From: "naruse (Yui NARUSE)" <naruse@...>
Date: 2013-04-25T06:49:19+09:00
Subject: [ruby-core:54558] [ruby-trunk - Bug #8299] Minor error in float parsing


Issue #8299 has been updated by naruse (Yui NARUSE).


phasis68 (Heesob Park) wrote:
> I found the issue is due to the difference of the floating point precision.
> 
> Here is a patch for this issue:
> 
>     diff --git a/numeric.c b/numeric.c.new
>     index 97ba104..c548c0f 100644
>     --- a/numeric.c
>     +++ b/numeric.c.new
>     @@ -3767,6 +3767,9 @@ Init_Numeric(void)
>          _control87(MCW_EM, MCW_EM);
>          _control87(_control87(0,0),0x1FFF);
>      #endif
>     +#if defined(__MINGW32__)
>     +    _control87(_PC_53, _MCW_PC);
>     +#endif
>          id_coerce = rb_intern("coerce");
>          id_to_i = rb_intern("to_i");
>          id_eq = rb_intern("==");

It specify fraction as 53 bit (other IEEE 754 FPU's 52 bit with economized form), but its internal exponent is still 15 bit.
For example following result should be still different (I don't tested on mingw32).

  [0x0008008000000000,0x3ff0000000000001].pack("Q2").unpack("d2").inject(&:*)

Therefore use SSE2 rather than such workaround.
----------------------------------------
Bug #8299: Minor error in float parsing
https://bugs.ruby-lang.org/issues/8299#change-38875

Author: bobjalex (Bob Alexander)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: trunk
Backport: 


I encountered a float that either parses [slightly] differently (or converts to string differently) in Ruby than it does in Python or Java. This looks like a  Ruby bug since the result "looks" incorrect.

It is easily reproduced by entering the magic number (-1.1505945E-5) into irb. It behaves the same in 2.0 and 1.9. I'm using Windows.

Below is an irb session that demonstrates. Also included are JRuby and Python trials that show better behavior.

This issue is not causing me any problems, but just in case someone there is interested in looking into it...

Bob

>ruby -v
ruby 2.0.0p0 (2013-02-24) [i386-mingw32]

>irb
irb(main):001:0> RUBY_VERSION
=> "2.0.0"
irb(main):002:0> -1.1505945E-5
=> -1.1505945000000001e-05

>ruby19 -v
ruby 1.9.3p392 (2013-02-22) [i386-mingw32]

>irb19
irb(main):001:0> RUBY_VERSION
=> "1.9.3"
irb(main):002:0> -1.1505945E-5
=> -1.1505945000000001e-05
irb(main):002:0>

>jirb
irb(main):001:0> -1.1505945E-5
=> -1.1505945e-05

>python
Python 2.7.4rc1 (default, Mar 24 2013, 14:34:32) [MSC v.1500 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> -1.1505945E-5
-1.1505945e-05
>>> repr(-1.1505945E-5)
'-1.1505945e-05'




-- 
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