Saving factorial in long integer

Question

I am using the following code to get the Factorial, but it seems to be limited for me.

private Int64 GetFactorial(Int64 value)
{
    if (value <= 1)
    {
        return 1;
    }
    return value * GetFactorial(value - 1);
}

But it seems to only allow the values upto 65 on 66th value, it provides a 0 as a result and the result is negative too if the value is 65 or near. What can I do to allow more values to work with, and get result with having the System.StackOverflowException?

Duplicate questions check this http://stackoverflow.com/questions/4202626/factorial-method-resulting-in-error?rq=1 — Satish, Jul 31 '14 at 20:29
mildly curious how you get something up to 65/66, as 65! ~= 10^90 — Kevin L, Jul 31 '14 at 20:30
@KevinL I was testing this out, I tried 100, it gave me error, I tried 60 it worked, so I started counting down and I got 65 as the limit. — Afzaal Ahmad Zeeshan, Jul 31 '14 at 20:31
If you were seriously interested in using factorials, I suggest that you would not be using the naïve method of recursion to calculate them. If you look at what you need the factorial for, you may be able to eliminate overflow problems. — Andrew Morton, Jul 31 '14 at 20:34
@AfzaalAhmadZeeshan Yes, a `for` loop. But consider if you wanted to calculate 99!/97! - you only need to calculate 99*98. — Andrew Morton, Jul 31 '14 at 20:36
@AndrewMorton, nah it is just a simple 99! calculator, :) division and other are not involved here. and yes, I would change the logic here. :) — Afzaal Ahmad Zeeshan, Jul 31 '14 at 20:38

score 3 · Accepted Answer · answered Jul 31 '14 at 20:30

3

You may want to check out the BigInteger struct, as it allows integers of an arbitrarily-large size: http://msdn.microsoft.com/en-us/library/system.numerics.biginteger(v=vs.110).aspx

answered Jul 31 '14 at 20:30

Keith Kurak

662
1
6
11

score 0 · Answer 2 · answered Jul 31 '14 at 20:29

0

Uint64 will get you more, not sure how many though.

answered Jul 31 '14 at 20:29

kidshaw

3,423
2
16
28

Didn't expect much, but thought it might be a bit more. I trust your math more than mine! – kidshaw Jul 31 '14 at 20:31
actually less, since uint64 is what, double the size? and with the value being in the range of (supposedly) 60s, it'd overflow just as fast... (i was thinking fibbonaci numbers) – Kevin L Jul 31 '14 at 20:32
Yep - big integer looks better +1 – kidshaw Jul 31 '14 at 20:33

Saving factorial in long integer

2 Answers2