From: "marcandre (Marc-Andre Lafortune)" <ruby-core@...>
Date: 2013-12-05T00:44:08+09:00
Subject: [ruby-core:58855] [ruby-trunk - Bug #9192] Inconsistent comparison	between Float and BigDecimal


Issue #9192 has been updated by marcandre (Marc-Andre Lafortune).


duerst (Martin D��rst) wrote:
> ...
> But simply converting BigDecimal to Float may not make sense in general. Imagine a BigDecimal with much higher precision, like
> BigDecimal('706.06059999999999'), and some corresponding floats. We may want to actually say that the BigDecimal is bigger or smaller than the float created from the same string, because the float looses precision.
> Currently, I get
>    706.06059999999999 <=> BigDecimal('706.06059999999999') #=>  0
>    BigDecimal('706.06059999999999') <=> 706.0605999999999  #=> -1
> which is of course inconsistent. We should in any case make sure that
>    (a <=> b) + (b <=> a)
> is always 0 wherever the comparison makes sense.

Agreed.

The float we write as 706.0606 represents a small range of real values that includes the real number 706.06059999999999. So the first result is correct:

    706.0606 <=> BigDecimal('706.06059999999999') #=>  0, ok

The only strange effect is that we can get:

    a == x # => true
    b == x # => true
    a == b # => false

when x is a Float and a or b are higher precision numerics.
This can be explained because the first two comparison involve floats and thus dealing with approximate values.

----------------------------------------
Bug #9192: Inconsistent comparison between Float and BigDecimal
https://bugs.ruby-lang.org/issues/9192#change-43412

Author: vatsu (Gustavo Sales)
Status: Assigned
Priority: Normal
Assignee: mrkn (Kenta Murata)
Category: ext
Target version: 
ruby -v:  2.0.0p353 (2013-11-22 revision 43784) [x86_64-darwin11.4.2]
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


I was checking a possible inconsistency on ActiveModel and I found situations where comparing BigDecimal to Float differs to comparing Float to BigDecimal.

I have create a simple ruby script to exemplify the problem. The script is attach to this issue and output is as follows:

Instance number 706.05
###########
Instance number 706.06
Inconsistence:
0.70606E3 > 706.06
706.06 < 0.70606E3
=========
Inconsistence:
0.70606E3 <= 706.06
706.06 >= 0.70606E3
=========
###########
Instance number 706.07
Inconsistence:
0.70607E3 < 706.07
706.07 > 0.70607E3
=========
Inconsistence:
0.70607E3 >= 706.07
706.07 <= 0.70607E3
=========
###########
Instance number 706.08
###########


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