How to Efficiently Extract Every Nth Element from a List in Python?

Extract Every Nth Element from a List in Python

When faced with the task of extracting every Nth element from a list, such as selecting every 10th item from a list of integers ranging from 0 to 1000, a straightforward approach may involve using a for loop to iterate through the list and check the modulus of each element's index by N. However, there exists a more efficient and elegant method that can accomplish the same result with a single line of code.

Using Python's slicing syntax, we can extract every Nth element by specifying a step size of N. In the example provided, we can obtain the desired list using the following code:

<code class="python">xs = list(range(165))
result = xs[0::10]</code>

This method is considerably faster than iteratively checking each element's index, as demonstrated by the following time comparison:

$ python -m timeit -s "xs = list(range(1000))" "[x for i, x in enumerate(xs) if i % 10 == 0]"
500 loops, best of 5: 476 usec per loop

$ python -m timeit -s "xs = list(range(1000))" "xs[0::10]"
100000 loops, best of 5: 3.32 usec per loop

By utilizing the slicing syntax, we can efficiently extract every Nth element from a list in Python, which is particularly beneficial for large lists where speed is a concern.

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