Re: Modulo operator produces incorrect results with negative operand(s).

["Soni L." <fakedme@...>]

On 24/01/17 09:47 PM, Milo Christiansen wrote:
> Hmmm... So why does my programmers calculator, Go, JavaScript, etc, 
> etc all return -3 in this case?
>
> Personally I think it's a bug, as "2" is not the correct result for 
> that expression (in any language but Lua it seems).

Don't top post. And read this:
http://hisham.hm/2015/04/18/a-small-practical-example-where-luas-behavior-is-better-than-cs/
(That's from the LuaRocks developer, I think. So they kinda know what 
they're saying.)

And check mailing list archives. This isn't the first time this has come up.

> On Tue, Jan 24, 2017 at 6:40 PM, Soni L. <fakedme@gmail.com 
> <mailto:fakedme@gmail.com>> wrote:
>
>     On 24/01/17 09:21 PM, Milo Christiansen wrote:
>
>         In Lua 5.3 "-3%5 == 2", according to my programmers calculator
>         and several other programing languages I tried this expression
>         in, the real result is -3.
>
>         On the other hand "5%-3 == 2", so maybe the result is correct,
>         and the bug is simply reversed operands if one is negative?
>
>         I discovered this because I am adding tests (based on the
>         official Lua tests) to my Go Lua VM, and one of the tests
>         failed because the incorrect result is coded into the test.
>
>
>     First of all your modulo is not my modulo (or at least this was a
>     thing back in C89... looks like it isn't in C99+).
>
>     Secondly, -3%5 == 2 because floor(-3/5) = -1, and (-1)*5+2 = -5+2
>     = -3.
>
>     See also:
>
>     https://www.microsoft.com/en-us/research/publication/division-and-modulus-for-computer-scientists/
>     <https://www.microsoft.com/en-us/research/publication/division-and-modulus-for-computer-scientists/>
>
>     http://hisham.hm/2015/04/18/a-small-practical-example-where-luas-behavior-is-better-than-cs/
>     <http://hisham.hm/2015/04/18/a-small-practical-example-where-luas-behavior-is-better-than-cs/>
>
>     https://github.com/sp614x/optifine/issues/288
>     <https://github.com/sp614x/optifine/issues/288>
>
>     etc. This is an endless argument.
>
>     Thirdly, in case you're still reading this for some unknown
>     reason, RTFM always applies:
>
>     http://www.lua.org/manual/5.3/manual.html#3.4.1
>     <http://www.lua.org/manual/5.3/manual.html#3.4.1>
>
>     "Modulo is defined as the remainder of a division that rounds the
>     quotient towards minus infinity (floor division)."
>
>     -- 
>     Disclaimer: these emails may be made public at any given time,
>     with or without reason. If you don't agree with this, DO NOT REPLY.

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