Python | Element repetition in list

Sometimes we require to add a duplicate value in the list for several different utilities. This type of application is sometimes required in day-day programming. Let’s discuss certain ways in which we add a clone of a number to its next position. 
Method #1 : Using list comprehension In this method, we just iterate the loop twice for each value and add to the desired new list. This is just a shorthand alternative to the naive method. 

Python3

# Python3 code to demonstrate
# to perform element duplication
# using list comprehension

# initializing list
test_list = [4, 5, 6, 3, 9]

# printing original list
print ("The original list is : " + str(test_list))

# using list comprehension
# to perform element duplication
res = [i for i in test_list for x in (0, 1)]

# printing result
print ("The list after element duplication " +  str(res))

Output :

The original list is : [4, 5, 6, 3, 9]
The list after element duplication [4, 4, 5, 5, 6, 6, 3, 3, 9, 9]

Method #2 : Using reduce() + add We can also use the reduce function to act the function to perform the addition of a pair of similar numbers simultaneously in the list. 

Python3

# Python3 code to demonstrate
# to perform element duplication
# using reduce() + add
from operator import add

# initializing list
test_list = [4, 5, 6, 3, 9]

# printing original list
print ("The original list is : " + str(test_list))

# using reduce() + add
# to perform element duplication
res = list(reduce(add, [(i, i) for i in test_list]))

# printing result
print ("The list after element duplication " +  str(res))

Output :

The original list is : [4, 5, 6, 3, 9]
The list after element duplication [4, 4, 5, 5, 6, 6, 3, 3, 9, 9]

Method #3 : Using itertools.chain().from_iterable() from_iterable function can also be used to perform this task of adding a duplicate. It just makes the pair of each iterated element and inserts it successively. 

Python3

# Python3 code to demonstrate
# to perform element duplication
# using itertools.chain.from_iterable()
import itertools

# initializing list
test_list = [4, 5, 6, 3, 9]

# printing original list
print ("The original list is : " + str(test_list))

# using itertools.chain.from_iterable()
# to perform element duplication
res = list(itertools.chain.from_iterable([i, i] for i in test_list))

# printing result
print ("The list after element duplication " +  str(res))

Output :

The original list is : [4, 5, 6, 3, 9]
The list after element duplication [4, 4, 5, 5, 6, 6, 3, 3, 9, 9]

 Method #4 : Using repeat

You can use the repeat method of the itertools module to achieve the desired result of adding a duplicate element to the list. Here is how you can do it:

Python3

from itertools import repeat

# Initializing the list
test_list = [4, 5, 6, 3, 9]

# Printing the original list
print("The original list is:", test_list)

# Using the repeat method to add a duplicate element
res = [i for i in test_list for _ in repeat(None, 2)]

# Printing the result
print("The list after element duplication:", res)
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is: [4, 5, 6, 3, 9]
The list after element duplication: [4, 4, 5, 5, 6, 6, 3, 3, 9, 9]

Time complexity: O(n)

Auxiliary Space: O(n)