Скачать или смотреть How to Randomly Replace Values in a Vector in R

Learn how to efficiently replace values in a vector in R with random choices, ensuring varied outputs each time.
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This video is based on the question https://stackoverflow.com/q/65835591/ asked by the user 'Seydou GORO' ( https://stackoverflow.com/u/11652655/ ) and on the answer https://stackoverflow.com/a/65835809/ provided by the user 'ThomasIsCoding' ( https://stackoverflow.com/u/12158757/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Randomly Replace Values in a Vector in R

When working with data in R, there may come a time when you want to replace certain values in a vector with random values. This guide addresses a common problem faced by R users, particularly when trying to replace a specific value in a vector (in this case, 2) with either 0 or 1, based on a defined probability. The challenge lies in achieving varied replacements each time the code is executed rather than receiving a consistent output. Let’s dive into how to effectively solve this issue!

The Problem

You have a vector of a specific length filled primarily with the number 2, and your goal is to randomly replace these occurrences of 2 with either 0 or 1. You would like the replacement to occur with a probability of 0.4 for replacing 2 with 1. However, using your current code results in every 2 being replaced with the same outcome, either all 1s or all 0s. Here’s the sample vector you’re working with:

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When you run the following code:

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

You end up receiving only one unique random replacement, which is repeated for every matching 2 in the vector.

Sample Outputs

All replaced values being 0:

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

All replaced values being 1:

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Clearly, this isn't the desired result! So, let’s explore the solution.

Understanding the Issue

To understand why this happens, we need to look closely at how the ifelse function in R operates. The ifelse function is created to take logical conditions and replace values based on those conditions. Once a yes or no value is decided, it is repeated for the entire vector. As per the code inside the function:

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

This means that whether the output for the yes statement is 0 or 1, that single realization will be repeated for as many occurrences of 2 as there are in the original vector.

The Solution

The solution to this problem is to adjust your approach so that you generate a random vector with the desired number of replacements. Instead of calling rbinom(1, 1, 0.40), you should generate a vector of random values that correspond to the number of 2s in your vector. Here’s how you can do this:

Revised Code

Use the following code to achieve varied random replacements:

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

Explanation:

rbinom(sum(vec == 2), 1, 0.40): This part generates a random vector of values where the length corresponds to the amount of 2s in the original vector. It uses the binomial function to return either 0 or 1 based on the specified probability (here, 0.4 for 1).

ifelse: The ifelse function now correctly substitutes the generated random vector back into the original vector for the positions where 2 was found.

Example Output

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

Now, each execution will yield different combinations of 0 and 1 replacing 2!

Conclusion

By adjusting your approach to generating a vector of random replacements specifically tailored to the number of occurrences in your original vector, you will successfully create a varied output. No longer will you be stuck with the same 0 or 1 replacing each 2. Instead, you can enjoy the dynamic randomness that R is capable of providing!

Happy coding!