Скачать или смотреть How to Properly Use map2 in R for Nested Lists

A detailed guide on how to effectively implement nested lists using `map2` in R with real-world examples for better understanding.
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This video is based on the question https://stackoverflow.com/q/63771069/ asked by the user 'Arthur Carvalho Brito' ( https://stackoverflow.com/u/7812142/ ) and on the answer https://stackoverflow.com/a/63771246/ provided by the user 'Ronak Shah' ( https://stackoverflow.com/u/3962914/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem

If you've ever found yourself struggling with R's purrr package, particularly map2, you're not alone. Many developers, especially those new to functional programming in R, often wonder how to effectively manage nested lists. In this guide, we're going to explore how to create a function that employs map2 to handle a data frame split into groups, run specific tests on those groups, and return the expected results.

The Question

The central question is: Do I need to nest lists in this function that uses map2? To provide clarity, let’s define the task at hand using a specific example of a loop function that processes a data frame.

Problem Context

You want to write a function named loop, which:

Takes a data frame df

Group splits it by a column (in this case, UF)

Runs another function called test on each group

The test function is designed to return a list based on a row index and the data frame.

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

The Desired Output

When you run the loop function, the output you expect from your test cases would look like this:

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

Analyzing the Current Attempt

You’ve attempted to implement the function loop1, but it hasn’t produced the expected results. Here’s the code snippet:

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

The problem lies in the usage of .x within nested mappings, which confuses the context in which it operates.

The Solution

To fix the issue, you can simplify your use of map2 while ensuring clarity in scope. Here’s how to adjust your code:

Updated Code Snippet

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

Explanation of the Changes

Anonymous Functions: The modification involves using map(.x, test, df = .y) instead of nesting another map inside map2. It enhances readability and clarifies the scope.

Output Validation: When you run this function with the defined df, it returns the desired nested list structure effectively.

Sample Output

Running the revised code will yield an output similar to:

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

Conclusion

In summary, dealing with nested lists in R, especially when using tools like purrr's map2, can seem daunting at first. However, with a clear understanding of functional scopes and the utility of anonymous functions, you can effectively manage your data. Remember, clarity in your code will not only help you but also others who might read or use your scripts in the future.

So, the next time you're stuck with map2, try following these guidelines, and you'll be on your way to mastering nested lists in R!