From: "marcandre (Marc-Andre Lafortune)" <ruby-core@...>
Date: 2013-12-03T14:57:20+09:00
Subject: [ruby-core:58827] [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:
> nobu (Nobuyoshi Nakada) wrote:
> > Sounds like the old usual floating point number error.
> 
> I don't think so. BigDecimal('706.06') is exact. 706.06 is a float, and therefore can't be represented exactly. That means that it's either represented as too small or too big than 706.06. If it's represented as too small, then we should get false twice; if it's represented as too big, we should get true twice.
> This would be "the old usual floating point number error" if Gustavo had asked why 706.06 != BigDecimal('706.06'), but that's not the question here.

I never thought about comparison of float with other numeric types, but should we convert everything to float and compare this way? This way we insure that:

    any_big_decimal.to_f == any_big_decimal

Here, the float 706.06 stands for a small range of real numbers that includes BigDecimal('706.06'). Thus shouldn't we have 706.06 == BigDecimal('706.06')?



----------------------------------------
Bug #9192: Inconsistent comparison between Float and BigDecimal
https://bugs.ruby-lang.org/issues/9192#change-43391

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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