Python – Insert after every Nth element in a list
Sometimes we need to perform a single insertion in python, this can be easily done with the help of the insert function. But sometimes, we require to insert in a repeated manner after every n numbers, for that there can be numerous shorthands that can be quite handy. Lets discuss certain ways in which this can be done.
Method #1 : Using join() + enumerate()
We can use the join function to join each of nth substring with the digit K and enumerate can do the task of performing the selective iteration of list.
# Python3 code to demonstrate # inserting K after every Nth number# using join() + enumerate() # initializing listtest_list = ['g', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'g', 'e', 'e', 'k', 's'] # printing original listprint ("The original list is : " + str(test_list)) # initializing k k = 'x' # initializing NN = 3 # using join() + enumerate()# inserting K after every Nth number res = list(''.join(i + k * (N % 3 == 2) for N, i in enumerate(test_list))) # printing result print ("The lists after insertion : " + str(res)) |
The original list is : [‘g’, ‘e’, ‘e’, ‘k’, ‘s’, ‘f’, ‘o’, ‘r’, ‘g’, ‘e’, ‘e’, ‘k’, ‘s’]
The lists after insertion : [‘g’, ‘e’, ‘e’, ‘x’, ‘k’, ‘s’, ‘f’, ‘x’, ‘o’, ‘r’, ‘g’, ‘x’, ‘e’, ‘e’, ‘k’, ‘x’, ‘s’]
Method #2 : Using itertools.chain()
This method also has the ability to perform a similar task using an iterator and hence an improvement in the performance. This function performs a similar task but uses chain method to join the n substrings.
# Python3 code to demonstrate # inserting K after every Nth number# using itertool.chain()from itertools import chain # initializing listtest_list = ['g', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'g', 'e', 'e', 'k', 's'] # printing original listprint ("The original list is : " + str(test_list)) # initializing k k = 'x' # initializing NN = 3 # using itertool.chain()# inserting K after every Nth number res = list(chain(*[test_list[i : i+N] + [k] if len(test_list[i : i+N]) == N else test_list[i : i+N] for i in range(0, len(test_list), N)])) # printing result print ("The lists after insertion : " + str(res)) |
The original list is : [‘g’, ‘e’, ‘e’, ‘k’, ‘s’, ‘f’, ‘o’, ‘r’, ‘g’, ‘e’, ‘e’, ‘k’, ‘s’]
The lists after insertion : [‘g’, ‘e’, ‘e’, ‘x’, ‘k’, ‘s’, ‘f’, ‘x’, ‘o’, ‘r’, ‘g’, ‘x’, ‘e’, ‘e’, ‘k’, ‘x’, ‘s’]
