How to Find the Index of the First Occurrence in a String?

When working with strings in programming, one common task is to determine the index of the first occurrence of a specific substring. This problem often comes up during technical interviews, as it tests your understanding of string manipulation and algorithmic thinking. In this article, we will explore various methods to achieve this in Python, while keeping the explanation clear and straightforward.

Using the find() Method

One of the simplest ways to find the index of the first occurrence of a substring in a string is by using the built-in find() method. This method returns the lowest index at which the substring is found within the string. If the substring is not found, it returns -1.

Here is how you can use it:

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In the above code, the find_first_occurrence function takes a string and a substring as inputs and returns the index of the first occurrence of the substring. The result will be printed out as expected.

Using the index() Method

Another method to locate the index of a substring is to use the index() method. This is similar to find(), but if the substring is not found, it raises a ValueError instead of returning -1.

Here’s an example:

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With this approach, if the search term does not exist in the text, the function will catch the exception and return -1, maintaining a clean interface.

Manual Search with a Loop

If you want to implement the functionality without using any built-in methods, you can do so with a simple loop. This approach gives you a better understanding of how string indexing works at a lower level.

Here's how this can be done:

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In this example, the function find_first_occurrence_manual iterates through the string and compares slices of the string with the substring. When it finds a match, it returns the current index. If there are no matches, it returns -1.

Finding the index of the first occurrence of a substring in a string is a fundamental task that can be accomplished using various methods in Python. Whether you use built-in functions like find() or index(), or implement your own search algorithm, understanding these concepts will significantly improve your string manipulation skills and prepare you for technical interviews.