C# Biginteger for huge number of bits

Question

I would like to know if there is any efficient way to store a big number using C#. I would like to create number consisting of 960 bytes but BigInteger can't hold it. I would be grateful for any advice.

UPDATE: I am using random byte generator to fill up array needed for constructor of BigInteger. For 960 byte array i BigInteger is returning a negative number.

why can't `BigInteger` hold it? It should be able to hold as much data as you have memory available... — user1666620, Nov 23 '15 at 14:58
What error do you get ? How are you declaring it ? Can you show some sample. — Marshal, Nov 23 '15 at 15:00
This will help you: https://stackoverflow.com/questions/18444337/what-variable-type-can-i-use-to-hold-huge-numbers-30-digits-in-java — sharp, Nov 23 '15 at 15:00
This question need more details. It also needs to be more directed. What problems are you experiencing? Show your code. — tdbeckett, Nov 23 '15 at 15:04
if the number is negative, it's not because it is too big for `BigInteger` - it's because your number is actually a negative value. — user1666620, Nov 23 '15 at 15:07
@JamesThorpe: more like atoms in the known universe. 960 bytes is about 2311 decimal digits. The number of atoms in the known universe is estimated at between 10^80 and 10^85. — Rudy Velthuis, Nov 24 '15 at 08:55

Jens · Accepted Answer · 2015-11-23T15:21:46.790

2

static void Main(string[] args)
{
    var arr = new byte[960];

    for (int i = 0; i != arr.Length; i++)
    {
        arr[i] = byte.MaxValue;
    }

    var big = new BigInteger(arr);
}

is working pretty fine and the result is -1 because the representation of the number is in the two's complement. That means a number with just 1s in binary always resolves to -1 as you can see in the article.

if you add one Length more and set the last element of the array to zero you should get a positive number which represents your binary number (this one byte will not hurt you):

var arr = new byte[961];
arr[arr.Length-1] = 0;
var big2 = new BigInteger(arr);

but then you really should be sure in what format your binary number is and what BigInteger is "reading"

edited Nov 23 '15 at 15:21

answered Nov 23 '15 at 15:07

Jens

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thanks, indeed it is working ok. I just thought i am getting overflow. – Dago Nov 23 '15 at 15:19
on the first five seconds i was also suprised but then i realized the complement. i will also add a solution – Jens Nov 23 '15 at 15:22
to sum up. If my BigInteger has a negative sign that means my 960 byte representing a negative number right? – Dago Nov 23 '15 at 15:53
@Dago yes. When the MSB (most significant bit) is 1 then your number is negative. That means doing (this sets the MSB to 1): `arr[arr.Length - 1] |= 0b1000 0000` will result always in a negative number. – Jens Nov 24 '15 at 06:16

C# Biginteger for huge number of bits

1 Answers1