Re: [Boost-users] converting to float128 from double

3 May 2016

      Hi John,

Thanks for the response.

Here are some more snippets from my code.

#include 
#include 
#include 
#include 
#include 
#include 
#include 
extern "C"{
#include 
}
#include <iostream>
#include <sstream>
#include <vector>
#include <iterator>

        using namespace boost::multi precision;

 double stieltjes_gamma(int num, const std::vector<double> & e8, const std::vector<double> & g8, int printlevel )
{

       std::vector<float128> epoint(num), gpoint(num), bcoef(num);

       for (int i=0; i < num; i++)
                {
                epoint.at(i)=(float128(e8.at(i)));
                gpoint.at(i)=(float128(g8.at(i)));
                }

 bcoef.at(0)=0.0Q;
        for (int i = 0; i < num; i++)
                {
                bcoef.at(0)+=gpoint.at(i)
                }
       std::cout << std::setprecision(std::numeric_limits<float128>::max_digits10)  << bcoef.at(0)<< std::endl;
    return(bcoef.at(0))
}

When I compare this to fortran code that does the same I don't see the same value for bcoef.at(0).

0.0345217724606016853085032291117978629  <- from above c++ code
3.452175962187407162907246287483402E-0002   <- Fortran correctly utilising quad precision on same input vector

The input vectors in this case are 7976 so errors in precision are really quickly apparrant. If I print  the gpoint vector, it gets filled with the same garbage for additional precision from double  as is for the fortran.

Thanks,
________________________________________
From: Boost-users  on behalf of John Maddock 
Sent: Tuesday, May 3, 2016 5:06:47 PM
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] converting to float128 from double

On 03/05/2016 13:44, Cooper, Bridgette R D wrote:
...
Hi,
I have a function that takes as an argument a vector of doubles and
tries to convert it to a vector of float128.
When I accumulate the values in the float128 vector it looks like it
does double additions and only casts up at the last step.
The initial vector is defined as conststd::vector<double>
The float128 vector is defined as std::vector<float128> epoint(num)
And I try to do the casting (inside a loop) via:
epoint.at(i)=(float128(e8.at(i)));
If I do the accumulation on the original vector with no casting into a
float128, I get the same as accumulating the vector that should be of
float128 type.
How should I be doing the casting?
Sorry but you'll have to post way more code than that to see where the
mistake is.

John.
...
Thanks,
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