Скачать или смотреть How to Sort an ArrayList in Kotlin Based on Another ArrayList Values

Learn how to efficiently sort an `ArrayList` in Kotlin based on values from another `ArrayList`, shifting matching elements to the front with clear explanations and code examples.
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This video is based on the question https://stackoverflow.com/q/73358471/ asked by the user 'Aris Paskalov' ( https://stackoverflow.com/u/17700577/ ) and on the answer https://stackoverflow.com/a/73358667/ provided by the user 'Jolan DAUMAS' ( https://stackoverflow.com/u/13115358/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Sort an ArrayList in Kotlin Based on Another ArrayList Values

Sorting lists can often become tricky, especially when one list's content determines the order of items in another list. In this guide, we will tackle a common problem faced by Kotlin developers: How to sort an ArrayList based on values from another ArrayList.

We will walk through a practical example, analyzing the issue with existing code and providing a clean solution.

The Problem

Imagine you have two ArrayLists in Kotlin:

A main list of food items (foodList).

A list of food items that have been added (addedFoodList).

You want to sort foodList so that any food item that appears in addedFoodList is prioritized and moved to the front of foodList. Here’s the initial structure:

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

The goal is to have the foodList sorted such that the item Avocado Oil appears first, like this:

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

The Initial Sorting Method

You attempted a sortedBy method to achieve this, as shown below:

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

However, this resulted in an incorrect order:

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

Understanding the Issue

Two Cases to Examine

Food Not Found:
When a food item is not found in addedFoodList, the indexOf method returns -1.

Food Found:
If the food is found, indexOf returns the item's index within addedFoodList.

How sortedBy Works

The sortedBy function expects a comparator that determines the order based on an integer. When an item is missing and returns -1, it’s treated as a higher priority than any found item (index 0 or more). Thus, the result is counterintuitive.

The Solution

To sort the foodList correctly, we need to adjust how we determine the order based on the presence of items in addedFoodList.

Quick Fix

Here’s a corrected version of the getSortedFoodList function:

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

How It Works

We check if the food item is contained in addedFoodList using any().

If it is found, we return -1 to prioritize those items; otherwise, we return 0.

This simple adjustment ensures that the foods in addedFoodList are placed first, while the rest follow.

Expected Output

Now, when you run the above sorting method, you should get the expected result:

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

Conclusion

Sorting an ArrayList based on another ArrayList is straightforward once you understand the index behavior in Kotlin's sorting functions. With the provided solution, you can effectively manage the order of items as needed.

Feel free to reach out with questions or share your experiences with Kotlin sorting challenges! Happy coding!