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

On 03/05/2016 17:35, Cooper, Bridgette R D wrote:

Hi John,

Thanks for the response.

Here are some more snippets from my code.

I don't see any obvious errors, but in any case you must have extracted that from something else but it doesn't compile! Not least you can't implicitly return from a float128 to a double in return(bcoef.at(0)) So if that's compiling for you, then I suspect your declaration of bcoef. If I re-write and simplify in a slightly more C++ idiom how does it compare to your fortran code now: double stieltjes_gamma(const std::vector<double> & e8, const std::vector<double> & g8, int printlevel) { assert(e8.size() == g8.size()); // precondition? std::vector<float128> epoint(e8.begin(), e8.end()), gpoint(g8.begin(), g8.end()), bcoef(e8.size()); bcoef[0] = 0.0Q; for(int i = 0; i < e8.size(); i++) { bcoef[0] += gpoint[i]; } std::cout << std::setprecision(std::numeric_limits<float128>::max_digits10) << bcoef.at(0) << std::endl; return static_cast<double>(bcoef[0]); } And finally... note that summing at higher precision can not actually protect you from cancellation error (in any language) since the error is inherent in the input values. HTH, John.

#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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