Re: Comment about IEEE floating-point rounding... (was: Setting Float Precision in Lua.c)
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- Subject: Re: Comment about IEEE floating-point rounding... (was: Setting Float Precision in Lua.c)
- From: Roberto Ierusalimschy <roberto@...>
- Date: 2018年6月27日 10:50:52 -0300
> > I get 0 on all platforms I can test Lua 5.[12] on, which is of course as bad
> > as 3 but not *this* bad. Could any patches you use have affected this?
>
> On Mac OS X 10.11.6 I get
>
> Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> > print(1e97 % 97)
> 0
>
> Lua 5.2.4 Copyright (C) 1994-2015 Lua.org, PUC-Rio
> > print(1e97 % 97)
> 0
>
> Lua 5.3.4 Copyright (C) 1994-2017 Lua.org, PUC-Rio
> > print(1e97 % 97)
> 3.0
Lua 5.1/5.2 computed the module (a%b) with (a - floor(a/b) * b),
which may introduce other rouding errors. Lua 5.3 computes the module
with 'fmod'; at least in my machine, the only error is the one in the
representation of 1e97 itself (see my other messages).
-- Roberto