为什么这个费马素数测试仪给我一个例外？ [重复]

Question

此问题已经在这里有了答案：

Got hit by an OverflowException（3个答案）

关闭3分钟前。

为什么这个Fermat素数测试仪给我一个例外？

class PrimeTest
{
    public static bool IsPrime(long n, int iteration = 5)
    {
        Random r = new Random();
        long a = 0;
        long aPow = 0;

        for (int i = 0; i < iteration; i++)
        {
            a = r.Next(1, Convert.ToInt32(n - 1));

            double x = Convert.ToDouble(a);
            double y = Convert.ToDouble(n - 1);
            double p = Math.Pow(x, y);

            aPow = Convert.ToInt64(p);//<==== this line is giving an exception.

            if (1 % n == aPow % n)
            {
                return true;
            }
        }

        return false;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("{0}", PrimeTest.IsPrime(33));
        Console.ReadLine();
    }
}

输出

An unhandled exception of type 'System.OverflowException' occurred in mscorlib.dll

Additional information: Arithmetic operation resulted in an overflow.

Answer 1

运行您的程序，我得到：

3.4336838202925124E+30或3433683820292512400000000000000

[Int64或long的最大值为9223372036854775807

很容易看到为什么会收到Overflow Exception。如果您查看此消息，将会看到更多详细信息：

Arithmetic operation resulted in an overflow.

该数字太大，无法容纳long值。

Answer 2

您的a是随机数[1〜n-1]，并且a ^（n-1）可能容易大于Int64.Max例如，a = 10且10 ^ 32大于Int64。Max。

        Random r = new Random();
        long a = 0;
        long aPow = 0;

        for( int i = 0; i < iteration; i++ ) {
            a = r.Next( 1, Convert.ToInt32( n - 1 ) );

            // p is 1E32; if a==10
            double p = Math.Pow( Convert.ToDouble( a ), Convert.ToDouble( n - 1 ) );

            // Int64 is 9223372036854775807, which is less than 1E32
            aPow = Convert.ToInt64( p );giving exception

            if( 1 % n == aPow % n ) {
                return true;
            }
        }

        return false;