How to Efficiently Find the Nth Occurrence of a Substring in Python?

Locating the Nth Occurrence of a Substring in Python

Identifying the position of a specific substring occurrence within a larger string is a frequent programming task. For novice Python developers, achieving this efficiently and idiomatically can pose a challenge. This article aims to elucidate the various methods available for finding the nth occurrence of a substring, focusing on the most Pythonic approach.

Iterative Approach

A straightforward iterative solution involves searching for the substring multiple times. By default, Python's find() method locates the first occurrence only. To obtain the nth occurrence, we can successively search starting from the position immediately following the previous occurrence:

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

This method is straightforward and memory-efficient.

Overlapping Occurrences

If the nth overlapping occurrence is desired, the increment in the find() method should be adjusted:

<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>

Pythonic Considerations

Compared to solutions using regular expressions or string splitting, the iterative approach adheres to Python's design principles of simplicity, flatness, and readability:

• Simplicity: It avoids unnecessary complexity and maintains a clear logical flow.
• Flatness: The absence of nested loops or conditional statements enhances code clarity.
• Readability: The straightforward syntax and intuitive variable names facilitate easy comprehension.

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