Calculate factorials in C#

Question

How can you calculate large factorials using C#? Windows calculator in Win 7 overflows at Factorial (3500). As a programming and mathematical question I am interested in kno

Answer 1

Have you looked at System.Numerics.BigInteger?

Answer 2

Using System.Numerics BigInteger

var bi = new BigInteger(1);
var factorial = 171;
for (var i = 1; i <= factorial; i++)
{
    bi *= i;
}

will be calculated to

1241018070217667823424840524103103992616605577501693185388951803611996075221691752992751978120487585576464959501670387052809889858690710767331242032218484364310473577889968548278290754541561964852153468318044293239598173696899657235903947616152278558180061176365108428800000000000000000000000000000000000000000

For 50000! it takes a couple seconds to calculate but it seems to work and the result is a 213237 digit number and that's also what Wolfram says.

Answer 3

Have a look at the BigInteger structure:

http://msdn.microsoft.com/en-us/library/system.numerics.biginteger.aspx

Maybe this can help you implement this functionality.

CodeProject has an implementation for older versions of the framework at http://www.codeproject.com/KB/cs/biginteger.aspx.

Answer 4

Since they don't give you the result down to the last digit, they may be "cheating" using some approximation. Check out http://mathworld.wolfram.com/StirlingsApproximation.html Using Stirling's formula you can calculate (an approximation of) the factorial of n in logn time. Of course, they might as well have a dictionary with pre-calculated values of factorial(n) for every n up to one million, making the calculator show the result extremely fast.