C program to find reverse of a string

Reversing a string is a common task in programming, often used in text processing and data manipulation. In this article, we'll walk through two methods to find the reverse of a given string in C: using basic string manipulation techniques and using the strrev function.

Introduction

Reversing a string involves creating a new string that has the characters of the original string in reverse order. We'll explore both manual and library-based approaches to accomplish this task.

Method 1: Manual String Reversal

Algorithm

1. Initialize Variables:
   • An array to hold the original string.
   • Another array to hold the reversed string.
   • Variables to track the length of the string and loop indices.
2. Input the String:
   • Use standard input functions to read the string from the user.
3. Reverse the String:
   • Calculate the length of the original string.
   • Use a loop to copy characters from the original string to the reversed string in reverse order.
4. Print the Reversed String:
   • Output the reversed string to the console.

Write a C program to find reverse of a string

Here is the C program to find the reverse of a string manually:

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

int main() {
    char str[1000], rev[1000];
    int len, i, j;

    // Input string from user
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove the newline character at the end if present
    str[strcspn(str, "\n")] = '\0';

    // Calculate the length of the string
    len = strlen(str);

    // Reverse the string
    for (i = 0, j = len - 1; j >= 0; i++, j--) {
        rev[i] = str[j];
    }
    rev[len] = '\0'; // Null-terminate the reversed string

    // Print the reversed string
    printf("Reversed string: %s", rev);

    return 0;
}

We can also run a for loop using just single variable, just replace line 19-21 above to get same result

for (i = 0; j >= 0; i++) {
    rev[i] = str[len-i-1];
}

Output

Enter a string: ProCoding
Reversed string: gnidoCorP

Explanation

1. Include Headers:
   • We include the standard input-output header (stdio.h) for input and output functions.
   • The string.h header is included for string manipulation functions like strlen and strcspn.
2. Main Function:
   • We declare two character arrays: str to hold the original string and rev to hold the reversed string.
   • We also declare integer variables len to store the length of the string, and loop indices i and j.
3. Input the String:
   • Using fgets, we read the input string from the user.
   • We use strcspn to remove the newline character that fgets may include at the end of the string.
4. Calculate Length:
   • We use strlen to calculate the length of the input string.
5. Reverse the String:
   • We use a for loop to copy characters from the original string to the reversed string in reverse order. The loop runs from the end of the original string to the beginning.
   • We ensure the reversed string is null-terminated by adding '\0' at the end.
6. Print the Reversed String:
   • We use printf to output the reversed string to the console.

Reverse the string in place

In the previous program, we have used a separate array to store the reverse of array but if required we can modify the reverse of string in place.

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

int main() {
    char str[1000], temp;
    int len, i, j;

    // Input string from user
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove the newline character at the end if present
    str[strcspn(str, "\n")] = '\0';

    // Calculate the length of the string
    len = strlen(str);

    // Reverse the string
    for (i = 0, j = len - 1; i <= j; i++, j--) {
        temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }

    // Print the reversed string
    printf("Reversed string: %s", str);

    return 0;
}

We can also use while loop ti get same result just replace the line 19-23 from -

i = 0;
j = len-1;
while (i <= j) {
   temp = str[i];
   str[i] = str[j];
   str[j] = temp;
   i++;
   j--;
}

Method 2: Using strrev Function

The strrev function from string.h provides a convenient way to reverse a string. Note that strrev is not part of the C standard library, but it is available in some environments.

Write a C program to find reverse of a string using strrev Function

Here is the C program to find the reverse of a string using the strrev function:

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

int main() {
    char str[1000];

    // Input string from user
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove the newline character at the end if present
    str[strcspn(str, "\n")] = '\0';

    // Reverse the string using strrev
    strrev(str);

    // Print the reversed string
    printf("Reversed string: %s", str);

    return 0;
}

Explanation

1. Include Headers:
   • We include the standard input-output header (stdio.h) for input and output functions.
   • The string.h header is included for string manipulation functions like strlen and strcspn.
2. Main Function:
   • We declare a character array str to hold the original string.
   • Using fgets, we read the input string from the user.
   • We use strcspn to remove the newline character that fgets may include at the end of the string.
   • We call strrev to reverse the string in place.
   • We use printf to output the reversed string to the console.

Edge Cases

• Empty String:
  • If the input string is empty, the reversed string will also be empty.
• Single Character:
  • If the input string contains only one character, the reversed string will be the same as the input string.
• Special Characters:
  • Both programs correctly handle special characters and white space within the string.

Reverse string using pointers

If you want to learn how to reverse a string using pointers then check this: