Re: [Boost-bugs] [Boost C++ Libraries] #5796: Casting from float to string using "lexical_cast<string>" gives inaccurate values.

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5796: Casting from float to string using "lexical_cast<string>" gives inaccurate values.
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-08-22 15:38:32

#5796: Casting from float to string using "lexical_cast<string>" gives inaccurate
values.
-----------------------------------------------------+----------------------
  Reporter:  Mikko Partonen <mikkopartonen@…>        |       Owner:  nasonov            
      Type:  Bugs                                    |      Status:  new                
 Milestone:  To Be Determined                        |   Component:  lexical_cast       
   Version:  Boost 1.47.0                            |    Severity:  Cosmetic           
Resolution:                                          |    Keywords:  lexical_cast, float
-----------------------------------------------------+----------------------

Comment (by apolukhin):

 Replying to [ticket:5796 Mikko Partonen <mikkopartonen@…>]:
 > When casting floats to strings with lexical_cast<string>() for use in
 wxTextCtrl, the new boost versions adds a lot of nines or zeroes along
 with some random numbers at the end of the float values.
 > For example, with an older boost version, I get "0.96" and "0.8" and
 with new versions, I get "0.959999979" and "0.800000012".[[BR]] [[BR]]


 It is not a random number, it is the exact representation of float number:
 http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.16 [[BR]] [[BR]]


 > This can easily be fixed by using an older version of lexical_cast.hpp
 file. Wouldn't it be better to fix this issue, so that I wouldn't need to
 use the old version of lexical_cast.hpp? [[BR]] [[BR]]


 Think about all the people, who used an updated version of this library
 (years passed since 2005, so there will be plenty of them). They relay on
 current behavior, and such changes will break their code. [[BR]] [[BR]]
 Current behavior was chosen because more people wanted to see precise
 numbers. When you convert number "1.000000099" in most cases you would
 like to see number "1.000000099" in output, not "1".

 You can always use std::stringstream to convert numbers with specified
 precision.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5796#comment:1>
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