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Python | List expansion by K
  • Last Updated : 14 Mar, 2019

Sometimes, we require to reduce the length of python list but other times we might also require to increase its size and that too by repeating its each element N times. This type of utility can come in day-day programming. Let’s discuss certain ways in which this can be achieved.

Method #1 : Using list comprehension
This task can be performed using the list comprehension method which is just a shortened version of the generic loop method in which we repeat each element for K times using the iteration.




# Python3 code to demonstrate
# List extension by K 
# using list comprehension
  
# initializing list
test_list = [4, 5, 2, 8]
  
# printing original list 
print("The original list : " + str(test_list))
  
# initializing K
K = 3
  
# using list comprehension 
# to extend list 
res = [i for i in test_list for j in range(K)]
  
# printing result
print("The resultant list after extension is : " + str(res))
Output :

The original list : [4, 5, 2, 8]
The resultant list after extension is : [4, 4, 4, 5, 5, 5, 2, 2, 2, 8, 8, 8]

 



Method #2 : Using itertools.chain() + itertools.tee() + zip()
The combination of the above three function can also help to achieve a solution to this particular problem. The tee function repeats in the list K times, nested in zip which links the iteration with particular element and chain function performs this task for all the elements.




# Python3 code to demonstrate
# List extension by K 
# using itertools.chain() + itertools.tee() + zip()
from itertools import chain, tee
  
# initializing list
test_list = [4, 5, 2, 8]
  
# printing original list 
print("The original list : " + str(test_list))
  
# initializing K
K = 3
  
# using itertools.chain() + itertools.tee() + zip()
# to extend list 
res = list(chain(*zip(*tee(test_list, K))))
  
# printing result
print("The resultant list after extension is : " + str(res))
Output :

The original list : [4, 5, 2, 8]
The resultant list after extension is : [4, 4, 4, 5, 5, 5, 2, 2, 2, 8, 8, 8]

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