----- Original Message -----
Sent: Sunday, August 11, 2013 5:19
PM
Subject: Re: Float numbers
equality.
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
