Palindrome Program in C | Program to Check the Number is Palindrome or not

Here in this program, we will code to check whether an entered number is a palindrome number or not. let's discuss the Palindrome Program in C.

Palindrome Program in C

What is a Palindrome number?

A palindrome number is a special number because it remains the same when its digit gets reversed. For example, 16461 is a palindrome number if you reverse this number still, we get 16461. By default, all the 1-digit numbers are palindrome numbers.

There are various syntaxes and algorithms to find a palindrome in C, and in this tutorial, we have discussed some of those.
Method 1: Using while loop and basic mathematical operations
Method 2: Using Recursion
Method 3: Using string and for loop
Method 4: Using string and halfway for loop

Method 1: Palindrome program in C with while loop and Basic Math

Statements we will use in this program

• While loop
• % (arithmetic modules)
• If…else statement

Logic

• First, we will ask the user to enter a number

• Then using a while loop we will try to reverse the entered number using % and other athematic operation

• At last, we will compare the reversed number with the entered number

• If the number match, we will print the number is a palindrome if not we print its not a palindrome number.

Palindrome Program in C

#include<stdio.h>
int main()
{
int num,x, y=0,rev;
printf("Enter a Number: ");
scanf("%d",&num);
rev= num;
while(rev!=0)
  {
   x= rev%10;
   y=x+(y*10);
   rev=rev/10;
   }

if(num==y)
                printf("%d is a palindrome number",num);
else
                printf("%d is not a palindrome number",num);
}

Output:

Enter a Number: 14641
14641 is a palindrome number

Analysis
The time complexity of this approach is O(N) and space complexity is O(1).

Method 2: Palindrome program in C using recursion function

Statements we will use in this program

• Recursion

• % (arithmetic modules)

• If…else statement

Logic

• Ask the user to enter a number

• Set the initial value of the reverse number to 0.

• Call the function that reverses the number entered by the user.

• In the function, set the base condition if the value of the number is 0, return the reverse.

• If not, calculate the remainder of the number by dividing it by 10, this will give us the last digit of the number and pop out the last digit.

• Multiply the current value of the reverse number and add the reminder to it.

• Then Call the function recursivly, by passing the value of the number divided by 10 and the current reverse number.

Program in C

#include<stdio.h> 

// Recursive function to get a reverse number
int reverseNum(int num, int rev){
    if(num == 0)
        return rev;
    
    int remainder = num % 10;
    rev = rev * 10 + remainder;
    
    // call the function recursively
    return reverseNum(num / 10, rev);
}

int main ()
{
    int num, rev = 0;
    
    printf("Enter a Number: ");
    scanf("%d",&num);
    
    // get the reverse of number
    rev = reverseNum(num, rev);
    
    // palindrome if num and reverse are equal
    if (num == rev)
        printf("%d is a palindrome number",num);
    else
        printf("%d is not a palindrome number",num);
}

Output

Enter a Number: 1234321
1234321 is a palindrome number

Analysis

The time complexity of this approach is the same as the while loop, which is O(N) and the space complexity is O(1).
But it is a recursive function, and by calling the function N times, it brings some additional space complexity of O(N), which means it is less efficient than the while loop approach.

Method 3: Palindrome program in C using String for loop

Statements we will use in this program

• C String
• For loop
• If statement

Logic

• Accept the number from the user as a string.

• Set two pointers, one point to the index 0 and the second point to the last index of the string.

• Create a for loop that starts from 0 upto the last index of the string.

• Inside the for loop check, if the pointer value of the first is not equal to the last pointer.

• If they are not equal, set the flag to 1, print the number is not a palindrome and break out of the loop.

Program in C

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

int main ()
{
   char num[20];
   int len, ptr1, ptr2, flag=0;
   
   printf("Enter the number: ");
   //get the number as a string
   gets(num);
   
   //length of the number
   len = strlen(num);
   
   if(len<=1)
   {
       printf("%s is a palindrome", num);
   }
   else{
       ptr2 = len-1;
       for(ptr1= 0; ptr1<len; ptr1++)
       {
           if( num[ptr1] != num[ptr2])
           {
               printf("%s is not palindrome", num);
               flag=1;
               break;
           }
           ptr2--;
       }
       if(flag==0)
       {
           printf("%s is palindrome", num);
       }
       
   }
}

Output

Enter the number: 1235321
1235321 is palindrome

Analysis

The time and space complexity of this program is, O(N), and O(1)

Method 4: Palindrome program in C using String function half for loop

In the above program, we loop through the complete string, but to verify the palindrome, we can check if the first half equals the last half.

Statements we will use in this program.

• C String
• For loop
• If statement

Logic

• Accept the number from the user as a string.
• Set two pointers, one point to the index 0 and the second point to the last index of the string.
• Create a for loop that starts from 0 upto the half of the length of the string.
• Check if the value of the pointer matches or not.

Program in C

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

int main ()
{
   char num[20];
   int len, ptr1, ptr2, flag=0;
   
   printf("Enter the number: ");
   //get the number as a string
   gets(num);
   
   //length of the number
   len = strlen(num);
   
   if(len<=1)
   {
       printf("%s is a palindrome", num);
   }
   else{
       ptr2 = len-1;
       
       for(ptr1= 0; ptr1<=len/2; ptr1++)
       {
           if( num[ptr1] != num[ptr2])
           {
               printf("%s is not palindrome", num);
               flag=1;
               break;
           }
           ptr2--;
       }
       if(flag==0)
       {
           printf("%s is palindrome", num);
       }
       
   }
}

Output

Enter the number: 1221
1221 is palindrome

Conclusion

Checking a palindrome number in C is very easy. The only logic it tasks is reversing the number that we want to check for palindrome. And to reverse a number in C we can use the simple logic where we keep dividing the number by 10 until its quotient becomes 0, which can be achieved with this algorithm

while(rev!=0)  
{    
x= rev%10;  
y=x+(y*10);   
rev=rev/10;    
}

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