[Boost-bugs] [Boost C++ Libraries] #8292: [multiprecision] et_on with uBLAS become bad result

Subject: [Boost-bugs] [Boost C++ Libraries] #8292: [multiprecision] et_on with uBLAS become bad result
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-03-14 09:21:20

#8292: [multiprecision] et_on with uBLAS become bad result
-------------------------------------------------------+--------------------
 Reporter:  Akira Takahashi <faithandbrave@…>          |        Type:  Bugs            
   Status:  new                                        |   Milestone:  To Be Determined
Component:  None                                       |     Version:  Boost 1.53.0    
 Severity:  Problem                                    |    Keywords:                  
-------------------------------------------------------+--------------------
 `cpp_dec_float` with `et_on` with uBLAS become bad result.

 following example is basic vector processing with `et_on`:
 {{{
 #include <iostream>
 #include <boost/multiprecision/cpp_dec_float.hpp>
 #include <boost/numeric/ublas/vector.hpp>
 #include <boost/numeric/ublas/io.hpp>

 namespace mp = boost::multiprecision;
 namespace ublas = boost::numeric::ublas;

 using float_type = mp::number<mp::cpp_dec_float<50>>;
 using vec = ublas::vector<float_type>;

 vec make_vector3(float_type x, float_type y, float_type z)
 {
     vec v(3);
     v[0] = x;
     v[1] = y;
     v[2] = z;
     return v;
 }

 vec normalize(vec v)
 {
     return v / ublas::norm_2(v);
 }

 int main()
 {
     vec p = make_vector3(0.0f, 0.0f, 0.0f); // now position
     vec q = make_vector3(3.0f, 0.0f, 6.0f); // target position

     vec v = normalize(q - p);
     std::cout << "vector : " << v << std::endl;

     float_type speed = 0.1f;
     float_type accel = 0.1f;
     for (int i = 0; i < 20; ++i) {
         p += v * speed;
         speed += accel;
     }

     std::cout << "result pos : " << p << std::endl;
 }
 }}}

 result:
 {{{
 vector : [3](1,0,1)
 result pos : [3](1,0,1)
 }}}
 this result is bad. should be:
 {{{
 vector : [3](0.447214,0,0.894427)
 result pos : [3](9.39149,0,18.783)
 }}}

 following code is using `et_off`:
 {{{
 #include <iostream>
 #include <boost/multiprecision/cpp_dec_float.hpp>
 #include <boost/numeric/ublas/vector.hpp>
 #include <boost/numeric/ublas/io.hpp>

 namespace mp = boost::multiprecision;
 namespace ublas = boost::numeric::ublas;

 using float_type = mp::number<mp::cpp_dec_float<50>, mp::et_off>;
 using vec = ublas::vector<float_type>;

 vec make_vector3(float_type x, float_type y, float_type z)
 {
     vec v(3);
     v[0] = x;
     v[1] = y;
     v[2] = z;
     return v;
 }

 vec normalize(vec v)
 {
     return v / ublas::norm_2(v);
 }

 int main()
 {
     vec p = make_vector3(0.0f, 0.0f, 0.0f); // now position
     vec q = make_vector3(3.0f, 0.0f, 6.0f); // target position

     vec v = normalize(q - p);
     std::cout << "vector : " << v << std::endl;

     float_type speed = 0.1f;
     float_type accel = 0.1f;
     for (int i = 0; i < 20; ++i) {
         p += v * speed;
         speed += accel;
     }

     std::cout << "result pos : " << p << std::endl;
 }
 }}}
 result:
 {{{
 vector : [3](0.447214,0,0.894427)
 result pos : [3](9.39149,0,18.783)
 }}}
 this result is correct.

 And, associated issue is here:
 http://stackoverflow.com/questions/14948102/boost-matrix-product-for-

 multiprecision-numbers
 this is `et_on`'s issue too.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8292>
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