How to Extract Every Nth Element from Lists Using Python Slicing

Nth Element Extraction from Lists Using Python

The task is to extract every subsequent Nth item from a given list. For instance, given a list of integers [0, 1, 2, 3, ..., 997, 998, 999], the goal is to obtain a new list containing the first and every subsequent 10th item, resulting in [0, 10, 20, 30, ..., 990].

While a for loop with modulus checking is a straightforward approach, a more concise and efficient solution exists using Python's slicing operator.

<code class="python">xs = list(range(1000))
new_list = xs[0::10]</code>

This line accomplishes the task in a single line of code. The syntax xs[0::10] is a slice expression where:

• 0 specifies the starting index (inclusive).
• ::10 is a step, which indicates to take every 10th element.

The resulting new_list contains the desired elements, [0, 10, 20, 30, ..., 990].

Performance Comparison

Benchmarking reveals that this slicing approach is significantly faster than the for loop solution:

$ python -m timeit -s "xs = list(range(1000))" "[x for i, x in enumerate(xs) if i % 10 == 0]"
517 loops, best of 5: 471.776 usec per loop

$ python -m timeit -s "xs = list(range(1000))" "xs[0::10]"
100000 loops, best of 5: 4.03 usec per loop

This demonstrates the efficiency and simplicity of the slicing approach for extracting every Nth item from a list in Python.

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