C program to find reverse of a string

Write a C program to find reverse of a given string using loop. How to find reverse of any given string using loop in C programming. Logic to find reverse of a string without using strrev() function in C. C program to reverse a string using strrev() string function.

Reverse string: olleH

Required knowledge

Basic C programming, For loop, String

Must know – Program to find reverse of an array

Logic to find reverse of a string

There are numerous ways to find reverse of a string. Here in this lesson I am going to explain few of them. First let us see the easiest method to find reverse of a string. Below is the step by step descriptive logic to find reverse of a string.

1. Input a string from user, store it in some variable say str.
2. Declare another array that will store reverse of the string, say char reverse[SIZE].
3. Find length of the string and store it in some variable say len.
4. Initialize two variables that will keep track of original and reverse string. Here we will access original string from last and reverse array from first. Hence, initialize strIndex = len – 1 and revIndex = 0.
5. Run a loop from len – 1 to 0 in decremented style. The loop structure should look like while(strIndex >= 0).
6. Inside the loop copy current character from original string to reverse string. Say reverse[revIndex] = str[strIndex];.
7. After copying, increment revIndex and decrement strIndex.

Read more – Program to reverse order of words in a given string

Program to find reverse of a string

/**
 * C program to find reverse of a string
 */

#include <stdio.h>
#define MAX_SIZE 100 // Maximum string size

int main()
{
    char str[MAX_SIZE], reverse[MAX_SIZE];
    int i, strIndex, revIndex, len;

    /* Input string from user */
    printf("Enter any string: ");
    gets(str);

    /* Find length of string */
    i = 0;
    while(str[i] != '\0') i++;

    len = i;

    /* 
     * Store each character from end of original string 
     * to reverse string
     */
    revIndex = 0;
    strIndex = len - 1;
    while(strIndex >= 0)
    {
        reverse[revIndex] = str[strIndex];

        strIndex--;
        revIndex++;
    }
    reverse[revIndex] = '\0';

    printf("\nOriginal string = %s\n", str);
    printf("Reverse string = %s", reverse);

    return 0;
}

Once you got the above approach, you can easily transform the program in pointers context. Let us re-write the above program more efficiently using pointers.

Program to find reverse of a string using pointers

/**
 * C program to find reverse of a string using pointers
 */

#include <stdio.h>
#define MAX_SIZE 100 // Maximum string size

int main()
{
    char str[MAX_SIZE], reverse[MAX_SIZE];
    char *s = str;
    char *r = reverse;
    int len = 0;

    /* Input string from user */
    printf("Enter any string: ");
    gets(str);

    /* Find length of string */
    while(*(s++)) len++;

    /* 
     * Store each character from end of original string 
     * to reverse string
     */
    s--;
    while(len >= 0)
    {
        *(r++) = *(--s);
        len--;
    }
    *r = '\0';

    printf("\nOriginal string = %s\n", str);
    printf("Reverse string = %s", reverse);

    return 0;
}

The above program is little geeky. However, in real life it is recommended to use built-in library function strrev() to find reverse of any string. strrev() is a string library function defined under string.h header file.

Program to find reverse of a string using strrev() function

/**
 * C program to find reverse of a string using strrev() function
 */
#include <stdio.h>
#include <string.h>
#define MAX_SIZE 100 // Maximum string size

int main()
{
    char str[MAX_SIZE];

    /* Input string from user */
    printf("Enter any string: ");
    gets(str);

    printf("Original string = %s\n", str);

    /* Find the reverse of string */
    strrev(str);

    printf("Reverse string = %s", str);

    return 0;
}

Read more – Program to find reverse a number

Enter any string: Codeforwin

Original string = Codeforwin
Reverse string = niwrofedoC

Happy coding 😉