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Python – Multiply K to every Nth element
  • Last Updated : 27 Feb, 2020

We generally wish to employ a particular function to all the elements in a list. But sometimes, according to requirement we would wish to employ a particular functionality to certain elements of the list, basically to every Nth 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 Nth number of list can be done with the help of list comprehension and enumerate function helps in the iteration of the whole list.




# Python3 code to demonstrate
# Multiply K to every Nth element
# 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))
  
# initializing N
N = 3
  
# initializing K 
K = 2
  
# using list comprehension + enumerate()
# Multiply K to every Nth element
res = [i * K if j % N == 0 else i for j, i in enumerate(test_list)]
  
# printing result
print ("The list after multiplying K to every Nth element : " + str(res))
Output :
The original list is : [1, 4, 5, 6, 7, 8, 9, 12]
The list after multiplying K to every Nth element : [2, 4, 5, 12, 7, 8, 18, 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 Nth element.






# Python3 code to demonstrate
# Multiply K to every Nth element
# 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))
  
# initializing N
N = 3
  
# initializing K 
K = 2
  
# using list comprehension + list slicing
# Multiply K to every Nth element
test_list[0::3] = [i * K for i in test_list[0 :: N]]
  
# printing result
print ("The list after multiplying K to every Nth element : " 
                                            + str(test_list))
Output :
The original list is : [1, 4, 5, 6, 7, 8, 9, 12]
The list after multiplying K to every Nth element : [2, 4, 5, 12, 7, 8, 18, 12]

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