Re: [boost] test_fp_comparisons and rounding errors

From: Beman Dawes (bdawes_at_[hidden])
Date: 2003-07-05 14:08:53

At 01:29 PM 7/5/2003, Guillaume Melquiond wrote:
 >On Sat, 5 Jul 2003, Beman Dawes wrote:
 >
 >> fpt_abs may do a unary "-" operation. I'm assuming, perhaps 
incorrectly,
 >> that is essentially subtraction and subject to a possible rounding 
error.
 >
 >I don't think there is a system where the opposite of a representable
 >number would not be a representable number. So there should be no 
rounding
 >error when changing signs. At least not with IEEE-754 floating point
 >numbers.
 >
 >Moreover, 0-x may lead to results different from -x. So changing sign is
 >essantially "not" a subtraction :-).

So where does that leave us? There must be some reason Gennadiy's code is 
producing results one or two epsilons greater than expected.

--Beman