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

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

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/

http://hisham.hm/2015/04/18/a-small-practical-example-where-luas-behavior-is-better-than-cs/

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

"Modulo is defined as the remainder of a division that rounds the 
quotient towards minus infinity (floor division)."

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