Repeat Each Element in a List in Python

In python, we use lists for various tasks. We have already discussed various operations on lists like counting the frequency of elements in a list or reversing a list. In this article, we will discuss three ways to Repeat each element k times in a list in python.

How to Repeat Each Element k Times in a List in Python?

To repeat the elements in a list in python, we insert the elements of a given list to a new list repeatedly. To repeat each element k times in a list, we will insert each element of the existing k times in the new list. 

For instance, If we have a list myList=[1,2,3,4,5] and we have to repeat the elements of the list two times, the output list will become [1,1,2,2,3,3,4,4,5, 5].

To perform this operation, we can use the for loop or list comprehension with the append() method. Alternatively, we can use itertools.chain.from_iterable() and itertools.repeat() method. We will discuss each of these ways in the upcoming sections.

Repeat Each Element k Times in a List Using For Loop and append() Method

The append() method is used to append elements to the end of a list. When invoked on a list, the append() method takes an element and adds it to the list as shown below.

myList = [1, 2, 3, 4, 5]
print("The original list is:", myList)
element = 6
myList.append(element)
print("List after appending {} is: {}".format(element, myList))

Output:

The original list is: [1, 2, 3, 4, 5]
List after appending 6 is: [1, 2, 3, 4, 5, 6]

To repeat each element k times in a list, We will first create an empty list named newList. After that, we will traverse each element of the input list. During traversal, we will add each element of the existing list to the newly created list k times using a for loop, range() method, and the append() method. After execution of the for loop, each element of the input list will be repeated k times in the new list as shown in the following example.

myList = [1, 2, 3, 4, 5]
print("The original list is:", myList)
k = 2
newList = []
for element in myList:
    for i in range(k):
        newList.append(element)
print("The output list is:", newList)

Output:

The original list is: [1, 2, 3, 4, 5]
The output list is: [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]

Repeat Each Element k Times in a List Using List Comprehension

Instead of using for loop, we can use list comprehension to repeat each element k times in a list as follows.

myList = [1, 2, 3, 4, 5]
print("The original list is:", myList)
k = 2
newList = [element for element in myList for i in range(k)]
print("The output list is:", newList)

Output:

The original list is: [1, 2, 3, 4, 5]
The output list is: [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]

Using itertools.chain.from_iterable() and itertools.repeat() Method

Instead of explicitly adding elements to the new list to repeat them, we can use the itertools.chain.from_iterable() and itertools.repeat() method to repeat each element k times in a list.

The itertools.repeat() method takes a value and the number of times the value has to be repeated as input arguments and creates an iterator. For instance, we can create an iterator that repeats any given value five times as follows.

import itertools

element = 5
k = 10
newList = list(itertools.repeat(element, k))
print("The output list is:", newList)

Output:

The output list is: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

The itertools.chain.from_iterable() takes a list of iterable objects and creates an iterator using all the elements of the iterable objects passed as arguments as follows.

import itertools

myList1 = [1, 2, 3, 4, 5]
myList2 = [6, 7, 8, 9, 10]
newList = list(itertools.chain.from_iterable([myList1, myList2]))
print("The output list is:", newList)

Output:

The output list is: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

To repeat each element k times in a list, we will first create an list for each element of the existing list using the itertools.repeat() method and the list() constructor. Here, each iterator will have a distinct element k number of times. After that, we will create output list using the itertools.chain.from_iterable() method from the lists created using the itertools.repeat() method. In this way, we will get a list that will have all the elements repeated k times.

import itertools

myList = [1, 2, 3, 4, 5]
print("The original list is:", myList)
k = 2
listOfLists = [list(itertools.repeat(element, k)) for element in myList]
newList = list(itertools.chain.from_iterable(listOfLists))
print("The output list is:", newList)

Output:

The original list is: [1, 2, 3, 4, 5]
The output list is: [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]

Conclusion

In this article, we have discussed three ways to repeat each element k times in a list in python. To read more about lists in python, you can read this article on how to delete last element from a list in python.