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
401-847-4210 (voice)
401-849-1585 (fax)
roger.k.we...@saic.com
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