# Python | Increasing alternate element pattern in list

• Last Updated : 27 Mar, 2019

This particular article solves a very specific issue in which we need to insert every alternate element as the increased size pattern of repeated element to form a pattern. This can have a utility in demonstrative projects. Let’s discuss certain ways in which this can be done.

Method #1 : Using list comprehension + enumerate()
List comprehension with the help of enumerate function can be used to perform this particular task in which we use the tuple which increases its length every alternate element for the insertion using the enumerate function.

 # Python3 code to demonstrate# increasing alternate element pattern# using list comprehension + enumerate()  # initializing list test_list = [1, 2, 3, 4, 5]  # printing original list print("The original list : " + str(test_list))  # using list comprehension + enumerate()# increasing alternate element patternres = [j for sub in ((i, '*' * k)       for k, i in enumerate(test_list, 1))       for j in sub]  # print resultprint("The increasing element pattern list : " + str(res))

Output :

The original list : [1, 2, 3, 4, 5]
The increasing element pattern list : [1, '*', 2, '**', 3, '***', 4, '****', 5, '*****']

Method #2 : Using itertools.chain.from_iterable() + zip()
This task can be performed efficiently using the above functions. The zip function interleaves the formed strings using string multiplication with the lists and from_iterable function does the task of flattening the obtained tuples.

 # Python3 code to demonstrate# increasing alternate element pattern# using itertools.chain.from_iterable() + zip()import itertools  # initializing list test_list = [1, 2, 3, 4, 5]  # printing original list print("The original list : " + str(test_list))  # using itertools.chain.from_iterable() + zip()# increasing alternate element patternres = list(itertools.chain.from_iterable(            zip(test_list, ("*" * (i + 1)            for i in range(len(test_list))))))  # print resultprint("The increasing element pattern list : " + str(res))

Output :

The original list : [1, 2, 3, 4, 5]
The increasing element pattern list : [1, '*', 2, '**', 3, '***', 4, '****', 5, '*****']

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