7.33 Examples

The following example demonstrates the use of the error handling form of the special functions, in this case to compute the Bessel function J_0(5.0),

     #include <stdio.h>
     #include <gsl/gsl_errno.h>
     #include <gsl/gsl_sf_bessel.h>
     
     int
     main (void)
     {
       double x = 5.0;
       gsl_sf_result result;
     
       double expected = -0.17759677131433830434739701;
       
       int status = gsl_sf_bessel_J0_e (x, &result);
     
       printf ("status  = %s\n", gsl_strerror(status));
       printf ("J0(5.0) = %.18f\n"
               "      +/- % .18f\n", 
               result.val, result.err);
       printf ("exact   = %.18f\n", expected);
       return status;
     }

Here are the results of running the program,

     $ ./a.out
     status  = success
     J0(5.0) = -0.177596771314338292 
           +/-  0.000000000000000193
     exact   = -0.177596771314338292

The next program computes the same quantity using the natural form of the function. In this case the error term result.err and return status are not accessible.

     #include <stdio.h>
     #include <gsl/gsl_sf_bessel.h>
     
     int
     main (void)
     {
       double x = 5.0;
       double expected = -0.17759677131433830434739701;
       
       double y = gsl_sf_bessel_J0 (x);
     
       printf ("J0(5.0) = %.18f\n", y);
       printf ("exact   = %.18f\n", expected);
       return 0;
     }

The results of the function are the same,

     $ ./a.out
     J0(5.0) = -0.177596771314338292
     exact   = -0.177596771314338292