  lua-l archive

• Subject: Re: compute cubic root with negative argument
• From: Paco Zamora-Martínez <pakozm@...>
• Date: Thu, 14 Apr 2016 10:50:44 +0000

This problem is present in C pow and powf functions. So, this issue is related with standard math library of C , which is in the bones of Lua ;-)

 http://en.cppreference.com/w/c/numeric/math/pow says: pow(base, exponent) returns NaN and raises FE_INVALID if base is finite and negative and exponent is finite and non-integer

On Thu, 14 Apr 2016 at 12:30 Christopher Kappe <kappe@cs.uni-kl.de> wrote:
Hello,

I just noticed that it is not possible to compute the cubic root of a
negative number. See e.g.
print( math.pow( -27, 1/3 ) ) --> -nan instead of -3
The same is true for the ^ operator (not surprising).

This probably stems from the fact that the result of the n-th root of a
negative number is not real if n is even (e.g. sqrt(-1) = (-1)^(1/2) =
i). However, (-3)^3 = -27 and likewise should hold that (-27)^(1/3) = -3.
This works by the way with Google (try googling f(x) = x^(1/3) and you
will see the graph of the function also for the negative x-axis).

If such a correct behavior is not easy to implement in the generic
math.pow function, I would suggest offering a math.cbrt function as
exists e.g. in C, C++ or Octave.

The least one should do, is document the fact that exponentiation only
works for a non-negative base argument.

Regards,
Christopher

--
Pako ZM :)
http://pakozm.hol.es