list
list_ = [1, 2, 3, 4, 5, 6]

print("The original list is as follows: ")
print(list_)

# Shuffling the list first time
random.shuffle(list_)
print("After shuffling the list the first time: ")
print(list_)

# Shuffling the list a second time
random.shuffle(list_)
print("\nAfter shuffling the list a second time: ")
print(list_)

Output:

The original list is as follows:
[1, 2, 3, 4, 5, 6]
After shuffling the list the first time:
[4, 3, 1, 6, 5, 2]
After shuffling the list a second time:
[5, 6, 4, 2, 1, 3]

Shuffling The List Using a User-Defined Function

Code

# Python program to shuffle the list as per a function

# importing the random module
import random

# user-defined function to shuffle the list
def shuffling_function():
    return 0.5

list_ = [1, 2, 3, 4, 5, 6]
print("The original list is as follows: ")
print(list_)

# The shuffling_function will return the same value each time.
# hence the order of the shuffle will remain the same even after shuffling multiple times.

random.shuffle(list_, shuffling_function)
print("After shuffling the list the first time: ")
print(list_)

list_ = [1, 2, 3, 4, 5, 6]

random.shuffle(list_, shuffling_function)
print("After shuffling the list a second time: ")
print(list_)

Output:

The original list is as follows:
[1, 2, 3, 4, 5, 6]
After shuffling the list the first time:
[1, 6, 2, 5, 3, 4]
After shuffling the list a second time:
[1, 6, 2, 5, 3, 4]

Randomly Shuffle Not in Place

As we know, the Python shuffle() function operates on the given collection in place and does not return anything or returns a None, which means that it randomly rearranges the elements of the original list and modifies it in its memory location. However, we generally require our original list or collection.

The method described now allows us to retain our original list and get a shuffled list. This alternative returns a new, scrambled list rather than altering the original one.

Before altering the order, we will make a copy of our original list to preserve it.

Code

# Python program to shuffle a list using random.sample but not in place 

# Importing the random module
import random

# Creating a list
original_list = [1, 2, 3, 4, 5, 6]

# Creating a copy of the original list
copy_list = original_list.copy()

# shuffling the copy of the list
random.shuffle(copy_list)

print("The original list is as follows: ")
print(original_list)

print("After shuffling the list: ") 
print(copy_list)

Output:

The original list is as follows:
[1, 2, 3, 4, 5, 6]
After shuffling the list:
[5, 2, 6, 4, 3, 1]

Shuffling Two Lists Simultaneously in the Same Order

Let us assume we have two Python lists. We need to shuffle these two lists to maintain the order of shuffling. For instance, one list contains alphabets, and the other contains corresponding numbers. Let's see how we can shuffle two lists and maintain their order.

Code

# Pyhton program to shuffle two lists in the same order

# Importing the random module
import random

# Creating the first list
list1 = ['a', 'b', 'c', 'd', 'e']
# Creating a second list
list2 = [1, 2, 3, 4, 5]

# Both of the lists before shuffling them
print("The first list: ", list1)
print("The second list: ", list2)

# We will use the zip function to map the indices of the lists to maintain the same order
map_index = list(zip(list1, list2))

# Shuffling the indices
random.shuffle(map_index)

# making a separate list
list1_s, list2_s = zip(*map_index)

# Both the lists after shuffling them
print("The first list after shuffling: ", list1_s)
print("The second list after shuffling: ", list2_s)

# Elements at index 2 of both the lists
print(list1_s[2], list2_s[2])

Output:

The first list:  ['a', 'b', 'c', 'd', 'e']
The second list:  [1, 2, 3, 4, 5]
The first list after shuffling:  ('c', 'e', 'a', 'd', 'b')
The second list after shuffling:  (3, 5, 1, 4, 2)

Shuffling NumPy Multidimensional Array

The Numpy module of Python has a package named numpy.random. The package is used to generate random data. In this example, we will use the Numpy module of Python to create a 2D array. Then we will use the numpy.random.shuffle() function to shuffle this multidimensional array.

Code

# Python program to shuffle a multidimensional array

# Importing the numpy module
import numpy as np

# Creating a 2D array using numpy
print('The original 2D array')
array = np.arange(1, 24, 2)
array = array.reshape(6, 2)
print(array)

print('The shuffled 2D array')
np.random.shuffle(array)
print(array)

Output:

The original 2D array
[[ 1  3]
 [ 5  7]
 [ 9 11]
 [13 15]
 [17 19]
 [21 23]]
The shuffled 2D array
[[13 15]
 [ 1  3]
 [21 23]
 [17 19]
 [ 9 11]
 [ 5  7]]

Shuffling an Item and Getting the Same Result Every Time

In this section, we will see how to use the seed method of the random package to shuffle a list so that every time we call the shuffle function, it will shuffle the original list in the same order. Thus every time, we will get the same list after shuffling.

How Does a PRNG Work?

Python's random library is not completely random. It uses a method called Pseudo-Random Number Generator (PRNG). This means that it is a deterministic system. The seed value of the random module acts as the basis of random number generation. By default, the random package takes the current system time as its seed value. We can change this seed value, and thus we can change the output of the random.shuffle() function. Let us see how to use this value.

Code

# Python program to show how to use a seed value to produce the same result after shuffling

# Importing the random module
import random

# Creating a list of numbers
numbers = [1, 3, 5, 7, 9, 11, 13, 15]
print("The original list is: ", numbers)

# shuffling the original list 4 times
for i in range(4):
    # Specifying the seed value before every shuffling instance
    random.seed(4)
    # Shuffling the list with pre-defined seed value
    random.shuffle(numbers)
    print("The shuffled list is: ", numbers)

Output:

The original list is: 
[1, 3, 5, 7, 9, 11, 13, 15]
The shuffled list is: 
[3, 13, 11, 9, 15, 1, 5, 7]
The shuffled list is: 
[3, 13, 11, 9, 15, 1, 5, 7]
The shuffled list is: 
[3, 13, 11, 9, 15, 1, 5, 7]
The shuffled list is: 
[3, 13, 11, 9, 15, 1, 5, 7]

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