# 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 list ` `test_list ``=` `[``'g'``, ``'e'``, ``'e'``, ``'k'``, ``'s'``, ``'f'``, ``'o'``, ``'r'``, ` `                            ``'g'``, ``'e'``, ``'e'``, ``'k'``, ``'s'``] ` ` `  `# printing original list ` `print` `(``"The original list is : "` `+` `str``(test_list)) ` ` `  `# initializing k  ` `k ``=` `'x'` ` `  `# initializing N ` `N ``=` `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)) `

Output:

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 list ` `test_list ``=` `[``'g'``, ``'e'``, ``'e'``, ``'k'``, ``'s'``, ``'f'``, ``'o'``, ``'r'``, ` `                            ``'g'``, ``'e'``, ``'e'``, ``'k'``, ``'s'``] ` ` `  `# printing original list ` `print` `(``"The original list is : "` `+` `str``(test_list)) ` ` `  `# initializing k  ` `k ``=` `'x'` ` `  `# initializing N ` `N ``=` `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)) `

Output:

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’]

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