Write a C program to check if a positive integer is palindrome or not.

• C Program for Palindrome Number: String Version
• C Program for Palindrome Number: Integer Version

Here, we develop two C programs (string and integer versions) to check if a positive integer is palindrome or not. An integer number is palindrome when the original number is equal to its reversed form. By literal definition, palindrome is a word, phrase, or sequence that reads the same backwards as forwards.

For example, 121 is a palindrome integer because number 121 and its reversed form are exactly the same. But, what if an integer is negative; suppose we are given -121 to check if this is palindrome. In order to check -121 for palindrome, if we first convert it into string which will be "-121" and compare it with its reversed form "121-" then check will fail and the number will not be said palindrome.

In another solution, if we don't convert the number into string and simply reverse it into a separate integer variable and compare original number with the reversed one then both numbers will be equal and the number will be said palindrome. This is the ambiguity in both solutions and to avoid confusion we drop negative numbers for palindrome check.

We will develop two C programs in order to check if a positive integer is palindrome. First solution converts the given integer into string and checks if the string is read the same backwards and forwards. Second solution reverses the given integer in a different integer variable and checks if the original and reversed number are equal. We will analyze both solutions of palindrome check and will look at their pros and cons.

C Program for Palindrome Number: String Version

Following piece of code (Program 1) first converts the given integer into string and makes a check for palindrome. We convert the number into string with the library function sprintf. You can write your own routine, if you like to do so.

 /* Program 1: C program to check whether given integer is palindrome number */
/* palindrome_string_ver.c */ 
#include <stdio.h>
#include <string.h>
#define NUM_LENGTH 20
 
//String version
int isPalindrome(int n)
{
  int palindrome = 1, length = 0, i = 0;
  char number[NUM_LENGTH];
 
  //not checking for negative integers
  if (n < 0)
  {
    return -1;
  }
 
  //single digit numbers are always palindrome
  if (n >= 0 && n < 10)
  {
    return 1;
  }
  //convert n into string and store it in char array number.
  sprintf(number, "%d", n);
 
  if(number != NULL)
  {
    //get string length i.e. number of digits
    length = strlen(number);
    for(i = 0; i < length/2; i++)
    {
      if(number[i] != number[length - 1 - i])
      {
        palindrome = 0;
        break;
      }
    }
  }
  return palindrome;
}
 
int main (void)
{
  int n;
  printf ("Enter an integer: ");
  scanf ("%d", &n);
 
  if (isPalindrome(n) == -1)
  {
    printf("%d is negative. Palindrome check is done for +ve numbers.\n", n);
  }
  else if (isPalindrome(n) == 1)
  {
    printf("%d is palindrome.\n", n);
  }
  else
  {
    printf("%d is not palindrome.\n", n);
  }
}
 
OUTPUT
======
[root@host ~/cprogs]$ ./palindrome_string_ver
Enter an integer: 343
343 is palindrome.
[root@host ~/cprogs]$ ./palindrome_string_ver
Enter an integer: -121
-121 is negative. Palindrome check is done for +ve numbers.
[root@host ~/cprogs]$ ./palindrome_string_ver
Enter an integer: 1
1 is palindrome.

Above solution for checking whether a positive integer is palindrome or not is not very efficient for two reasons. First, it requires auxiliary memory to store the converted string. Second it takes more time than the solution we are going to present next. Above solution has to first convert the number into string then it checks if the string is palindrome.

C Program for Palindrome Number: Integer Version

As we know, it is easy to get digits from right to left via the / and % operators. For example, digits in the number 123 from right to left are 3, 2, and 1. A reversed number 321 is constructed with these three digits. Let's check whether the reversed number is identical to the original one. If it is, the original number is a palindrome. Following C program demonstrates that.

 /* Program 2: C program to check whether given integer is palindrome number */
/* palindrome_int_ver.c */ 
#include <stdio.h>
#include <string.h>
int isPalindrome (int n)
{
  int temp = n, revN = 0;
 
  //not checking for negative integers
  if (n < 0)
  {
    return -1;
  }
 
  //single digit numbers are always palindrome
  if (n >= 0 && n < 10)
  {
    return 1;
  }
 
  while (temp != 0)
  {
    revN = revN * 10 + temp % 10;
    temp = temp / 10;
  }
 
  if (n == revN)
  {
    return 1;
  }
  else
  {
    return 0;
  }
}
 
int main (void)
{
  int n;
  printf ("Enter an integer: ");
  scanf ("%d", &n);
 
  if (isPalindrome(n) == -1)
  {
    printf("%d is negative. Palindrome check is done for +ve numbers.\n", n);
  }
  else if (isPalindrome(n) == 1)
  {
    printf("%d is palindrome.\n", n);
  }
  else
  {
    printf("%d is not palindrome.\n", n);
  }
}
 
OUTPUT
======
[root@host ~/cprogs]$ ./palindrome_int_ver
Enter an integer: 343
343 is palindrome.
[root@host ~/cprogs]$ ./palindrome_int
Enter an integer: -121
-121 is negative. Palindrome check is done for +ve numbers.
[root@host ~/cprogs]$ ./palindrome_int
Enter an integer: 1
1 is palindrome.

Above solution is faster and consumes less memory than the former one. But, it may fail at times. Reversing an integer may cause overflow depending upon the input. Suppose a 16-bit integer can hold biggest positive value 32767, if you reverse it then it would be 76723 that will cause overflow.

Last Word

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