How To Find The Second Occurrence Of A Substring In A Python String?

As a Python developer working on a text analysis project for a publishing company in Chicago, I came across a scenario where I needed to identify the second occurrence of specific keywords within articles. After researching and testing different methods, I found several effective solutions. In this tutorial, I will explain how to find the second occurrence of a substring in a Python string with examples.

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Find the Second Occurrence of a Substring in a Python String

Let’s say you have a string that represents a list of American cities, separated by commas:

cities = "New York, Los Angeles, Chicago, Houston, New York, Philadelphia"

Your task is to find the index of the second occurrence of “New York” in this string.

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Method 1: Use the find() Function

Python’s built-in find() function can locate the first occurrence of a substring within a string. To find the second occurrence, we can use find() twice.

Here’s how it works:

cities = "New York, Los Angeles, Chicago, Houston, New York, Philadelphia"
first_occurrence = cities.find("New York")
second_occurrence = cities.find("New York", first_occurrence + 1)

if second_occurrence != -1:
    print(f"The second occurrence of 'New York' is at index {second_occurrence}")
else:
    print("'New York' does not occur twice in the string")

Output:

The second occurrence of 'New York' is at index 41

I have executed the above example code and added the screenshot below.

Find the Second Occurrence of a Substring in a Python String

In this code:

We use find() to locate the first occurrence of “New York” and store its index in first_occurrence.
We call find() again, but this time we start searching from first_occurrence + 1. This ensures we find the next occurrence after the first one.
If second_occurrence is not -1 (which indicates the substring was not found), we print its index. Otherwise, we print a message indicating there is no second occurrence.

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Method 2: Use a Custom Function

We can also create a reusable function to find the nth occurrence of a substring. This is helpful if you need to find occurrences beyond just the second one.

def find_nth_occurrence(string, substring, n):
    occurrences = 0
    index = -1

    while occurrences < n:
        index = string.find(substring, index + 1)
        if index == -1:
            break
        occurrences += 1

    return index

cities = "New York, Los Angeles, Chicago, Houston, New York, Philadelphia"
second_new_york = find_nth_occurrence(cities, "New York", 2)

if second_new_york != -1:
    print(f"The second occurrence of 'New York' is at index {second_new_york}")
else:
    print("'New York' does not occur twice in the string")

Output:

The second occurrence of 'New York' is at index 41

I have executed the above example code and added the screenshot below.

How to Find the Second Occurrence of a Substring in a Python String

In this custom find_nth_occurrence() function:

We initialize occurrences to keep track of how many times we’ve found the substring, and index to store the current index.
We start a loop that continues until we’ve found the nth occurrence (or run out of occurrences).
Inside the loop, we use find() to locate the next occurrence, starting from the previous index + 1.
If find() returns -1, we break the loop since there are no more occurrences.
Otherwise, we increment occurrences and continue the loop.
Finally, we return the index of the nth occurrence (or -1 if not found).

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Handle Edge Cases

When searching for occurrences, it’s important to consider edge cases. For example:

What if the substring doesn’t occur at all?
What if the substring occurs only once?
What if the substring is empty?

Here’s an example that demonstrates handling these cases:

def find_second_occurrence(string, substring):
    if not substring:
        return "Substring cannot be empty"

    first_occurrence = string.find(substring)
    if first_occurrence == -1:
        return "Substring not found"

    second_occurrence = string.find(substring, first_occurrence + 1)
    if second_occurrence == -1:
        return "Substring occurs only once"

    return second_occurrence

print(find_second_occurrence("John Smith, Jane Doe, John Doe", "John"))
print(find_second_occurrence("John Smith, Jane Doe, John Doe", "Doe"))
print(find_second_occurrence("John Smith, Jane Doe, John Doe", "Mike")) 
print(find_second_occurrence("John Smith, Jane Doe, John Doe", ""))

Output:

22
27
Substring not found
Substring cannot be empty

I have executed the above example code and added the screenshot below.

Find the Second Occurrence of a Substring in a Python String handle case

By adding these checks, we ensure our function provides informative feedback for different scenarios.

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Conclusion

In this tutorial, I have explained how to find the second occurrence of a substring in a Python string. I covered some important methods like using the find() function and using the custom function. I also covered how to handle edge cases.