How do I raise a really big number to a really big power?

Question

I have a ulong value that I need to raise to a high power, but Math.Pow does not successfully produce a correct output.

My c# code:

 ulong a = 9123456789;
 ulong b = (ulong)Math.Pow(a, 9999);
 Console.WriteLine(b);

The output to screen is 0. How can I perform this calculation and obtain a correct result?

Do you seriously think that 9123456789^9999 will fit in a ulong? — Blorgbeard, Dec 08 '16 at 19:04
What could you possibly be doing with a number that big anyway? — Servy, Dec 08 '16 at 19:04
[it's nearly 100,000 decimal digits long](http://www.wolframalpha.com/input/?i=9123456789%5E9999&dataset=) — Blorgbeard, Dec 08 '16 at 19:05
Read the [MSDN for `Math.Pow`](https://msdn.microsoft.com/en-us/library/system.math.pow(v=vs.110).aspx), it tells you when it returns 0, such as when the number is larger than can fit in the data type. Some methods like to do things like throw exceptions for errors like this, this one just returns `0` instead. — Quantic, Dec 08 '16 at 19:05
I'm intellectually curious as to the problem that requires raising a value to the 9999th power. Care to share? My worldview needs expanding. — Anthony Pegram, Dec 08 '16 at 19:14

score 9 · Accepted Answer · answered Dec 08 '16 at 19:11

Yes, it is possible to compute that number, using the specialized BigInteger class. It's hopeless to try and do this with a ushort, which has a width of 16 bits.

using System;
using System.Collections;
using System.Numerics;

public class Test 
{
    public static void Main(string[] args)
    {
        BigInteger a = 9123456789;
        BigInteger b = BigInteger.Pow(a, 9999);

        //Output this number if you're feeling lucky.
        Console.WriteLine(b);
    }
}

Outputs

43056151396124937171542222696555900626431494491043369510338912076154406943108
..
8614367506747618876018379290109

By the way, you'd need 330,837 bits to store the result.

You need explicitly include the `System.Numerics.dll` in your project. Go in visual studio, right-click on "References" in side the project, go to "Assemblies", then search for `System.Numerics` and add a tick-mark next the name. That should include the dll and the code should compile. — Maximilian Gerhardt, Dec 08 '16 at 19:16

user6516640 · Answer 2 · 2016-12-08T19:11:30.770

0

Look up for BigInteger if you're going to work with very big numbers.

edited Dec 08 '16 at 19:11

answered Dec 08 '16 at 19:10

user6516640

13
2

@Great.And.Powerful.Oz Haha. Sorry about that, fixed it now. Thanks! – user6516640 Dec 08 '16 at 19:11

How do I raise a really big number to a really big power?

2 Answers2