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A step-by-step guide on how to count unique values in a Python dictionary using keys, with examples and code snippets.
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How to Get Count by Key in Python Dictionary

Working with dictionaries in Python can sometimes be tricky, especially when it comes to counting unique values associated with different keys. In this guide, we will address a common problem: how to count unique values in a dictionary based on keys. We will take a look at a specific example and provide you with clear, step-by-step solutions to achieve your goal.

Understanding the Problem

Let's say you have the following Python dictionary:

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Here, we want to count the unique values in key B for each value in key A. This means that:

The value 'a' in A corresponds to '1' in B, resulting in one unique count.

The value 'b' in A has the value '2' (with duplicates), which again results in one unique count.

The value 'c' in A has values '1', '2', and '3' in B, giving us three unique counts.

The desired output should look like this:

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Step-by-Step Solution

Now, let’s break down the solution step by step. We will offer two methods for counting unique values: a basic method using a standard dictionary and a more advanced method using defaultdict from the collections module.

Method 1: Using Basic Dictionary

Initialize Your Lists and Mapping Dictionary

Begin by initializing your lists (A and B) and an empty mapping dictionary to store unique values.

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Zip Through the Lists

Use the zip function to iterate through both lists simultaneously. For each pair of values, add the value from B into a set corresponding to the key from A. This helps to keep only unique values.

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Count Unique Values

Create a new dictionary to count the length of the sets you've built, which represent unique values.

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Prepare Your Final Output

Finally, you can print the keys (values from A) and the corresponding counts of unique values from B:

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Method 2: Using defaultdict for Cleaner Code

For a more elegant solution, we can use the defaultdict from the collections module, which simplifies our previous steps.

Import defaultdict and Initialize Lists

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Zip and Add to defaultdict

Similar to the previous example, but this time we can skip the existence check thanks to defaultdict:

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Retrieve Unique Counts

Finally, you can extract the keys and the lengths of the sets with ease:

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Conclusion

In this guide, we tackled the problem of counting unique values in a Python dictionary based on corresponding keys. We presented two methods: using a standard dictionary and leveraging the convenience of defaultdict. Both approaches achieved the same result, so choose whichever suits your coding style better!

Feel free to experiment with the provided code snippets in your own projects and expand upon them. Happy coding!