| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of Victor A. | Wagner Jr. | Sent: 10 February 2006 06:08 | To: boost@lists.boost.org; boost@lists.boost.org | Subject: Re: [boost] Boost test - displaying float point numbers | | At 15:19 2006-02-09, Paul A Bristow wrote: | >Some of you have noted (with displeasure) the ugly display | of floating point | >values from | > | >BOOST_CHECK_CLOSE | > | >for example | > | >difference between +1e-6{9.9999999999999995e-007} and | >1e-5{1.0000000000000001e-005} exceeds 2.22045e-018% | > | >The number of decimal digits used is enough to show all that can be | >significant. | > | >For 64-bit doubles it is 17 decimal digits, (for 32-bit | float 9) and for | >long double, quad_float even more. | | I don't see where you're getting your # of digits | my calculations show that it's around 14 for 64bit floating and | around 7 for 32bit | numeric_limits<>::digits10 shows 6 for float, 15 for double on vc8
From formula given by Kahan, inventor of IEEE754, so he should know!
http://http.cs.berkeley.edu/~wkahan/ieee754status/IEEE754.PDF Page 4 and this was proposed http://www2.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1822.pdf and found favour with WG14 so may well be part of TR2. Note that the number of digits required to show all possibly significant digits which I have called max_digits10 (and and as max significant digits MACROs for C N1151 to WG14 FLT_DIG 6, FLT_MAXDIG10 9 DBL_DIG 15, DBL_MAXDIG10 17 LDBL_DIG 19, LDBL_MAXDIG10 21 is 2 or 3 more than numeric_limits<>::digits10 which is effectively the numerb that are guranteed correct - after being rounded. Paul -- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB Phone and SMS text +44 1539 561830, Mobile and SMS text +44 7714 330204 mailto: pbristow@hetp.u-net.com http://www.hetp.u-net.com/index.html http://www.hetp.u-net.com/Paul%20A%20Bristow%20info.html