Question

I’ve been playing with some bigger values with Java, and I’m running across something I don’t understand. For some reason, Java seems to like giving me bum data (although, it’s far more likely I’m telling it to give me bum data)

Here’s a snippet, edited for clarity:

        System.out.println(
                "2 == " + (Math.pow(51, 13) % (77))
        );

Which, according to both Wolfram Alpha (See link below), and the rest of my algorithm is wrong.

(Output:)

  2 == 70.0

http://www.wolframalpha.com/input/?i=51^13+mod+77

Any ideas?

Answer 1

I believe it’s because of a precision issue. doubles are only precise up to 15 or so digits. Math.pow(51,13) is a huge number (~20 digits) so when you try modding it by 77, you’re going to have numerical errors.

For arbitrary precision arithmetic, take a look at BigInteger and BigDecimal.