id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
8292	[multiprecision] et_on with uBLAS become bad result	Akira Takahashi <faithandbrave@…>	John Maddock	"`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."	Bugs	closed	To Be Determined	multiprecision	Boost 1.53.0	Problem	fixed