Re: Why do some math functions return -0

[KHMan <keinhong@...>]

On 9/20/2010 12:45 AM, Mike McGonagle wrote:
> I am running on a Mac OS X 10.4.11 with lua 5.1.2, comiled in 2007.
>
> here are the first few lines of output...
>
> i:0.000000 r:0.000000 c:-0.000000 b:0.000000 br:-0.000000 bc:-0.000000
> i:1.000000 r:0.000000 c:1.000000 b:0.000000 br:-0.000000 bc:1.000000
> i:2.000000 r:0.000000 c:2.000000 b:0.000000 br:-0.000000 bc:2.000000
> i:3.000000 r:0.000000 c:3.000000 b:1.000000 br:-0.000000 bc:-0.000000
> i:4.000000 r:0.000000 c:4.000000 b:1.000000 br:-0.000000 bc:1.000000
> i:5.000000 r:0.000000 c:5.000000 b:1.000000 br:-0.000000 bc:2.000000
> i:6.000000 r:0.000000 c:6.000000 b:2.000000 br:-0.000000 bc:-0.000000
> i:7.000000 r:0.000000 c:7.000000 b:2.000000 br:-0.000000 bc:1.000000
> i:8.000000 r:0.000000 c:8.000000 b:2.000000 br:-0.000000 bc:2.000000
> i:9.000000 r:1.000000 c:-0.000000 b:0.000000 br:1.000000 bc:-0.000000
>
> As you notice, not all the 0's are negative. I don't know, maybe this
> really isn't an issue, as so far I have not run into anything other
> than this output being not exactly what I expect. I do know that I
> have a later version of lua on my machine, but I don't really have the
> time at the moment to check it... so maybe I should just try that and
> then get back to the list...

Nothing negative on Lua 5.1.2/Cygwin. The common element for the 
-0 results looks to me like %. The code for the % hasn't changed 
at all from 5.1.2 to 5.1.4, I think. But there is a -ve in the 
definition:

luaconf.h:
    #define luai_nummod(a,b)	((a) - floor((a)/(b))*(b))

Seems like a platform issue, but -0 sure looks like the wrong 
answer. Would be interesting to check a calculation with a simple 
C program and look at the exact bit pattern that is generated.

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