How to Efficiently Extract Elements from a List Based on Indices?

Accessing Multiple List Elements by Index

To extract specific elements from a list based on their indices, various approaches are available. Consider the example of extracting elements with indices 1, 2, and 5 from the list [-2, 1, 5, 3, 8, 5, 6].

Using a List Comprehension

As demonstrated in the question, a list comprehension can be used effectively:

a = [-2, 1, 5, 3, 8, 5, 6]
b = [1, 2, 5]
c = [a[i] for i in b]

This approach efficiently iterates over the index list b and extracts the corresponding elements from list a.

Using operator.itemgetter

Python's operator.itemgetter function provides a concise solution:

from operator import itemgetter
a = [-2, 1, 5, 3, 8, 5, 6]
b = [1, 2, 5]
print(itemgetter(*b)(a))

Using NumPy

For those using NumPy, it offers an alternative approach:

import numpy as np
a = np.array([-2, 1, 5, 3, 8, 5, 6])
b = [1, 2, 5]
print(list(a[b]))

Performance Considerations

While the choice depends on factors such as the size of the list and the frequency of its use, the provided list comprehension typically performs well for small lists. However, if the list is large or accessed frequently, itemgetter or NumPy might be preferable due to their improved performance.

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