C# Find Nth Root

Question

I use below method to calculate Nth Root of double value, but it takes a lot of time for calculating the 240th root. I found out about Newton method, but was not able to implement it into a method. Any help would be appreciated.

static double NthRoot(double A, int N)
{
   double epsilon = 0.00001d;//
   double n = N;
   double x = A / n;
   while (Math.Abs(A-Power(x,N)) > epsilon)
   {
    x = (1.0d/n) * ((n-1)*x + (A/(Power(x, N-1))));
   }
   return x;
}

did you have a look at this? http://en.wikipedia.org/wiki/Newton%27s_method#Pseudocode should no be too difficult to translate into real code — DrCopyPaste, Sep 06 '13 at 12:13
What actually is the question here? Do you just want it to be faster? Or do you explicitly want to see how the newton method would look in real code? — DrCopyPaste, Sep 06 '13 at 12:16
I came across POW, but for some reason I though it's same as per the method I posted above. I'm not programmer, and would not have posted question here unless I was not able to figure it out myself. Thank you — illusion, Sep 06 '13 at 12:29
Where did you get that power-method from? Is it selfwritten or what namespace is it in? — DrCopyPaste, Sep 06 '13 at 12:34
From here http://stackoverflow.com/questions/3848640/n-th-root-algorithm , It's funny, I came across Pow but I thought it could not be so simple after learning the method mentioned above. I'm learning (C# by (learn what I need), I try to write indicators for a trading program NinjaTrader. — illusion, Sep 06 '13 at 12:38

score 63 · Accepted Answer · answered Sep 06 '13 at 12:15

63

static double NthRoot(double A, int N)
{
    return Math.Pow(A, 1.0 / N);
}

From Wikipedia:

In calculus, roots are treated as special cases of exponentiation, where the exponent is a fraction:

\sqrt[n]{x} \,=\, x^{1/n}

answered Sep 06 '13 at 12:15

Stefan Dragnev

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But I want the 3th root that use 1.0/3==0.333333333 !=1/3. – lindexi Apr 21 '17 at 08:08
8

Computer arithmetic is always bound by the precision of the underlying data type. To within enough digits of precision, it's equal. – Stefan Dragnev Apr 23 '17 at 03:49

score 2 · Answer 2 · edited Apr 16 '18 at 16:51

2

You can use the same function used to find the power of a number, just use reciprocal of the number instead of the number itself.

To find N root of X you can write,

int root = Convert.ToInt32(Math.Pow(X, (1 / N));

edited Apr 16 '18 at 16:51

LarsTech

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answered Apr 16 '18 at 16:47

Abhas Bhoi

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C# Find Nth Root

2 Answers2

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