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Python | Operate on every Kth element of list

  • Last Updated : 26 Feb, 2019

We generally wish to employ a particular function to all the elements in a list. But sometimes, according to the requirement, we would wish to employ a particular functionality to certain elements of the list, basically to every Kth element in list. Let’s discuss certain ways in which this can be performed.

Method #1 : Using list comprehension + enumerate()
The functionality of getting every Kth number of list can be done with the help of list comprehension and enumerate function helps in the iteration of the whole list.

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# Python3 code to demonstrate
# Edit every Kth element in list 
# using list comprehension + enumerate()
  
# initializing list 
test_list = [1, 4, 5, 6, 7, 8, 9, 12]
  
# printing the original list
print ("The original list is : " + str(test_list))
  
# using list comprehension + enumerate()
# Edit every Kth element in list 
# add 2 to every 3rd element
res = [i + 2 if j % 3 == 0 else
       for j, i in enumerate(test_list)]
  
# printing result
print ("The list after editing every kth element : " + str(res))
Output:
The original list is : [1, 4, 5, 6, 7, 8, 9, 12]
The list after editing every kth element : [3, 4, 5, 8, 7, 8, 11, 12]

 
Method #2 : Using list comprehension + list slicing
Above mentioned functions can help to perform these tasks. The list comprehension does the task of iteration in list and list slicing does the extraction of every Kth element.




# Python3 code to demonstrate
# Edit every Kth element in list 
# using list comprehension + list slicing 
  
# initializing list 
test_list = [1, 4, 5, 6, 7, 8, 9, 12]
  
# printing the original list
print ("The original list is : " + str(test_list))
  
# using list comprehension + list slicing
# Edit every Kth element in list 
# add 2 to every 3rd element
test_list[0::3] = [i + 2 for i in test_list[0 :: 3]]
  
# printing result
print ("The list after editing every kth element : "
                                   + str(test_list))
Output:
The original list is : [1, 4, 5, 6, 7, 8, 9, 12]
The list after editing every kth element : [3, 4, 5, 8, 7, 8, 11, 12]



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