```Pea wrote:
--snip--
> irb(main):010:0> (0.05+0.01) - 0.06
> => 6.93889390390723e-18
>
> as mentioned by Dan, careful on comparing floats. And as to any
> precision subject, there is what we call significant digits..
>
> this floating problem is a faq and is very surprising on such a
> very high language such as ruby.  can we address this? maybe create
> flag like \$EPSILON=0 or something, or may flag to revert to rational
> or bigdeci like \$FLOAT_PROCESSOR=RATIONAL...
>

Unfortunately, there is no easy solution to this problem. Here is a
catalog of often proposed solutions and why they do not work:

1 (proposed by doug meyer in this thread) Always use
(x-y).abs < Float::EPSILON
as a test for equality.

This won't work because the rounding error easily can get bigger than
Float::EPSILON, especially when dealing with numbers that are bigger
than unity. e.g.
y = 100.1 + 0.3
y - 100.4 # => -1.421e-14, while Float::EPSILON = 2.22e-16

2 Always use
(x-y).abs < (x.abs + y.abs) * Float::EPSILON)
as a test for equality.

Better than the first proposal, but won't work if the rounding error
gets too large after a complex computation.
In addition, (1) and (2) suffer from the problem that x==y and y==z do
not imply x==z.

3 Use Bigdezimal

This only shifts the problem a few decimal places down, and tests for
equality will fail as with the normal floats.

4 Use Rationals

Works if you only have to deal with rational operations. But doesn't
solve the following
x = sqrt(2)
y = x + 1
x + 0.2 == y - 0.8  # => false
In addition, rational arithmetic can produce huge numbers pretty fast,
and this will slow down computations enormously.

5 Use a symbolic math package

This could in theory solve the issue with equality, but in practice there
is no way to decide that two symbolic representations of a number are the
same, like
1 / (sqrt(2) - 1) == sqrt(2) + 1
Also, very, very slow.

6 Use interval arithmetic