Python | Repeat each element K times in list

Last Updated : 08 Apr, 2019

Many times we have this particular use-case in which we need to repeat each element of list K times. The problems of making a double clone have been discussed but this problem extends to allow a flexible variable to define the number of times the element has to be repeated. Let’s discuss certain ways in which this can be performed.

Method #1 : Using list comprehension
This particular task requires generally 2 loops and list comprehension can perform this particular task in one line and hence reduce the lines of codes and improving code readability.

# Python3 code to demonstrate 
# repeat element K times
# using list comprehension
  
# initializing list of lists
test_list = [4, 5, 6]
  
# printing original list 
print("The original list : " +  str(test_list))
  
# declaring magnitude of repetition
K = 3
  
# using list comprehension
# repeat elements K times
res =  [ele for ele in test_list for i in range(K)]
  
# printing result 
print("The list after adding elements :  " + str(res))

Output :

The original list : [4, 5, 6]
The list after adding elements :  [4, 4, 4, 5, 5, 5, 6, 6, 6]

Method #2 : Using itertools.chain.from_iterable() + itertools.repeat()
This particular problem can also be solved using python inbuilt functions of itertools library. The repeat function, as name suggests does the task of repetition and grouping into a list is done by the from_iterable function.

# Python3 code to demonstrate 
# repeat element K times
# using itertools.chain.from_iterable() + itertools.repeat()
import itertools
  
# initializing list of lists
test_list = [4, 5, 6]
  
# printing original list 
print("The original list : " +  str(test_list))
  
# declaring magnitude of repetition
K = 3
  
# using itertools.chain.from_iterable() 
# + itertools.repeat() repeat elements K times
res = list(itertools.chain.from_iterable(itertools.repeat(i, K)
                                           for i in test_list))
  
# printing result 
print("The list after adding elements :  " + str(res))

Output :

The original list : [4, 5, 6]
The list after adding elements :  [4, 4, 4, 5, 5, 5, 6, 6, 6]

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