How to Find the Nth Occurrence of a String in Python?

Finding the Nth Occurrence of a String in Python

Identifying the index of the n'th occurrence of a substring within a string is a common task in programming. However, Python's built-in find method doesn't provide a straightforward way to specify the occurrence number.

Solution:

Instead of relying on the find method directly, here's a Pythonic solution that iteratively locates the substring and increments the occurrence count until it reaches the desired value:

<code class="python">def find_nth(haystack: str, needle: str, n: int) -> int:
    """Find the index of the nth occurrence of needle in haystack."""
    start = haystack.find(needle)
    while start >= 0 and n > 1:
        start = haystack.find(needle, start+len(needle))
        n -= 1
    return start</code>

Usage:

To find the index of the second occurrence of the substring "foofoo" in the string "foofoofoofoo", you would call:

<code class="python">find_nth("foofoofoofoo", "foofoo", 2)</code>

This would return the index 6, corresponding to the starting position of the second "foofoo" substring.

Considerations for Overlapping Occurrences:

If you need to find the index of the n'th overlapping occurrence of the substring, you can modify the find_nth function to increment the starting position by 1 instead of the length of the needle:

<code class="python">def find_nth_overlapping(haystack, needle, n):
    """Find the index of the nth overlapping occurrence of needle in haystack."""
    start = haystack.find(needle)
    while start >= 0 and n > 1:
        start = haystack.find(needle, start+1)
        n -= 1
    return start</code>

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