Re: Incorrect values of sin and cos

From: anshu khare (anshukhare1998_at_[hidden])
Date: 2020-01-17 12:32:34

I ran the following code :

#include <iostream>
using namespace std;
#include <boost/multiprecision/cpp_dec_float.hpp>
using boost::math::constants::pi;
using boost::multiprecision::cpp_bin_float_50;

int main()
{
cout << cos(3 * pi<cpp_bin_float_50>/2) << endl;

cout << cos(5 * pi<cpp_bin_float_50>/2) << endl;

cout << cos(7 * pi<cpp_bin_float_50>/2) << endl;

cout << cos(9 * pi<cpp_bin_float_50>/2) << endl;

return 0;
}

Output is as follows:

1.06911e - 50
1.06911e - 50
-1.06911e - 50
1.06911e - 50

Anshu

On Fri, 17 Jan, 2020, 3:29 PM John Maddock via Boost, <boost_at_[hidden]>
wrote:

>
> On 17/01/2020 01:52, anshu khare via Boost wrote:
> > On running cos(3*pi/2) , cos(5*pi/2) , cos(7*pi/2) , cos(9*pi/2)  all
> give
> > the same answer, which is less than epsilon except cos(7*pi/2) which
> gives
> > a highly negative value.
>
> Not seeing it, I'm running:
>
>     for (unsigned i = 3; i < 20; i += 2)
>     {
>        std::cout << std::setw(20) << i << std::setw(20) <<
> ceil(fabs(cos(i *
> boost::math::constants::half_pi<boost::multiprecision::cpp_bin_float_50>())
>
> /
> std::numeric_limits<boost::multiprecision::cpp_bin_float_50>::epsilon()))
> << std::endl;
>     }
>
> Which gives:
>
>                    N                 abs error / epsilon
>
>                     3                   2
>                     5                   2
>                     7                   2
>                     9                   2
>                    11                   6
>                    13                  10
>                    15                  10
>                    17                   6
>                    19                   6
>
> And if I switch to testing type double I see:
>
>                     3                   1
>                     5                   2
>                     7                   2
>                     9                   3
>                    11                  12
>                    13                   5
>                    15                  13
>                    17                   4
>                    19                  14
>
> Best, John.
>
>
>
>
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