Tim Peters added the comment:
If someone has a more recent version of MS's compiler, I'd be interested
to know what this does:
inc = 2.0**-43
base = 1024.0
xs = ([base + i*inc for i in range(-4, 0)] +
[base] +
[base + 2*i*inc for i in (1, 2)])
print xs
print ["%.16g" % x for x in xs]
That creates 7 distinct doubles all of which map to "1024" when
correctly rounded to 16 significant digits. And that's what the Cygwin
Python 2.5.1 (which presumably uses David Gay's correct-rounding
conversion routines from glibc) displays for the last line:
['1024', '1024', '1024', '1024', '1024', '1024', '1024']
The released Windows Python 2.5.1 displays this instead:
['1024', '1024', '1024', '1024', '1024', '1024', '1024.000000000001']
That's a pretty gross rounding error, since the exact value of the last
element is
1024.00000000000045474735088646411895751953125
and so the 16'th digit should indeed not round up to 1. It's a "pretty
gross" error because the rounded-off part isn't particularly close to a
half unit in the last (16'th) place.
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Tracker <[EMAIL PROTECTED]>
<http://bugs.python.org/issue1580>
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