I am new to Wolfram Language and wolfram engine. I use wolfram engine development kit to increase the precision of the output from Spherical Bessel function from WSTP in c++. Part of codes that transfer functions and parameters to the link is

int order = 1;
long double x = 15;
long double result;
WSPutFunction( lp, "N", 2);
    WSPutFunction( lp, "SphericalBesselY", 2);
        WSPutInteger( lp, order);
            WSPutReal128(lp, x);
                WSPutInteger(lp, 30);
 WSEndPacket( lp);
 WSGetReal128( lp, &result);
    printf( "%.30Lf \n ", result);

However, whatever precision number I alter, the result is the same and

  • 1
    $\begingroup$ (1) N[SphericalBesselY[2, x], 30] will return a machine-precision number if x is machine precision. I have not tried quad-precision numbers, but probably it is either unsupported or N[..] returns a quad-precision number. For your N[..] call to work, the input x should be exact or of a high-enough precision that SphericalBesselY evalutes to at least 30 digits of precision.... $\endgroup$
    – Michael E2
    Jul 16, 2020 at 13:16
  • 2
    $\begingroup$ (2) From the docs: "Note that you can conveniently exchange arbitrary-precision numbers with external programs by converting them to lists of digits in the Wolfram Language using IntegerDigits and RealDigits." It appears you're trying to read a quad-precision FP number, not an arbitrary-precision number. If N[..] works as you seem to intend, it should return an arbitrary-precision number. $\endgroup$
    – Michael E2
    Jul 16, 2020 at 13:18
  • $\begingroup$ I guess I should add (3) your question ends mid sentence. $\endgroup$
    – Michael E2
    Jul 16, 2020 at 13:19
  • 1
    $\begingroup$ Okay, my first two comments may be wrong if Real128 is supported as it is in Import/Export. Nonetheless, N[expr, prec] cannot be used to raise the precision. Also N[y, newprec] might not change the underlying point-estimate used to compute the Real128 number (if quad behaves like double). E.g. N of the Real128 value of E is N[2.718281828459045235360287471352662314358421867194`33.71535951436589, 30] resulting in 2.718281828459045235360287471352662314358421867194`30., which has exactly the same numerical value. $\endgroup$
    – Michael E2
    Jul 16, 2020 at 13:35
  • $\begingroup$ thanks, my question then should be directed to how to change the a machine precision digits into arbitrary precision digit and parse to link within WSTP. @MichaelE2 $\endgroup$
    – Wasabi
    Jul 16, 2020 at 21:35

1 Answer 1


You're feeding it with the same precision so it's hard to expect different results, I think.

In Mathematica itself:

SphericalBesselY[1, 15.] // Accuracy

will give you 17.35 [decimal] digits of accuracy. I'm not sure how WSTP treats 128bit floating numbers though

By contrast:

SphericalBesselY[1, 15.] (* -0.0399761*)
SphericalBesselY[1,15`100] (* telling it 15 is precise up to 100 digits will answer -0.0399761319533241409493242562719989970404441275224595705839764978076\
671781562996219519312380539715616 *)

So the problems seems to be with passing arbitrary precision numbers over WSTP.

Probably someone can suggest how to do it.


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