Raising a decimal to a power of decimal ?

Question

The .net framework provides in the Math class a method for powering double. But by precision requirement I need to raise a decimal to a decimal power [ Pow(decimal a, decimal b) ]. Does the framework have such a function? Does anyone know of a library with this kind of function?

Answer 1

Are you sure you actually want to do this? A decimal multiply is about 40 times slower than double's, so I'd expect a decimal Math.Pow() to be practically unusable.

If you expect only integer powers, though, I suggest you use the integer-based power algorithm that was already discussed here on SO.

Answer 2

I think it depends a lot on the number you plan on plugging in. If 'a' and 'b' are not 'nice' number then you'll likely get a value which is non-terminating that is impossible to store and if C# BigDecimal behaves at all like Java BigDecimal it probably throws an exception in such a case.

Answer 3

log(c^d) = d * log(c)

further...

c^d = antilogarithm(d * log(c))

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("169^(1/2) = " + RaiseToPower((169), (.5)));
            Console.ReadLine();
        }

        public static double RaiseToPower(double a, double b)
        {
            return Math.Exp(b * Math.Log(a));
        }
    }
}

Answer 4

To solve my problem I found some expansion series, and them I had them implemented to solve the equation X^n = e^(n * ln x).

  // power series
  int iteration = 27; // Adjust this to modify the precision
  decimal result = 1; 
  while (iteration > 0)
  {
    fatorial = Factorial(iteration);
    result += (Pow(power, iteration) / fatorial);
    iteration--;
  }

  // natural logarithm series
  decimal aux = (value - 1);
  decimal result = 0;
  while (iteration > 0)
  {
    result += Pow(aux, iteration) / iteration;
    iteration--;
  }

The Pow() and Factorial() functions are simple because the power is always an int (inside de power series).