On 7/7/21 2:41 PM, Steven D'Aprano wrote:
> Do you know for a fact that Gnumeric implements its rounding by
> multiplying 0,3000000000000000044 * 1E16 or are you guessing?
Use the source, Luke.
There's no need for guessing. Here's what gnumeric does:
> double
> go_fake_floor (double x)
> {
> x = go_d2d (x);
>
> if (x == floor (x))
> return x;
>
> return (x >= 0)
> ? floor (go_add_epsilon (x))
> : floor (go_sub_epsilon (x));
> }
That's from .../goffice/goffice/math/go-math.c
Three comments:
1) If I were doing it, I wouldn't do it that way. I would stick
with POSIX floor().
2) It shouldn't matter! If your algorithm is sensitive to plus
or minus one machine epsilon, you need a better algorithm.
For each positive integer, there is exactly one representable
floating point number where this differs from POSIX floor().
(For negative numbers, it's wildly non-POSIX, but that's a
whole different story.)
3) You can work around this if you really care:
if(floor(x)<=x,floor(x),floor(x)-1) # for positive x
But again, it shouldn't matter.
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