[Boost-bugs] [Boost C++ Libraries] #9579: conversion error from boost cpp_rational to int

Subject: [Boost-bugs] [Boost C++ Libraries] #9579: conversion error from boost cpp_rational to int
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-01-16 00:13:08

#9579: conversion error from boost cpp_rational to int
-------------------------------------------+----------------------
 Reporter:  W Randolph Franklin <boost@…>  |      Owner:  turkanis
     Type:  Bugs                           |     Status:  new
Milestone:  To Be Determined               |  Component:  rational
  Version:  Boost 1.55.0                   |   Severity:  Problem
 Keywords:  cpp_rational conversion        |
-------------------------------------------+----------------------
 boost cpp_rational seems to convert wrong to int when the numerator and
 denominator have many digits.
 {{{
  #include <iostream>  #include <boost/multiprecision/cpp_int.hpp>

  using namespace boost::multiprecision;
  using namespace std;

  int main() {
   cpp_rational a("4561231231235/123123123123");
   std::cout << "bad convert: " << a << ' '  <<
      float(a) << ' ' << int(a) << ' ' <<
      a.convert_to<int>() << endl;
   a = (cpp_rational)"456/123";
   std::cout << "good convert: " << a << ' '  <<
      float(a) << ' ' << int(a) << ' ' <<
      a.convert_to<int>() << endl;
  }
 }}}
 The output is:
 {{{
  bad convert: 651604461605/17589017589 37.0461 -3 -3
  good convert: 152/41 3.70732 3 3
 }}}
 Also, attempts to convert cpp_rational to cpp_int fail to compile, e.g.,
 using
 {{{
  cpp_int b = static_cast<cpp_int> (a);
  cpp_int b = a.convert_to<cpp_int>();
 }}}
 What I want to happen is to divide and round down and never get it wrong
 even close to an int.

 Help? Thanks.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9579>
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