C＃Math.pow（x，0.333）

Question

private double f(double x, double zn = 1)
    {
        double X = - zn;
        X *= x * x * (x + 1);
        X *= Math.Pow((x - 2), 0.333);
        return funct ? x : X;
    }

I have this code. When I try to find Math.Pow((x-2), 0.333) - i have NaN. How to solve it? Why NaN?

Rewritten...

private double f(double x, double zn = 1)
    {
        double answer = - zn;
        answer *= x * x * (x + 1);
        answer *= Math.Pow((x - 2), 0.333);
        return answer;
    }

Answer 1

My guess is that you're taking the cube root of a negative number. That seems the most likely cause, but your code is really hard to read due to having both x and X as local variables...

After closer examination, as you're not actually modifying x at any point, it really depends on the incoming value of x . If it's a finite value greater than or equal to 2, it should be fine. But if x is smaller than 2, it will fail (well, return NaN) for reasons of simple maths...

Answer 2

You can see there all 3 cases when Math.Pow returns NaN:

http://msdn.microsoft.com/en-us/library/system.math.pow.aspx

public static double Pow(double x, double y)

1) x or y = NaN.

2) x < 0 but not NegativeInfinity; y is not an integer, NegativeInfinity, or PositiveInfinity.

3) x = -1; y = NegativeInfinity or PositiveInfinity.

Answer 3

Math.Pow is not defined for numbers less than 0 for given power. So you function will fail for some x.

x < 0 but not NegativeInfinity; y is not an integer, NegativeInfinity, or PositiveInfinity.
Result: NaN

Answer 4

If you look here it explains all the situations where Math.Pow will give NaN. I suspect this case may be your problem:

x < 0 but not NegativeInfinity; y is not an integer, NegativeInfinity, or PositiveInfinity.