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- Subject: Re: Float numbers equality.
- From: Andrew Starks <andrew.starks@...>
- Date: Sun, 11 Aug 2013 10:19:45 -0500
On Saturday, August 10, 2013, Luiz Henrique de Figueiredo wrote:
> Given that this subject comes up every couple of months, and that there are some subtleties to it, that this warrants a new function in the math library to do it the right way?
I don't think so. Floating-point arithmetic is not simple. I think it'd be
a disservice to try to hide its complexities in a official function that
may give the impression that it solves the problem reliably, when it can't.
On the other hand, there is nothing instrinsically wrong with floating-point
equality. The only catches come from numbers that should be equal but are
computed in two different ways and from naive expectations such as
300 * 0.7 == 21, due to a misunderstanding that floating-point representation
is a binary one, not a decimal one.
In an effort to understand (not debate), where would:
return math.abs(lhs - rhs) < epsilon*rhs
...be unsuitable? Imagine that I'm dealing with float equality and it isn't working. Also imagine that I don't know very much about the issue. I dig into the math library and find a function that does this.
How am I likely to be confused further? What knowledge would I still lack that would keep me from understanding, provided that PiL offered some additional context?
As a post script, I took the advice found in Programming In Lua[1] and found myself utterly lost (although ever so slightly closer to my Calculus Through Googling degree). I read the part of the discussion related to the epsilon, but since so much of it was aimed at an audience that wasn't me, I failed to identify the solution.
As a result, I believe that once I feel like I understand the common issues, I should (or someone like me should) write a wiki on it. I was unable to find a really good document aimed at someone who can't read calculus notation. Several Microsoft articles came very close, however.
- Andrew
