Re: [boost] Incorrect values of sin and cos

17 Jan 2020

      I ran the following code :

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

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

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

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

cout << cos(9 * pi/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, 
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_piboost::multiprecision::cpp_bin_float_50())
/
std::numeric_limitsboost::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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