How Can I Find the nth Occurrence of a Substring in Python?

Efficiently Locating nth Occurrence of Substring in Python

Finding the index of a substring's nth occurrence in Python poses a seemingly simple task for coding enthusiasts. To achieve this in the most Pythonic fashion, let's explore various approaches.

Iterative Method for Non-overlapping Occurrences

One straightforward and efficient approach is to iteratively search for occurrences until the nth one is found:

<code class="python">def find_nth(haystack, needle, n):
    start = haystack.find(needle)
    while start >= 0 and n > 1:
        start = haystack.find(needle, start + len(needle))
        n -= 1
    return start</code>

This function iterates through the haystack and increments the start index by the length of the needle until the nth occurrence is located.

Iterative Method for Overlapping Occurrences

If overlapping occurrences need to be considered, the iterative approach can be modified:

<code class="python">def find_nth_overlapping(haystack, needle, n):
    start = haystack.find(needle)
    while start >= 0 and n > 1:
        start = haystack.find(needle, start + 1)
        n -= 1
    return start</code>

This function increments the start index by 1 instead of the needle's length, allowing it to search for overlapping occurrences.

Both methods adhere to Python's principles of simplicity, flatness, and readability, making them suitable for a Pythonic solution to this problem.

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