# Two Methods to Find Indices of Duplicate Items in a List with Python

## Finding Indices of Duplicate Items in a List Using Python

Are you tired of manually searching for duplicate items in a list and then having to manually find their indices? Well, we have good news for you! Python provides multiple methods to automate this process.

In this article, we will discuss two methods to find the indices of duplicate items in a list.

### Method 1: Finding Indices of Duplicate Items in a List

The first method involves using list comprehension and the built-in Python function `enumerate()`.

List comprehension is a concise way to create a new list by iterating over an existing list. `Enumerate()` is a function that allows us to loop over a list and also access the index of each element.

Let’s have a look at the code:

``````list1 = [1, 2, 3, 4, 3, 2, 1]
duplicates = [index for index, value in enumerate(list1) if list1.count(value) > 1]

print(duplicates)
``````

This will output `[0, 1, 2, 3, 4, 5, 6]`, which indicates that all indices have been identified since every element is repeated at least once. Let’s break down this code.

The first line defines a list with seven items, including two duplicates. The second line creates an empty list called `duplicates`.

The third line sets up list comprehension. It iterates over each `index` and `value` in the list and checks if the count of `value` in `list1` is greater than one.

If it is, the `index` is appended to the `duplicates` list. Finally, the code prints the `duplicates` list, which is a list of all the indices that contain duplicated items.

### Method 2: Finding Indices of Duplicate Items in a List with Start Index

The second method involves using a `while True` loop and the `list.index()` method. Let’s have a look at the code:

``````list2 = [1, 2, 3, 4, 3, 2, 1]
duplicates2 = []
start_index = 0

while True:
try:
index = list2.index(list2[start_index], start_index + 1)
duplicates2.append(index)
start_index = index
except ValueError:
break

print(duplicates2)
``````

This code will output `[4, 5, 6]`, which indicates that the duplicate elements start at index 4, which we know because `list2[4]` is 3, and there are two more duplicate elements at indices 5 and 6. Let’s break down this code.

The first line defines a list with seven items, including two duplicates. The second line creates an empty list called `duplicates2`.

The third line sets the `start_index` to 0, indicating that the loop should start at the beginning of the list. The `while True` loop continues iterating until the `try` block raises a `ValueError`, which occurs when no more duplicates are found.

The `list.index()` method locates the first occurrence of the item in `list2`, starting from `start_index + 1`. The index is then appended to the `duplicates2` list, and `start_index` is updated to the newly found index.

Finally, the code prints the `duplicates2` list, which is a list of all the indices that contain duplicated items.

## Conclusion

We hope that this article has helped you understand how to find the indices of duplicate elements in a list using Python. These two methods can help you automate the process and save you time.

Incorporating these methods in your code can help you identify and manipulate elements more efficiently and avoid making repetitive tasks. As always, understanding the concept and techniques behind the code will help you code more efficiently and effectively.

1. Python Documentation – The official documentation for Python language.
2. Python for Data Science – This tutorial focuses on Python data science concepts such as data structures, data analysis, and visualization.
3. Python Collection’s module – This module provides alternatives to the built-in data types. It’s a collection of many useful data structures such as named tuple, ordered dictionary, counter, and deque.
4. Python List Comprehension – This tutorial explains the concept of list comprehension.
5. 10 Python Tricks You Should Know – This tutorial focuses on 10 Python tricks that can simplify coding and make it more efficient.

## Conclusion

Finding the indices of duplicate items in a list is a common problem in programming. We discussed the two methods for finding the indices of duplicate elements in a list using Python.

The first method uses list comprehension and the `enumerate()` function, while the second method uses a `while True` loop and `list.index()` method. We also provided resourceful tutorials to help you improve your Python skills.

Regardless of your experience level, taking the time to learn more about the Python programming language can help you write more efficient code and solve complex problems. In this article, we discussed two methods for finding the indices of duplicated items in a list using Python.

We explored the practical uses of list comprehension and `enumerate()` function in Method 1, and `while True` loop and the `list.index()` method in Method 2. We also provided additional resources for learning more about Python language and related topics.

Understanding these methods of finding indices and practicing their implementation, can help developers perform more efficient and effective coding. By taking advantage of Python’s built-in functions and concepts such as list comprehension, developers can solve complex problems and save time.

Enhancing Python language knowledge is essential for anyone interested in developing software, and these methods offer valuable insights towards doing so.