From: "Matthias Wächter" <matthias@...>
Date: 2007-09-03T19:09:18+09:00
Subject: Re: 0.06 == 0.06 returns false in Ruby?

On 31.08.2007 16:07, Davor wrote:
> Hi,
> Try converting to strings e.g:
> irb(main):004:0> (0.05 + 0.01).to_s == 0.06.to_s
> => true

I think what puzzles is while 0.05+0.01 == 0.06 returns false, the
strings _do_ match.

def float_check(a,b)
  p [a==b,a.to_s == b.to_s]
end

float_check(0.06,0.06)       # -> [true,true]
float_check(0.05+0.01,0.06)  # -> [false,true]


Most other programming languages either have no format-less .to_s
function (like in C) or return values at inspection time so that
people get aware of the issue more easily. In Python it is obvious
that 0.05+0.01 == 0.06 is false.

$ python
>>> 0.05
0.050000000000000003
>>> 0.01
0.01
>>> 0.05+0.01
0.060000000000000005
>>> 0.06
0.059999999999999998

I don't understand why Ruby 'rounds' values with .inspect (and
.to_s) opposed to Marshal.dump. Compare its output with the one
Python gives (this code is for Ruby 1.8.6):

[0.05,0.01,0.05+0.01,0.06].each do |v|
  p Marshal.dump(v).sub(/....([^\000]*).*/,'\1')
end

Output:

"0.050000000000000003"
"0.01"
"0.060000000000000005"
"0.059999999999999998"


Did I miss something here, is there a way to make Float#to_s (and,
thus, Float#inspect) give the same output as Marshal?

- Matthias