Re: [boost] Boost test - displaying float point numbers

From: Victor A. Wagner Jr. (vawjr_at_[hidden])
Date: 2006-02-10 01:08:27

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

>This is undoubtedly ugly, but has the virtue that it avoids apparently
>nonsensical displays like
>
>1.000000 != 1.000000
>
>when comparing 1.f, with the next after, one least significant bit above
>1.00000012f,
>
>which is what you get if the standard 6 decimal digits for float is used.
>
>It has another great virtue - it highlights the limitations of storing
>decimal representations in binary.
>
>There are smarter algorithms for display (which Gennadiy is considering),
>but IMO this is the C++ standard way of doing these things and I suggest
>that this is the Right Thing To Do in this case.
>
>Are there any dissenting views?
>
>Paul
>
>--
>Paul A Bristow
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Victor A. Wagner Jr.      http://rudbek.com
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