Reverse a String using Pointers: C Program

Reversing a string is nothing but changing the order of characters where last character becomes first character and so on. There are various ways of reversing a string.

Example:
Original string : college.
Reversed string : egelloc

Two ways of reversing a string:

1. Reverse a string using pointers.( Without using library function)
2. Reverse a string using strrev() library function.

Description:
s1 – Original string.
s2 – Reversed string.
p1 – Pointer 1 which points to end of original string s1.
p2 – Pointer 2 which points to beginning of string s2.
‘\0’ – NULL character: which marks the end of string.
Input : String to reverse.
Output : Reverse of that string.

Algorithm : To Reverse a given string using Pointers

STEP 1: START
STEP 2: READ the original string which is to be reversed.
              READ s1
STEP 3: ASSIGN pointer 1 with address of last character of string s1
              p1 = address(last character of s1)  
STEP 4: ASSIGN pointer 2 with starting address of  string s2
              p2 = address( s2)  
STEP 5: LOOP until all the characters are stored from last to first
              p2 <- p1
              INCREMENT pointer p2
              p2 = p2 + 1
              DECREMENT pointer p1
              p1 = p1 – 1         
STEP 6: ASSIGN NULL character to the end of string s2
              p2 = NULL
 STEP 7: PRINT both original and reversed string
              PRINT s1, s2
STEP 8: END

Example Program 1: To Reverse a given string using Pointers in C

In the below program we are not using any library function to reverse the string.

#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
    char s1[100],s2[100];
    char *p1,*p2;
    printf("Enter a String:\n");
    gets(s1);
    p1=s1+strlen(s1)-1;
    p2=s2;
    while(p1>=s1)
    {
       *p2=*p1;
        p2++;
        p1--;
    }
    *p2='\0';
    printf("String before reverse: %s\n",s1);
    printf("String after reverse: %s",s2);
    getch();
}

Output:

Enter a String:
ICanTutorials
String before reverse: ICanTutorials
String after reverse: slairotuTnaCI

Example Program 2: To Reverse a string using strrev() library function in C

#include<stdio.h>  
#include<conio.h>
#include<string.h>  
int main()  
{  
    char s[100]; 
    printf ("Enter the string to be reversed: ");  
    gets(s); 
    printf ("After reversing the string: %s ", strrev(s));  
    getch();
    return 0;  
}  

Output:

Enter the string to be reversed: Learning
After reversing the string: gninraeL

QUIZ:

1] strrev() library function is present in header file.

1. math.h
2. string.h
3. stdio.h
4. conio.h

2] Pointers store memory addresses.

• TRUE
• FALSE