Boost C++ Libraries: Ticket #6627: nonfinite_num_put formatting of 0.0 is incorrect

owner changed

John Maddock — Wed, 29 Feb 2012 08:32:31 GMT

owner changed from John Maddock to Paul A. Bristow

Paul A. Bristow — Wed, 29 Feb 2012 09:43:51 GMT

OK, looks to be a bug. Will look at this in a day or few.

Paul A. Bristow — Wed, 29 Feb 2012 16:28:16 GMT

Well this looks a bit more complicated than I feared. There seems to be a difference between sending to cout (00000) and a stringstream (00.00).

 cout << std::fixed << std::setw( 5 ) << std::setfill( '0' ) << std::setprecision( 2 ) << 0.0 << endl; // 00000
  std::stringstream astream;
  astream << std::fixed << std::setw( 5 ) << std::setfill( '0' ) << std::setprecision( 2 ) << 0.0; // 00.00

which leaves me more than a bit wary of making any precipitate changes to Johan Rade's nonfinite_num_put code :-( (Before fools step in where angels fear to tread? - again!)

I'd like you to consider if it might be safer to circumnavigate this 'feature'?

Paul

krwalker@… — Thu, 01 Mar 2012 23:05:49 GMT

Do you have those reversed?

int main() {
  std::locale::global( std::locale( std::locale(),
    new boost::math::nonfinite_num_put< char >() ) );
  writeZero( std::cout );
  std::cout << std::endl; // 00.00
  std::stringstream astream;
  writeZero( astream );
  std::cout << astream.str() << std::endl; // 00000
  return 0;
}

"Changing the global locale does not change the locales of pre-existing streams. If you want to imbue the new global locale on cout, you should call std::cout.imbue(locale()) after calling std::locale::global()." -- http://stdcxx.apache.org/doc/stdlibug/24-3.html

krwalker@… — Thu, 01 Mar 2012 23:38:40 GMT

I may have a patch that makes the FP_ZERO case only output the '-' if ( flags_ & signed_zero ). It then reduces iosb.width() by 1 and delegates to std::num_put.

attachment set

krwalker@… — Fri, 02 Mar 2012 00:05:55 GMT

attachment → nonfinite_num_put_zero_formatting.patch

Paul A. Bristow — Fri, 02 Mar 2012 10:05:11 GMT

OK - if you are keen to fix this properly, I'll try to get to look at this more closely, including writing all the several test cases to check I haven't introduced another bug in fixing this one. You could also email me privately.

attachment set

Paul A. Bristow — Fri, 02 Mar 2012 11:51:42 GMT

attachment → nonfinite_num_facets_formatting_2.hpp.patch

Patch to try to treat unsigned zero as normal value.

Paul A. Bristow — Fri, 02 Mar 2012 11:54:48 GMT

I have yet to fully understand how this code is intended to work.

You patch seems rather complicated?

Does my 2nd patch (that simply aims to treat an unsigned zero as a normal value) work for you ?

(While I write some tests).

Paul A. Bristow — Thu, 08 Mar 2012 19:16:40 GMT

Replying to pbristow:

I have now written a much larger collection of tests (though there are an almost infinite number of possible combinations of ostream options like precision, width, letf right, internal, showpos showpoint :-(

And committed a revised version (using code from KR Walker) that passes on MSVC 10 these tests with the signed_zero flag set (and also handles inf and nan as before).

I am not entirely convinced that there is really a need for a signed_zero flag at all.

Are there really any platforms/libraries still out there that do not output negative zero correctly as -0?

Feedback welcome.

Paul A. Bristow — Fri, 23 Mar 2012 09:57:36 GMT

Tests reveal that, for some combinations of options, there is a difference in output between Dinkumware STL and other STL implementations, for example:

#if defined(_CPPLIB_VER) && (_CPPLIB_VER >= 306) Dinkumware outputs "0.00"

CHECKOUT(std::showpoint << 0., "0.000000"); std::setprecision(6) CHECKOUT(std::setprecision(2) << std::showpoint << 0., "0.00");

#else others output "0.0"

CHECKOUT(std::showpoint << 0., "0.00000"); std::setprecision(6) CHECKOUT(std::setprecision(2) << std::showpoint << 0., "0.0");

#endif

I am unclear if either of these is 'wrong' according the C/C++ Standards, but this is outside control of Boost.Math code, so I propose to declare this fixed in trunk.

(There are also some STL libraries that use two exponent digits rather than three - this is standards conformant).

status, version changed; resolution set

Paul A. Bristow — Sun, 22 Apr 2012 11:53:15 GMT

status new → closed
version Boost 1.48.0 → Boost 1.50.0
resolution → fixed

Since the C++ IO Library Standard is somewhat permissive in output, and the support for signed zero is platform dependent, it is not believed possible to provide a solution which is exactly portable (producing idential output) over all platforms. It is probably wise to avoid using the signed_zero flag.