Efficiently Finding Indices of Subarrays with Numpy.where() in Python

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Getting Indices of Subarrays in a 1D Numpy Array

If you work with time series data in Python, you may often deal with arrays and need to locate specific segments that meet certain criteria. One common task involves identifying the indices of subarrays within a larger 1D array that have a specific size. This guide is dedicated to helping you achieve that using numpy.

The Problem

Imagine you have a list of peaks (list_of_peaks) and a corresponding 1D array (data) containing the time series values. The challenge is to extract segments of the data array that are within an upstream and downstream range of ±100 units around each peak. After splitting the array based on these ranges, your goal is to find indices of resulting subarrays that are smaller than 200 elements.

The Solution Steps

To solve this issue, let's break the solution down into clear, manageable steps.

1. Define the Range

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

Here, you are creating a comprehensive list of points 100 units upstream and downstream from each peak.

2. Split the Data

Next, split the data using the defined ranges:

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

This step creates a list of subarrays from data, partitioned according to the defined peak ranges.

3. Get the Sizes of Each Subarray

Now, the trick is to get the size of each subarray where you want to apply the size condition. Instead of directly accessing .size on the split_data_test_w_extra list, you need to apply it to each subarray:

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

In this code snippet:

We use map() to apply a lambda function that retrieves the size of each subarray.

We then convert this to a NumPy array, enabling us to apply the condition for sizes less than or equal to 200.

4. An Alternative Method: List Comprehension

Alternatively, you can efficiently achieve the same result using list comprehension. This is often a more Pythonic approach:

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

This line iterates over enumerated subarrays, collecting indices where the size condition is met.

Conclusion

By following these organized steps, you now have a clearer roadmap for tackling similar challenges with arrays in your future Python projects. Happy coding!