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Python – Negative index of Element in List

  • Difficulty Level : Medium
  • Last Updated : 20 Oct, 2020

Given a list of elements, find its negative index in the List.

Input : test_list = [5, 7, 8, 2, 3, 5, 1], K = 2 
Output : -4 
Explanation : 2 is 4th element from rear.

Input : test_list = [5, 7, 8, 2, 3, 5, 1], K = 5 
Output : -2 
Explanation : 5 is 2nd element from rear. 

Method #1 : Using index() + len()

In this, we get the index of the element using index(), and then subtract it from the list length to get the required result.

Python3




# Python3 code to demonstrate working of
# Negative index of Element
# Using index() + len()
 
# initializing list
test_list = [5, 7, 8, 2, 3, 5, 1]
 
# printing original list
print("The original list is : " + str(test_list))
 
# initializing Element
K = 3
 
# getting length using len() and subtracting index from it
res = len(test_list) - test_list.index(K)
 
# printing result
print("The required Negative index : -" + str(res))

 
 Output:

The original list is : [5, 7, 8, 2, 3, 5, 1]
The required Negative index : -3

 

Method #2 : Using ~ operator + list slicing + index()

In this, we reverse the list using slicing, and use ~ operator to get negation, index() is used to get the desired negative index. 

Python3




# Python3 code to demonstrate working of
# Negative index of Element
# Using ~ operator + list slicing + index()
 
# initializing list
test_list = [5, 7, 8, 2, 3, 5, 1]
 
# printing original list
print("The original list is : " + str(test_list))
 
# initializing Element
K = 3
 
# -1 operator to reverse list, index() used to get index
res = ~test_list[::-1].index(K)
 
# printing result
print("The required Negative index : " + str(res))

 
Output:

The original list is : [5, 7, 8, 2, 3, 5, 1]
The required Negative index : -3


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