On Wed, 20 Oct 2010 12:57:11 +0200, Don wrote: > Lars T. Kyllingstad wrote: >> (This message was originally meant for the Phobos mailing list, but for >> some reason I am currently unable to send messages to it*. Anyway, >> it's probably worth making others aware of this as well.) >> >> In my code, and in unittests in particular, I use >> std.math.approxEqual() a lot to check the results of various >> computations. If I expect my result to be correct to within ten >> significant digits, say, I'd write >> >> assert (approxEqual(result, expected, 1e-10)); >> >> Since results often span several orders of magnitude, I usually don't >> care about the absolute error, so I just leave it unspecified. So far, >> so good, right? >> >> NO! >> >> I just discovered today that the default value for approxEqual's >> default absolute tolerance is 1e-5, and not zero as one would expect. >> This means that the following, quite unexpectedly, succeeds: >> >> assert (approxEqual(1e-10, 1e-20, 0.1)); >> >> This seems completely illogical to me, and I think it should be fixed >> ASAP. Any objections? > > I'm personally pretty upset about the existence of that function at all. > My very first contribution to D was a function for floating point > approximate equality, which I called approxEqual. It gives equality in > terms of number of bits. It gives correct results in all the tricky > special cases. Unlike a naive relative equality test involving > divisions, it doesn't fail for values near zero. (I _think_ that's the > reason why people think you need an absolute equality test as well). > And it's fast. No divisions, no poorly predictable branches. > > Unfortunately, somebody on the ng insisted that it should be called > feqrel(). Stupidly, I listened. And now nobody uses my masterpiece > because it has a totally sucky name.
That *is* a sucky name. Well, now that I'm aware of it, I'll be sure to check it out. :) However, I, like most people, am a lot more used to thinking in terms of digits than bits. If I need my results to be correct to within 10 significant digits, say, how (if possible) would I use feqrel() to ensure that? -Lars
