Re: numerical approximations can suck!

["Brett Bibby" <research@...>]

What he said, but do realize that once you fix that problem 0.0 can be
signed, nan, or a number of other things that may make you more unhappy
than you are now :-)  I highly recommend you study the IEEE floating
point specification as they were well aware of the issues involved and
had to make a number of tradeoffs in implementation that could frustrate
you.  Also, you can redefine what a lua number is if you want to....

----- Original Message ----- 
From: <jmckenna@reflectionsinteractive.com>
To: <lua@bazar2.conectiva.com.br>
Sent: Thursday, November 20, 2003 5:11 PM
Subject: RE: numerical approximations can suck!


> Either your computer's floating point is broken, or x and y are
genuinely
> slightly different.  Floating point calculations are exact for
integers (as
> long as you're in range.  The range of double precision floating point
is
> larger than that of 32 bit integers, so that's not usually a problem).
>
> x = 150
> y = 150
> print( x, y, x-y )
> -> 150  150  0
> x = 150 + 1.9895196601283e-13
> y = 150
> print( x, y, x-y )
> -> 150  150  1.9895196601283e-13
>
> It could be print's helpful truncating of numbers that's confusing
you.
>
> math.floor and math.ceil might be the functions you're looking for,
but I
> suspect you're blaming the wrong thing.
>
> > -----Original Message-----
> > From: Ando Sonenblick [mailto:ando@spritec.com]
> > Sent: 20 November 2003 07:19
> > To: Lua list
> > Subject: numerical approximations can suck!
> >
> >
> > Gang,
> >
> > I've recently begun to lament all floating point (be it floats or
> > doubles).
> >
> > Since when does 150 - 150 equal 1.9895196601283e-13?
> >
> > My lua code has dx = x - y and print(x); print(y); print(dx)
> > results in:
> >
> > 150
> > 150
> > 1.9895196601283e-13
> >
> > Now I know that to you and mean (and to integer arithmetic)
> > 1.9895196601283e-13 is effectively zero, but it's somehow screwing
up
> > my math and my programs functionality!
> >
> > Hypothetical rant: we've been computing as a species for 60+
> > years?!?1
> > can't we have accurate math!?!?!
> >
> > real world question: is there a way to convert a lua number
> > (ie double)
> > into its integer counterpart?  I'm simply (currently) working with
> > integer precision (or WANT to be doing so)...
> >
> > thx,
> > ando "don't even MENTION trying to work with numtostring and floats
> > with values of 1.9895196601283e-13" moon :)
> >