Скачать или смотреть How to Change the Value of a Key in a 2D Dictionary While Looping

Learn how to effectively change the value of a key in a 2D dictionary in Python while looping through its elements. Follow our guide and optimize your code for better results!
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This video is based on the question https://stackoverflow.com/q/73151531/ asked by the user 'Tea' ( https://stackoverflow.com/u/7554794/ ) and on the answer https://stackoverflow.com/a/73151974/ provided by the user 'thebjorn' ( https://stackoverflow.com/u/75103/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding How to Change Values in a 2D Dictionary in Python

When working with 2D dictionaries in Python, you might encounter scenarios where you want to update values while iterating through the dictionary. If you're facing unexpected outcomes in your code, you're not alone! Let's break down this common problem and how to solve it step by step.

The Problem

Consider a scenario where you initialized a 2D dictionary and tried to append incrementing values to a nested list. Here’s the original code you might have used:

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

Nested Loop to Update Values

You might have written a nested loop to change keys by appending values to lists as follows:

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

What Went Wrong?

The output you received was:

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

However, you expected each key to maintain its own list of values like this:

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

The issue arises because the second dictionary (dict2) is referencing the same list as dict1. When you append to one list, all references get updated, leading to unexpected behavior.

The Solution

To create distinct lists for each key in the nested dictionary, you need to re-initialize the list for each key in the loop itself. Here’s how you can do it correctly.

Solution Using Nested Loops

You can use a more structured approach by initializing dict2 while iterating:

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

Alternative Method: Using Custom Enumeration

You could also opt for a solution that combines the keys without nested loops:

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

Using itertools.product

For a cleaner and potentially more efficient solution, consider using itertools.product:

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

The One-liner Approach (Not Recommended)

As an advanced option, you could condense this into a one-liner, but for readability, it’s better to avoid this for complex structures:

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

Final Output

When you run the corrected code, you should get the expected output:

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

Conclusion

Understanding how to manage nested dictionaries in Python is crucial for avoiding unexpected results. By following the aforementioned solutions, you can ensure that each key in your 2D dictionary maintains its uniqueness. Whether using traditional nested loops or leveraging itertools, these techniques will help you manipulate dictionaries effectively. Happy coding!