How to reverse a number or string in C Programming

We, the C programmers, sometimes face a problem that we are to reverse a number or a string. On many online judges there are some problems like number reversion. On these cases, what will we do? We may consider the following code to reverse a number:

#include <stdio.h>
int main()
{
 int i, x = 12345, rev[5];
 for(i = 0; i < 5; i++){
 rev[i] = x % 10;
 x = x/10;
 printf("%d", rev[i]);
 }
 return 0;
}

Output:

54321

But this program can reverse only 12345. We can make it more efficient in this way:

#include <stdio.h>

int main()
{
 int i, x, rev[30];
 scanf("%d", &x);
 for(i = 0; x != 0 ; i++){
 rev[i] = x % 10;
 x = x/10;
 printf("%d", rev[i]);
 }
 return 0;
}

Input:234567Output:765432

But it’s a long process too. We can write the program as follows to make it more efficient:

#include <stdio.h>

int main()
{
 int x, rev = 0;
 scanf("%d",&x);
 while (x != 0)
 {
 rev = rev * 10 + x % 10;
 x = x/10;
 }
 printf("%d\n", rev);
 return 0;
}

Input:456789Output:987654

Here we have seen three programs which can be used to reverse any number. You may write more efficient programs. Please do that by yourself. Now we will write a program to reverse a string. Please consider the following program:

#include<stdio.h>

int main()
{
 char temp, str[100]= "Bangladesh";
 int i=0,j=9;
 while(i<j){
 temp=str[i];
 str[i]=str[j];
 str[j]=temp;
 i++;
 j--;
 }
 puts(str);
 return 0;
}

Output:hsedalgnab

But it’s not efficient program as only a fixed string “Bangladesh” can be reversed. We can make it efficient as follows:

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

int main()
{
    char str[100],temp;
    int i=0,j;
    gets(str);
    j=strlen(str)-1;
    while(i<j){
        temp=str[i];
        str[i]=str[j];
        str[j]=temp;
        i++;
        j--;
    }
    puts(str);
    return 0;
}

Input:dhakaOutput:akahd

But we can make our program more efficient using strrev() function defined in header file. If you aren’t familiar to this function, you can read this article - use of strrev() function in C programming. By using it, we can get one of the most efficient program of reversing any number or string. Please consider the following program:

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

int main()
{
    char str[100];
    gets(str);
    puts(strrev(str));
    return 0;
}

Input:coderOutput:redoc

By using it you can reverse any number or function. Hope you have understood the reversion of a number or string in C. Happy Coding!!!