Nested while loop not working in C - c

Question

I'm trying to succeed with project euler's 4th problem : What is the biggest palindrome-number you can make by multiplying 2 3-digit numbers ?
( https://projecteuler.net/problem=4 )
I have the following code but it is not working. For some reason it seems like the nested while loop is broken. It only print numbers 10000 to 99900, which means the nested while loop is executed only once...
#include <stdio.h>
#include <string.h>
int is_a_palindrome(int test_number);
int main (void)
{
long result;
long number_x_1;
long number_x_2;
long biggest_pal;
number_x_1 = 100;
number_x_2 = 100;
while (number_x_1 < 1000)
{
while (number_x_2 < 1000)
{
result = number_x_1 * number_x_2;
printf("%li\n", result);
if (is_a_palindrome(result) == 1)
{
biggest_pal = result;
printf("palindrome found : %li", biggest_pal);
}
number_x_2++;
}
number_x_1++;
}
return (biggest_pal);
}
int is_a_palindrome(int test_number)
{
int test_number_unchanged;
int reverse;
reverse = 0;
test_number_unchanged = test_number;
while (test_number != 0)
{
reverse = reverse * 10;
reverse = reverse + test_number % 10;
test_number = test_number / 10;
}
if (test_number_unchanged == reverse)
{
return (1);
}
else
{
return (0);
}
}

Answer 1

You don't set number_x_2 to 100 inside the outer loop... you do it only once outside all loops !
Thus when the 2nd loop on number_x_1 starts, number_x_2 is already at the maximum value and the inside loop (at while (number_x_2 < 1000)) does not start.

Answer 2

Check that simple mistake.. you must initialize number_x_2 = 100 in outer loop..
#include <stdio.h>
#include <string.h>
int is_a_palindrome(int test_number);
int main (void)
{
long result;
long number_x_1;
long number_x_2;
long biggest_pal;
number_x_1 = 100;
while (number_x_1 < 1000)
{
number_x_2 = 100;
while (number_x_2 < 1000)
{
result = number_x_1 * number_x_2;
printf("%li\n", result);
if (is_a_palindrome(result) == 1)
{
biggest_pal = result;
printf("palindrome found : %li", biggest_pal);
}
number_x_2++;
}
number_x_1++;
}
return (biggest_pal);
}
int is_a_palindrome(int test_number)
{
int test_number_unchanged;
int reverse;
reverse = 0;
test_number_unchanged = test_number;
while (test_number != 0)
{
reverse = reverse * 10;
reverse = reverse + test_number % 10;
test_number = test_number / 10;
}
if (test_number_unchanged == reverse)
{
return (1);
}
else
{
return (0);
}
}

Answer 3

The inner while loop sends number_x_2 to 1000, but then it stays at 1000 so the program never runs through the inner loop after the first cycle. The program then increments number_x_1 until it reaches 1000, and the program ends.
To solve this, move the statement number_x_2 = 100; To the line before while(number_x_2 < 1000).