How to get index of repeated elements in list Python

Python | Duplicate element indices in list

While working with Python list, sometimes, we require to check for duplicates and may also sometimes require to track their indices. This kind of application can occur in day-day programming. Let’s discuss certain ways in which this problem can be solved.

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Python | Duplicate element indices in list
Python: Get index of item in List
Find an index of duplicate elements in list Python Example
How do I get the index of a duplicated string in a list?
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Python List index()
Return Value
Example: To find the index of the given element.
Example: Using start and end in index()
Example: To test index() method with an element that is not present.
1. Using index() function
“is there any function who finds the duplicate and returns the index number of duplicate element in list in python” Code Answer’s
Python answers related to “is there any function who finds the duplicate and returns the index number of duplicate element in list in python”
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Method #1 : Using loop + set()
This task can be solved using the combination of above functions. In this, we just insert all the elements in set and then compare each element’s existence in actual list. If it’s the second occurrence or more, then index is added in result list.

# Python3 code to demonstrate working of 
# Duplicate element indices in list 
# Using set() + loop 

# initializing list 

test_list = [1, 4, 5, 5, 5, 9, 1]

# printing original list 

print("The original list is : " + str(test_list))

# Duplicate element indices in list 
# Using set() + loop 

oc_set = set()

res = []

for idx, val in enumerate(test_list):

if val not in oc_set:

oc_set.add(val)

else:

res.append(idx)

# printing result 

print("The list of duplicate elements is : " + str(res))

Output : The original list is : [1, 4, 5, 5, 5, 9, 1] The list of duplicate elements is : [3, 4, 6]

Method #2 : Using list comprehension + list slicing
This method is one liner alternative to perform this task. In this we just check for the non repeating element using list slicing and keep adding index in case it’s repeated.

# Python3 code to demonstrate working of

# Duplicate element indices in list

# Using list comprehension + list slicing

# initializing list

test_list = [1, 4, 5, 5, 5, 9, 1]

# printing original list

print("The original list is : " + str(test_list))

# Duplicate element indices in list

# Using list comprehension + list slicing

res = [idx for idx, val in enumerate(test_list) if val in test_list[:idx]]

# printing result

print("The list of duplicate elements is : " + str(res))

Output : The original list is : [1, 4, 5, 5, 5, 9, 1] The list of duplicate elements is : [3, 4, 6]

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Python: Get index of item in List

To find index of element in list in python, we are going to use a function list.index(),

list.index()

Python’s list data type provides this method to find the first index of a given element in list or a sub list i.e.

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list.index(x[, start[, end]])

Arguments :

x : Item to be searched in the list
start : If provided, search will start from this index. Default is 0.
end : If provided, search will end at this index. Default is the end of list.

Returns: A zero based index of first occurrence of given element in the list or range. If there is no such element then it raises a ValueError.

Important Point : list.index() returns the index in a 0 based manner i.e. first element in the list has index 0 and second element in index is 1.

Let’s use this function to find the indexes of a given item in the list,

Suppose we have a list of strings,

# List of strings list_of_elems = ['Hello', 'Ok', 'is', 'Ok', 'test', 'this', 'is', 'a', 'test', 'Ok']

Now let’s find the index of the first occurrence of item ‘Ok‘ in the list,

elem = 'Ok' # Find index position of first occurrence of 'Ok' in the list index_pos = list_of_elems.index(elem) print(f'First Index of element "{elem}" in the list : ', index_pos)

Output

First Index of element "Ok" in the list : 1

As in the list.index() we did not provided start & end arguments, so it searched for the ‘Ok‘ in the complete list. But returned the index position as soon as it encountered the first occurrence of ‘Ok‘ in the list.

But if searched item doesn’t exists in the list, then index() will raise ValueError. Therefore we need to be ready for this kind of scenario. For example,

list_of_elems = ['Hello', 'Ok', 'is', 'Ok', 'test', 'this', 'is', 'a', 'test', 'Ok'] elem = 'Why' try: index_pos = list_of_elems.index(elem) print(f'First Index of element "{elem}" in the list : ', index_pos) except ValueError as e: print(f'Element "{elem}" not found in the list: ', e)

Output

Element "Why" not found in the list: 'Why' is not in list

As ‘Why‘ was not present in the list, so list.index() raised ValueError.

Find an index of duplicate elements in list Python Example

Simple example code.

a = [1, 2, 3, 2, 1, 5, 6, 5, 5, 5] res = [idx for idx, item in enumerate(a) if item in a[:idx]] print(res)

Output:

Another example using collections

You have to import the collections module for this example.

import collections original = [1, 2, 3, 2, 1, 5, 6, 5, 5, 5] dup = [item for item, count in collections.Counter(original).items() if count > 1] for i in dup: print("index of", i, "=", original.index(i))

Output:

index of 1 = 0
index of 2 = 1
index of 5 = 5

How do I get the index of a duplicated string in a list?

Traversing through the length of the list so that we can keep a track of each index and print the index.

import collections original = ['A', 'B', 'A', 'B', 'C'] dup = [item for item, count in collections.Counter(original).items() if count > 1] for i in dup: print("index of", i, "=", original.index(i))

Output:

index of A = 0
index of B = 1

Do comment if you have any questions and doubts or suggestions on this Python list topic.

Note: IDE:PyCharm2021.3 (Community Edition)
Windows 10
Python 3.10.1
AllPython Examplesare inPython3, so Maybe its different from python 2 or upgraded versions.

Degree in Computer Science and Engineer: App Developer and has multiple Programming languages experience. Enthusiasm for technology & like learning technical.

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Python List index()

The list index() method helps you to find the first lowest index of the given element. If there are duplicate elements inside the list, the first index of the element is returned. This is the easiest and straightforward way to get the index.

Besides the built-in list index() method, you can also use other ways to get the index like looping through the list, using list comprehensions, enumerate(), filter methods.

The list index() method returns the first lowest index of the given element.

Syntax

list.index(element, start, end)

Parameters

Parameters	Description
element	The element that you want to get the index.
start	This parameter is optional. You can define the start: index to search for the element. If not given, the default value is 0.
end	This parameter is optional. You can specify the end index for the element to be searched. If not given, it is considered until the end of the list.

Return Value

The list index() method returns the index of the given element. If the element is not present in the list, the index() method will throw an error, for example, ValueError: ‘Element’ is not in the list.

Example: To find the index of the given element.

In the list my_list = [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’] , we would like to know the index for element C and F.

The example below shows how to get the index.

my_list = ['A', 'B', 'C', 'D', 'E', 'F'] print("The index of element C is ", my_list.index('C')) print("The index of element F is ", my_list.index('F'))

Output:

The index of element C is 2 The index of element F is 5

Example: Using start and end in index()

In this example will try to limit searching for index in a list using start and end index.

my_list = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J'] print("The index of element C is ", my_list.index('C', 1, 5)) print("The index of element F is ", my_list.index('F', 3, 7)) #using just the startindex print("The index of element D is ", my_list.index('D', 1))

Output:

The index of element C is 2 The index of element F is 5 The index of element D is 3

Example: To test index() method with an element that is not present.

When you try to search for index in the list for element that is not present ,you will get an error as shown below:

my_list = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J'] print("The index of element C is ", my_list.index('Z'))

Output:

Traceback (most recent call last): File "display.py", line 3, in <module> print("The index of element C is ", my_list.index('Z')) ValueError: 'Z' is not in list

1. Using index() function

A simple solution is to get iterate through the list with indices using list comprehension and check for another occurrence of each encountered element using the index() function. The time complexity of this solution would be quadratic, and the code does not handle repeated elements in output.

if __name__ == '__main__':

nums = [1, 5, 2, 1, 4, 5, 1]

dup = [x for i, x in enumerate(nums) if i != nums.index(x)]

print(dup)# [1, 5, 1]

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“is there any function who finds the duplicate and returns the index number of duplicate element in list in python” Code Answer’s

count the duplicates in a list in python

python by Repulsive Raven on Jan 30 2021 Comment

find duplicated entries present in a list

whatever by Troubled Thrush on Jul 09 2021 Comment

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