How can I calculate the cubic root of a negative number?

General Discussion
1

Hi!!
I am obtaining a “nan” every time that x takes a negative value in pow(x,1./3.)?

What am I doing wrong?

Thank you!!!

2

@glmiranda I think that is the definition of the power function. You can do the root of the absolute value, and times it with the sign function.

3

This function is directly interfaced with the C++ one. As your exponent is not an integer, it is the standard behavior.

4

Hi Simon!
You know, in c++ there exists the “float cbrt( float )” function which can handle negative arguments but this kind of function doesn’t exist in Freefem. This is strange because Freefem was created from c++, I think …

Any way, in my program, I had to define my own version as follows:
func real cbcrt( real z )
{
if(z<0)
return -pow(-z,1./3.);
else
return pow(z,1./3.);
}
And it seems that works!

Thank you for answering my question!!

5

Hi Robin!

Yeah, you are right:

func real cbcrt( real z )
{
if(z<0)
return -pow(-z,1./3.);
else
return pow(z,1./3.);
}

Thank you!