How to Count Substring Occurrences in C Using Recursive Functions

Learn how to create a recursive function in C that counts how many times the substring "hi" appears in a given string. This blog outlines the problem and provides a step-by-step solution.
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Counting Substrings with Recursive Functions in C

When working with strings in C, you might find yourself needing to count how many times a specific substring appears. For instance, in this blog, we will focus on how to count occurrences of the substring "hi" within input strings. If you've encountered an issue where your function returns incorrect results, this guide aims to provide clarity and solutions.

The Problem

The problem involves creating a recursive function that accepts a string and its length and returns the total count of the substring "hi". This can quickly become frustrating, especially if your function is responding with ordered sequences rather than the intended count of occurrences.

Input Requirements

First Input: The number of test cases, n.

Subsequent Inputs: n lines, each containing a string with a maximum length of 5000 characters, all in lowercase letters.

Example

For instance, let's say you have the following input:

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The expected output would be:

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The Solution

To solve this issue, we need to ensure our recursive function is accurately defined and properly iterates over the string to count the occurrences of "hi".

Step 1: Correcting the Loop

One common mistake often encountered is incorrect loop boundaries. Consider updating your for loops from this:

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To this:

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This adjustment ensures you don't access out-of-bounds memory.

Step 2: Function Definition

The function ocorrencias needs to be correctly declared. Instead of taking the length as an argument, simply use the string pointer:

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Step 3: Implementing the Recursive Logic

A simple and effective way to define the recursive function is as follows:

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Here, strstr efficiently searches for the substring. If found, it increments the count by 1 and continues searching from the next character after the found position.

Step 4: Update the Main Function

Next, you need to adjust the main function:

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Alternative: Counting Variants

If you also want to count occurrences of both "hi" and "hy", your recursive function can be modified like this:

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This update checks for both characters that follow 'h' and counts them accordingly.

Conclusion

By following the steps outlined above, you should now have a functioning recursive function that accurately counts the occurrences of the substring "hi" in any provided string. Remember to always validate your loop boundaries, correctly declare your functions, and implement robust logic to traverse through strings. Happy coding!