prime no

Question

Sir/Madam How to find the prime number between 2^511 and 2^512.I have tried the below code but its not giving proper output. using System; using System.Collections.Generic; using System.Text; namespace ConsoleApplication3 { class Program { static void Main(string[] args) { bool prime = false; Console.WriteLine((Math.Pow(2511)).ToString()); Console.WriteLine((Math.Pow(2 512)).ToString()); double a = Math.Pow(2 511); double b=Math.Pow(2 512); Console.WriteLine((b-a).ToString()); while (a { for (double i = 2; i { if (a % i == 0) { prime = true; //Console.WriteLine(i.ToString()); break; } } if (prime == false) { // Console.WriteLine(a.ToString()); break; } a = a + 1; } Console.ReadLine(); } } } Thanks in advance...

Josip Juric · Answer

In .net 4 you have new class called BigInteger and it is in namespace System.Numerics(first you need to add this namespace using AddReference dialog) and you can't use Math.Pow() because it return's long and we need BigInteger so we need our Function for potentiation and that function should look like this: private BigInteger BetterPow(int x int y) { BigInteger res=x; for (int i = 1; i { res*=x; } return res; } EDIT!!! And you code should look like this: bool prime = false; Console.WriteLine((BetterPow(2 511)).ToString()); Console.WriteLine((BetterPow(2 512)).ToString()); BigInteger a = BetterPow(2 511); BigInteger b = BetterPow(2 512); Console.WriteLine((b - a).ToString()); Console.WriteLine(); //This is remainder what lefd dividing two BigIneger numbers(in BigInteger % operator isn't implemented) BigInteger remainder; for (BigInteger number = a; number { for (BigInteger divisor = 2; divisor { remainder = BigInteger.Remainder(number divisor); if (BigInteger.Compare(remainder 0) == 0) { prime = false; break; } else { prime = true; break; } } if (prime) { Console.WriteLine(number.ToString()); } } Console.ReadLine(); BTW. I put source so if download and try it.

Crish · Answer

Hi I have same problem.Thanks for this code it will help me to solve the issue of prime no.

Mike Gold · Answer

that does sound like the way to go (BigInt). Alsoc check out http://www.fractal-landscapes.co.uk/bigint.html

Mike Gold · Answer

Also check out the following http://en.wikipedia.org/wiki/Primality_test . This may guide you a little bit and give you some ideas.

Mike Gold · Answer

that problem does not sound easy to me but I suspect you would do it with a set of 8 long values (or 8 64 bit values) representing each number. You can't do it with doubles (e.g. Math.Pow) because you'll get rounding errors in the higher numbers) -Mike