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Re^2: When is a 2 not a 2? (eq)

by halley (Prior)
on Jan 31, 2008 at 13:43 UTC ( #665343=note: print w/replies, xml ) Need Help??


in reply to Re: When is a 2 not a 2? (eq)
in thread When is a 2 not a 2?

tye, you may already know of this, but I'll reiterate that anyone who is interested in implementing a "comparing floating point numbers" feature should read this article. http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm

In the olden days, people would compare using a chosen EPSILON. if (abs($a - $b) < $EPSILON) { ... } If you know about the uneven resolution of floats, you learn that EPSILON must be chosen carefully for each comparison. Better to know something about the format of IEEE floats (the most common implementation on modern computers) and some fast and flexible ways to make a suitable AlmostEqual() function that lets you use a tolerance that is tied to the resolution, not the decimal position of the error.

Comparing floats is a huge gotcha for newcomers or writers of quick ad-hoc code, and easy to do wrong. I would rather that high-level programming languages melt such comparison features into the language, say, with an A =~ B floating point operator. (Whether regex or float, that can be read as "does A bind with B.") Then, the default tolerance can be set to a suitable "about one decimal place" and overridden through a pragma or language variable. But I digress.

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[Corion]: ambrus: Hmmm. It makes sense that AnyEvent would implement the push_write itself, but I think I don't have a good idea of where the boundary between AnyEvent and the underlying event system lies... Implementing the timer should give me a better idea
[ambrus]: Corion: push_write is in the higher level abstraction of AnyEvent::Handle, not in the watcher
[Corion]: ambrus: Hmm - rereading Prima::File, that merrily coincides with what Prima does - it tells you "you can read", and you're supposed to read from the fh yourself. I thought it called you with the data already read, which would've been harder to integrate
[ambrus]: you just need an io watcher, created by &AnyEvent::Impl:: Whatever::io(...)
[Corion]: So after talking it through with you even while I'm still not entirely clear on where AE ends and my implementation begins, I think I understand that I only need to implement some smaller parts for each functionality I want to support.
[Corion]: Yeah... and you might even be able to mix and match additional functionality if you have additional async suppliers, like from a separate thread
[ambrus]: You hvae to be careful with the timer, because apparently Prima::Timer insists on being periodic, wheras AnyEvent::Impl:: Whatever::timer should give a one-shot timer watcher

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