On Friday, 6 December 2013 at 13:47:12 UTC, Joseph Rushton
Wakeling wrote:
A dangerous topic for everyone :-)
I've been working with some unittests involving comparing the
output of different but theoretically equivalent versions of
the same calculation. To my surprise, calculations which I
assumed would produce identical output, were failing equality
tests.
It seemed unlikely this would be due to any kind of different
rounding error, but I decided to check by writing out the whole
floating-point numbers formatted with %.80f. This confirmed my
suspicion that the numbers were indeed identical.
You can read the detailed story here:
https://github.com/D-Programming-Language/phobos/pull/1740
It seems like I can probably use isIdentical for the unittests,
but I am more concerned about the equality operator. I
completely understand that equality comparisons between FP are
dangerous in general as tiny rounding errors may induce a
difference, but == in D seems to see difference in
circumstances where (so far as I can see) it really shouldn't
happen.
Can anybody offer an explanation, a prognosis for improving
things, and possible coping strategies in the meantime (other
than the ones I already know, isIdentical and approxEqual)?
When you print out, you print out at type-precision. The
comparison could be happening at higher precision with trailing
precision from the last calculation.
I'm pretty sure D is free to do this, it goes with the whole
more-precision-is-better-precision philosophy.
