[Mplapack-devel] mpcomplex.h: lost precision in extraction of real and imag parts

[Edwin H. <ew...@st...>]

Hello,

The code for real() and imag() in "mpcomplex.h", starting from line 141:

    mpreal real()
    {
        mpreal tmp;
        tmp = mpc_realref(mpc);
        return tmp;
    }

results in lost precision if mpc has higher precision than the default used
for tmp.



For example, the following code:

#include <mpcomplex.h>
using namespace mpfr;

int main(void)
{
const mp_prec_t prec = 256;

mpreal pi(0.0, prec, MPFR_RNDN);
pi = const_pi(prec, MPFR_RNDN);
mpfr_printf("pi, mpreal:\t%.60Rf\n", mpfr_ptr(pi));

mpcomplex pi_complex(0.0, prec, prec, MPC_RNDNN);
pi_complex = pi;
mpfr_printf("pi, mpcomplex:\t%.60Rf\n", mpfr_ptr(pi_complex.real()));

return 0;
}

gives this output:

pi, mpreal: 3.141592653589793238462643383279502884197169399375105820974945
pi, mpcomplex:
3.141592653589793115997963468544185161590576171875000000000000



I believe this may be resolved without any negative side effects by this
simplification:
    mpreal real()
    {
        return mpc_realref(mpc);
    }

and likewise for imag().


Best,
Edwin