C Program to Reverse a String Using Pointers

Introduction

Reversing a string involves changing the order of characters in the string so that the first character becomes the last, the second becomes the second-to-last, and so on. This guide will show you how to write a C program to reverse a string using pointers.

Example:

• Input: "Hello"
• Output: "olleH"

Problem Statement

Create a C program that:

• Takes a string as input from the user.
• Uses pointers to reverse the string in place.
• Displays the reversed string.

Solution Steps

1. Include the Standard Input-Output Library: Use #include <stdio.h> for standard input-output functions.
2. Declare a String and Pointer Variables: Declare a character array to store the string and pointers to help reverse the string.
3. Input the String: Use gets or fgets to take input from the user.
4. Reverse the String Using Pointers: Use pointers to swap characters from the beginning and end of the string, moving toward the center.
5. Display the Reversed String: Use printf to display the reversed string.

C Program to Reverse a String Using Pointers

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

int main() {
    // Step 2: Declare a string and pointer variables
    char str[100];
    char *start_ptr, *end_ptr, temp;

    // Step 3: Prompt the user to enter the string
    printf("Enter a string: ");
    gets(str);  // Use fgets(str, 100, stdin) if gets is deprecated

    // Initialize pointers to the start and end of the string
    start_ptr = str;
    end_ptr = str + strlen(str) - 1;

    // Step 4: Reverse the string using pointers
    while (start_ptr < end_ptr) {
        // Swap the characters at start_ptr and end_ptr
        temp = *start_ptr;
        *start_ptr = *end_ptr;
        *end_ptr = temp;

        // Move the pointers toward the center
        start_ptr++;
        end_ptr--;
    }

    // Step 5: Display the reversed string
    printf("Reversed string: %s\n", str);

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

Explanation

Step 2: Declare a String and Pointer Variables

• The character array str is declared to store the string entered by the user.
• The pointers start_ptr and end_ptr are declared to point to the beginning and the end of the string, respectively.
• The temporary variable temp is used to help swap the characters.

Step 3: Input the String

• The program prompts the user to enter a string using gets. Note that gets may be unsafe; for safer input, you can use fgets(str, 100, stdin).

Step 4: Reverse the String Using Pointers

• start_ptr is initialized to point to the first character of the string.
• end_ptr is initialized to point to the last character of the string (str + strlen(str) - 1).
• A while loop is used to reverse the string:
  • The loop continues until start_ptr is no longer less than end_ptr.
  • Inside the loop, the characters pointed to by start_ptr and end_ptr are swapped using the temporary variable temp.
  • The pointers start_ptr and end_ptr are then moved towards the center by incrementing and decrementing them, respectively.

Step 5: 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 Output:

Enter a string: Hello
Reversed string: olleH

Conclusion

This C program demonstrates how to reverse a string using pointers. It covers basic concepts such as pointer manipulation, swapping values, and string handling. This example is useful for beginners learning how to work with pointers and strings in C programming.