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Python | List consisting of all the alternate elements

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  • Difficulty Level : Basic
  • Last Updated : 24 Dec, 2018
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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.




# 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.




# 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.




# 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]

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