Re: Modulo infinity

[Roberto Ierusalimschy <roberto@...>]

>     a = 100
>     b = a / 3
>     print(a/b, a%b, math.fmod(a,b))
> 
> This prints "3 0 33.333333333333". I have no idea why, but it breaks the 
> guarantee that a=floor(a/b)*b+a%b

Because 100/3 has no exact representation, a/b can result in something
a little smaller than 3 (in which case its floor is 2) or something a
little larger than 3 (in which case its floor is 3). With floor(a/b)=2,
fmod gives 33.33333; with floor=3, a%b gives 0. (I would say that a%b
sounds more correct than fmod(a,b) in this case.)


> By that reason, though, it's a bug for N%math.huge to return anything other 
> than N since 0=floor(N/math.huge)

It is correct that floor(N/math.huge)=0, but 0*math.huge is not zero in IEEE
arithmetic; it is NaN. So, floor(N/math.huge)*math.huge gives NaN.

-- Roberto