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Python | List consisting of all the alternate elements

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  • Difficulty Level : Basic
  • Last Updated : 30 Dec, 2022
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Some of the list operations are quite general and having shorthands without needing to formulate a multiline code is always required. Wanting to construct the list consisting of all the alternate elements of the original list is a problem that one developer faces in day-day applications. Let’s discuss certain ways to print all the alternate elements of the given list. 

Method #1 : Using list comprehension Shorthand to the naive method, list comprehension provides a faster way to perform this particular task. In this method, all the indices which are not multiple of 2, hence odd are inserted in the new list. 

Python3




# Python code to demonstrate
# to construct alternate element list
# using list comprehension
 
# initializing list
test_list = [1, 4, 6, 7, 9, 3, 5]
 
# printing original list
print("The original list : " + str(test_list))
 
# using list comprehension
# to construct alternate element list
res = [test_list[i] for i in range(len(test_list)) if i % 2 != 0]
 
# printing result
print("The alternate element list is : " + str(res))

Output

The original list : [1, 4, 6, 7, 9, 3, 5]
The alternate element list is : [4, 7, 3]

  Method #2 : Using enumerate() This is just a variation to the list comprehension method but does the similar internal working as list comprehension but uses different variables to keep track of index along with its value. 

Python3




# Python code to demonstrate
# to construct alternate element list
# using enumerate()
 
# initializing list
test_list = [1, 4, 6, 7, 9, 3, 5]
 
# printing original list
print("The original list : " + str(test_list))
 
# using enumerate()
# to construct alternate element list
res = [i for j, i in enumerate(test_list) if j % 2 != 0]
 
# printing result
print("The alternate element list is : " + str(res))

Output

The original list : [1, 4, 6, 7, 9, 3, 5]
The alternate element list is : [4, 7, 3]

   Method #3 : Using Slice notation This the most pythonic and elegant way to perform this particular task and enhances to the full power of python. We can use the skip utility provided by slice to get the alternate elements from start to end in the list. 
 

Python3




# Python code to demonstrate
# to construct alternate element list
# using Slice notation
 
# initializing list
test_list = [1, 4, 6, 7, 9, 3, 5]
 
# printing original list
print("The original list : " + str(test_list))
 
# using Slice notation
# to construct alternate element list
res = test_list[1::2]
 
# printing result
print("The alternate element list is : " + str(res))

Output

The original list : [1, 4, 6, 7, 9, 3, 5]
The alternate element list is : [4, 7, 3]

Method #4 : Using for loop

Python3




# Python code to demonstrate
# to construct alternate element list
 
# initializing list
test_list = [1, 4, 6, 7, 9, 3, 5]
 
# printing original list
print("The original list : " + str(test_list))
res = []
for i in range(1, len(test_list), 2):
    res.append(test_list[i])
 
# printing result
print("The alternate element list is : " + str(res))

Output

The original list : [1, 4, 6, 7, 9, 3, 5]
The alternate element list is : [4, 7, 3]

Method #5 : Using itertools

One other approach that you can use to get the alternate elements from a list is to use the itertools module. The itertools.islice() function allows you to slice an iterator by specifying a start, stop, and step. You can use this function to get the alternate elements from a list by specifying the start index as 1, the step as 2, and the stop index as the length of the list.

Here’s an example of how you can use itertools.islice() to get the alternate elements from a list:

Python3




import itertools
 
# Initialize the list
test_list = [1, 4, 6, 7, 9, 3, 5]
 
# Get the alternate elements using itertools.islice()
res = itertools.islice(test_list, 1, len(test_list), 2)
 
# Print the result
print(list(res))
 
#This code is contributed by Edula Vinay Kumar Reddy

Output

[4, 7, 3]

Time complexity: O(n)

Auxiliary Space: O(n)


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