C++ program to find the largest palindrome which is product of two two digit numbers

Question

I have seen this, but this is not what I am looking for.

The problem is same that is to find the largest palindrome which is the product of two three digit numbers.

Since my program was not working so I made a little change, instead of finding the largest palindrome which is the product of two three digit numbers I have written the program to find the largest palindrome which is the product of two two digit numbers.

Kindly see the program:

#include <iostream>
using namespace std;
int main() {
    int i, j, n, s, m, w;
    for (i = 99; i > 9; i--) {
        for (j = 99; j > 9; j--)
            n = i * j;
        s = n;
        while (n != 0) {
            w = 0;
            m = n % 10;
            w = w * 10 + m;
            n = n / 10;
        }
        if (s == w)
            cout << s << endl;
        break;
    }
    return 0;
}

The problem with this program is that it is neither showing any error nor giving any result.

So kindly help me to find the problem in my program.

1) use **meaningful names** for your variables. One-letter variables are confusing at best. 2) choose **one style of indentation**. Not two or three mixed up completely. If you do that, you would see that one of your loops might be missing curly braces around its block. 3) Instead of just dumpig your code for others to find the error, try to narrow it down and debug it. — Arne Mertz, Sep 08 '14 at 14:43

score 2 · Answer 1 · answered Sep 08 '14 at 14:42

Right now you are missing the curly braces for the j-loop. The current code is doing 99! * i.

Then you would have to focus on storing the largest palindrome value instead of just printing all those values to the screen (this is considering your implementation, it is not the most efficient one by any means).

NetVipeC · Accepted Answer · 2014-09-08T15:17:54.347

Some modified version of your code:

#include <iostream>
using namespace std;
int main() {
    int max_product = 0;
    for (int i = 99; i > 9; i--) {
        for (int j = i; j > 9; j--) {
            int product = i * j;
            if (product < max_product)
                break;
            int number = product;
            int reverse = 0;
            while (number != 0) {
                reverse = reverse * 10 + number % 10;
                number /= 10;
            }
            if (product == reverse && product > max_product) {
                max_product = product;
            }
        }
    }
    cout << "Solution: " << max_product << endl;
    return 0;
}

You have various problems:

Need one more pair of {, }. After the for-loop of j. The only instruction the for-loop of j is executing is: n = i * j; with the braces the rest of the instruction (testing if it's a palindrome) are out of the loop.
the variable w the reverse of the number to test for palindrome is reset his value to 0 in every execution of while (n != 0) loop resulting in incorrect reverse value (and never find the palindrome).
The max palindrome product of 2 two digits number don't have to be the first one found with this 2 for-loop, eg: suppose that there is 2 valid solutions i = 98, j = 2 and i = 70, j = 65 in this case i*j would be in first solution = 196 in the second = 4550 and when you found the first you could not stop the search. In your code using the break don't do what I think you are waiting for (stop the search), only stop the search with the actual i value.

Some notes about the modified code:

The two for-loop don't need to be from 99..9, in this case you are testing a lot of product two times (eg: 98*99, are testing when i == 98 and j == 99 and i == 99 and j == 98), you could restrict j to be always less or equal to i.
Using max_product to maintain the maximum palindrome product found. And use that info in the inner loop (if (product < max_product)) for early exit when no better solution could be found.

C++ program to find the largest palindrome which is product of two two digit numbers

2 Answers2