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Date: Mon, 07 Mar 2011 10:44:51 -0500
From: "Roger K. Wells" <ROGER DOT K DOT WELLS AT saic DOT com>
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Subject: Re: 1.7.8: Fortran I/O rounding inaccuracy
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On 03/07/2011 04:39 AM, Thomas Henlich wrote:
> Hi,
>
> I found the following bug in cygwin 1.7.8 on Windows XP:
>
> Fortran I/O rounding truncates the result after a certain number of
> digits. The following program:
> ===
> write(*, '(f35.32)') 0.14285714285714285d0
> end
> ===
> gives this output:
>   0.14285714285714284921269000000000
> The expected output is:
>   0.14285714285714284921269268124888
>
> This is in violation of the Fortran 2008 standard which demands:
>
> ===
> 10.7.2.3.7 I/O rounding mode
>
> 2. In what follows, the term "decimal value" means the exact decimal number as
> given by the character string, while the term "internal value" means the number
> actually stored in the processor.  For example, in dealing with the decimal
> constant 0.1, the decimal value is the mathematical quantity 1/10, which has no
> exact representation in binary form.  Formatted output of real data involves
> conversion from an internal value to a decimal value; formatted input involves
> conversion from a decimal value to an internal value.
>
> 3.  When the I/O rounding mode is UP, the value resulting from conversion shall
> be the smallest representable value that is greater than or equal to the
> original value. When the I/O rounding mode is DOWN, the value resulting
> from conversion shall be the largest representable value that is less than or
> equal to the original value. ...
> ===
>
> Applied to the example this means, 0.14285714285714284921269268124888
> is the largest representable
> (with 32 decimal digits) value that is less than the original value (binary
> 1.001001001001001001001001001001001001001001001001001 * 2^-3 = decimal
> 0.1428571428571428492126926812488818...), but
> 0.14285714285714284921269000000000 is NOT.
>
> The problem seems limited to the Fortran language, because the C call
> printf("%35.32f\n", 0.14285714285714285) prints the expected result:
>   0.14285714285714284921269268124888
>
> --
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>
uname -a: CYGWIN_NT-6.1 rwells-w7 1.7.8(0.236/5/3) 2011-03-01 09:36 i686 
Cygwin

round.f:
       program round
       write(*,'(f35.32)') 0.14285714285714285d0
       end

output:
  0.14285714285714284921269268124888

did I miss something?

HTH,
roger wells



-- 
Roger Wells, P.E.
SAIC
221 Third St
Newport, RI 02840
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401-849-1585 (fax)
roger DOT k DOT wells AT saic DOT com


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