How to Uniquify Lists and Get Indices of Matching Elements in Python

Discover how to easily extract indices of duplicate elements in a list using Python's built-in features.
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How to Uniquify Lists and Get Indices of Matching Elements in Python

In the world of programming, working with lists is a common task. Sometimes, however, we find ourselves needing to solve a specific problem: we want to identify duplicate elements in a list and get the indices of their occurrences. For example, given a list like [a, b, c, a, a, b], we would like to extract a structure that indicates where each duplicate is located, such as [[0, 3, 4], [1, 5]] for this list.

In this guide, we will break down the solution to this problem step-by-step, showing you how to achieve this using Python's built-in features. Follow along to learn how to effectively handle duplicate indices in lists.

The Problem Statement

You have a list of items, and your goal is to find the indices of the elements that occur more than once. The list might look like this:

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The expected output for this list would be:

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Here, indices 0, 3, and 4 correspond to the letter 'a', while indices 1 and 5 correspond to the letter 'b'. Now let's explore how we can achieve this.

Solution: Using defaultdict from the collections Module

To tackle this problem efficiently, we'll make use of the defaultdict feature from Python's collections module. This allows us to create a dictionary that defaults to a list whenever a new key is encountered. Here’s a clear breakdown of our approach:

Step-by-Step Implementation

Import the Required Library: We will first import the defaultdict class from the collections module.

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Initialize the List: Define your list of items.

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Create the Default Dictionary: Initiate a defaultdict to store the indices of each unique element.

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Enumerate Through the List: Loop through the list using enumerate to get both the index and the corresponding value.

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Extract Matching Indices: Finally, filter this dictionary to get the lists of indices where the length is greater than 1 (indicating duplicates).

[[See Video to Reveal this Text or Code Snippet]]

Complete Code Example

Bringing it all together, the complete code looks as follows:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In this guide, we explored how to efficiently find the indices of duplicate elements in a list using Python. By leveraging the defaultdict from the collections module, we could easily gather and filter indices of matching items. This technique is quite powerful, especially when dealing with lists in data processing and analysis.

Now, you can apply this method to your own lists to quickly identify duplicate indices. Happy coding!