Thread: largest palindrome number (error in code)

Hey all..

Code:

/* A palindromic number reads the same both ways.  The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 x 99.
 *
 * Find the largest palindrome made from the product of two 3-digit numbers. */


#include <stdio.h>


int main (void)
{
	
	int reverse_num = 0, remainder, temp;
	int a = 100, b = 100; 
	int sum = 0;
	int ctr;
	int large;
	large = sum;


	for (a = 100; a < 999; a++) // create loop for var a from 100 to 999
	{
		for(b = 100; b < 999; b++) // create loop for var b from 100 to 999
		{
			do
			{
				sum = a * b;
				temp = sum;
				while (temp != 0) // loop to check digits in temp (separate the digits)
				{
					remainder = temp % 10;
					reverse_num = reverse_num * 10 + remainder;
					temp /= 10;
				}


				// printf("%d", reverse_num); // with this commented out this isn't printing the correct sum out in reverse order 
				++ctr;
				
				if (sum % sum == 0) // find largest palindrome number and store it in var large
				{
					if(sum > large)
					{
						large = sum;
					}
				}
			} while (ctr < 'C'); // infinite loop
		}
	}


	printf("%d is largest palindrome number.\n", large); 	// print largest palindrome number


	
	return 0;
}

Program output:

Code:

996004 is largest palindrome number.

With

Code:

	// printf("%d", reverse_num);

code comments removed, the number (sum) is not being calculated and displayed correctly in reverse and the algorithm is being copied correctly (the source code I used) i've found the modulus source code for the reverse number in the while loop on the internet a few places.

Also, is this nested for loop, correct? To try all calculations for a palindrome number. Thanks.

a = 100; a < 999; a++ ?
b = 100; b < 999; b++ ?

Problem 4 - Project Euler

Originally Posted by Salem

Create a utility function to do some work for you, and then make sure it works.

Code:

int reverseNum ( int num ) {
  // some calculation goes here
}

int main ( ) {
  int x = 12345;
  printf("Reverse of %d is %d\n", x, reverseNum(x) );
}

When you're happy that little bit of code works, then you can do this.

Code:

    for (a = 100; a < 999; a++) // create loop for var a from 100 to 999
    {
        for(b = 100; b < 999; b++) // create loop for var b from 100 to 999
        {
            sum = a * b;
            if ( sum == reverseNum(sum) )
            {
                // do something
            }

Code:

#include <stdio.h>


int reverseNum (int num)
{
	int reverse_num = 0, remainder, temp;


	while (num != 0)
	{
		remainder = num % 10;
		reverse_num = reverse_num * 10 + remainder;
		num /= 10;
	}


	return reverse_num;


}


int main (void)
{
	int a = 100;
	int b = 100;
	int sum = 0;
	int large;


	for(a = 100; a < 999; a++)
	{
		for(b = 100; b < 999; b++)
		{
			sum = a * b;
			if ( sum == reverseNum(sum) )
			{
				if (sum > large)
				{
					large = sum;
				}
			}
		}
	}


	printf("%d\tLargest palindrome match\n", large);	// do something


	return 0;
}

This code works and finds the correct answer