Re: [ANN] Lua 5.2.0 (rc7) now available

[KHMan <keinhong@...>]

On 12/10/2011 12:20 AM, Roberto Ierusalimschy wrote:
> > and the simple tests:
> >
> > >   print(2^3 - 8)
> > -1.7763568394003e-015
> > >   print(3^4 - 81)
> > 2.8421709430404e-014
> > > =2^4-16
> > -1.7763568394003e-015
>
> These all should be zero. It really seems that the 'pow' function
> from the C library is broken.

The above results can be calculated as follows:

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

int main()
{
    // from cygwin's newlib.c
    // calculate x ** y as 2 ** (y log2(x))
    double x, y, z;
    x = 3; y = 4;
    z = pow(2, y*log2(x));
    printf("%g\n", z-81);
    return 0;
}

This gives (MinGW gcc Win32):
$ gcc -o test test.c ; ./test
-2.84217e-014

Looking at newlib's math sources, more robust implementations of 
pow() checks for integer exponents and handles that properly.

--
Cheers,
Kein-Hong Man (esq.)
Kuala Lumpur, Malaysia