C Program to Reverse a String Using Recursion

Introduction

Reversing a string means rearranging the characters of the string such that the first character becomes the last, the second character becomes the second-to-last, and so on. This guide will show you how to write a C program to reverse a string using recursion.

Example:

• Input: "hello"
• Output: "olleh"

Problem Statement

Create a C program that:

• Takes a string as input from the user.
• Uses a recursive function to reverse the string.
• Displays the reversed string.

Solution Steps

1. Include the Standard Input-Output and String Libraries: Use #include <stdio.h> for standard input-output functions and #include <string.h> for string manipulation.
2. Write the Recursive Function: Define a recursive function that reverses the string by swapping characters.
3. Write the Main Function: Define the main function to take user input, call the recursive function, and display the reversed string.
4. Input the String: Use scanf or gets to take input from the user for the string.
5. Call the Recursive Function: Pass the string and its indices to the recursive function to reverse it.
6. Display the Reversed String: Use printf to display the reversed string.

C Program to Reverse a String Using Recursion

#include <stdio.h>
#include <string.h>

// Step 2: Define the recursive function to reverse the string
void reverseString(char str[], int start, int end) {
    if (start >= end) {
        return;  // Base case: When start index is greater than or equal to end index
    }
    
    // Swap the characters at start and end indices
    char temp = str[start];
    str[start] = str[end];
    str[end] = temp;
    
    // Recursively call the function with the next set of indices
    reverseString(str, start + 1, end - 1);
}

int main() {
    // Step 3: Declare a variable to hold the string
    char str[100];

    // Step 4: Prompt the user to enter the string
    printf("Enter a string: ");
    gets(str);  // Using gets to read the string including spaces

    // Step 5: Call the recursive function to reverse the string
    reverseString(str, 0, strlen(str) - 1);

    // Step 6: Display the reversed string
    printf("Reversed string: %s\n", str);

    return 0;  // Return 0 to indicate successful execution
}

Explanation

Step 2: Define the Recursive Function

• The reverseString function takes three parameters:
  • str[]: The string to be reversed.
  • start: The starting index for the current recursion step.
  • end: The ending index for the current recursion step.
• Base Case: If the start index is greater than or equal to the end index, the recursion stops, as the string has been fully reversed.
• Recursive Case:
  • Swap the characters at the start and end indices.
  • Call reverseString recursively with the start index incremented by 1 and the end index decremented by 1.

Step 3: Declare a Variable

• The variable str is declared to store the input string.

Step 4: Input the String

• The program prompts the user to enter a string using gets, which reads the entire line of input, including spaces.

Step 5: Call the Recursive Function

• The program calls the reverseString function with the string, the starting index 0, and the ending index strlen(str) - 1.

Step 6: Display the Reversed String

• The program uses printf to display the reversed string.

Return 0

• The return 0; statement indicates that the program executed successfully.

Output Example

Example 1:

Enter a string: hello
Reversed string: olleh

Example 2:

Enter a string: C programming
Reversed string: gnimmargorp C

Conclusion

This C program demonstrates how to reverse a string using a recursive function. It covers basic concepts such as recursion, string manipulation, and user input, making it a useful example for beginners learning C programming.