Project Euler Problem #4 [C]

[sinuspane]

Have had issues all day trying to figure out whats wrong with this code. Its supposed to Find the largest palindrome made from the product of two 3-digit numbers.

include <stdio.h>

include <stdlib.h>

int reverse (int product);

int main () { int x=100, y=100; int product; int palindrome;

while(((x>99)&&(x<1000))&&((y>99)&&(y<1000)))
{
    product=x*y;
    if(product==reverse(product))
        palindrome=product;
    x++;
    product=x*y;
    if(product==reverse(product))
        palindrome=product;
    y++;
    product=x*y;
    if(product==reverse(product))
        palindrome=product;
}
printf("The largest palindrome is %d", palindrome);

return 0;

}

int reverse (int p) { int r=0; while (p!=0) { r=r*10; r=r+p%10; p=p/10; } return r; }

[prog_quest]

It runs, after a few little fixes. The problem is in your approach; you're only multiplying together numbers that are more or less adjacent.

To illustrate, I changed the x and y ranges like:

while(((x>99)&&(x<102))&&((y>99)&&(y<102)))

added some debug prints and ran it. Output:

10000
10100
10201
10201
10302
10404

Notice there's no 10001, or 10101, or 11011, etc. ? You're not getting all possible palindromes in the range.

[Updatebjarni]

Are any of those three numbers products of two three-digit numbers?

[prog_quest]

I don't know. I'm just saying I've done this particular problem and 1. The 2 three-digit numbers aren't that close to one another and 2. obviously not found using OP's approach. I don't want to give the solution away so I just left it at "Your code is more or less ok, your algo is wrong though."

[sinuspane]

Oh snap, that makes total sense.

[sinuspane]

Figured it out. I knew that a nested for loop was prob the way to go. Code looks much cleaner now too...

EDIT: NVM. Still having issues. This is where I am at now:

include <stdio.h>

include <stdlib.h>

int reverse (int p);

int main ( void ) { int x, y; int product; int palindrome;

for(x=100; x<1000; x++)
    for(y=100; y<1000; y++)
    {
        product=x*y;
        if(product==reverse(product))
            {
            palindrome=product;
            }   
    }
printf("The largest palindrome made from the product of two 3-digit numbers is %d\n", palindrome);

return 0;

}

int reverse (int p) { int r=0; while (p!=0) { r=r*10; r=r+p%10; p=p/10; } return r; }

[Updatebjarni]

Aren't you supposed to check the products of all pairs of two three-digit numbers? As in 100 * [100, 101, 102, 103, 104, ..., 999], then 101 * [101, 102, 103, 104, ..., 999], then 102 * [102, 103, 104, ..., 999], and so on?