Reverse String using Pointer

0 June 15, 2021

In this example, We will learn a program to Reverse String using Pointer. Before that, You must have knowledge of the following topics.

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Algorithm

This is the algorithm of this program

Step 1: Start
Step 2: Declare string variables str and rev
Step 3: Declare pointer variables sptr and rptr
Step 4: *sptr = str and *rptr = rev
Step 5: Declare integer variables and assign -1
Step 6: Take input from user and store in str
Step 7: REPEAT this step until sptr is NULL
        Increase sptr by 1
        Increase i by 1
Step 8: REPEAT this step unitl i >= 0
        Decrease sptr by 1
        *rptr = *sptr
        Increase rptr by 1
        Decrease i by 1
Step 9: *rptr = \'\\0\' and rptr = rev
Step 10: REPEAT this step until rptr is NULL
        *sptr = *rptr;
        Increase sptr by 1
        Increase rptr by 1
Step 11: Print str
Step 12: End

Flowchart

This is the flowchart of this program

Flowchart : Reverse String using Pointer

CODE: Reverse String using Pointer

Now, We will see the code to reverse a string.

#include <stdio.h>

int main(){

    char str[100];
    char rev[100];
    char *sptr = str;
    char *rptr = rev;

    int i = -1;

    printf("\nEnter a string: ");
    scanf("%s", str);

    // storing the ending address of str in sptr
    while(*sptr)
    {
        sptr++;
        i++;
    }

    // storing the string str in rev in reverse order
    while(i >= 0)
    {
        sptr--; 
        *rptr = *sptr;
        rptr++;
        i--;
    }
    
    //string should always end with '\0'
    *rptr = '\0'; 
    rptr = rev;

    // storing the reverse string in the original string
    while(*rptr)
    {
        *sptr = *rptr;
        sptr++;
        rptr++;
    }

    // printing the reverse string
    printf("\nReverse of the string is: %s ", str);
    
    return 0;
}

The output of above program is

Enter a string: Hello

Reverse of the string is: olleH
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