/* code computes a derivative using both the
   forward step and central difference algorithms */

#include <iostream>
#include <iomanip>
#include <gsl/gsl_math.h>
#include <gsl/gsl_deriv.h>

using namespace std;

double f (double x, void * params)
{
  return pow (x, 1.5);   // <----- CHANGE ME
}
     
int main ()
{
  gsl_function F;
  double result, abserr;
  
  F.function = &f; // no touch
  F.params = 0;    // no touch
  
  cout << "f(x) = x^(3/2)" << endl;
  
  gsl_deriv_central (&F, 2.0, 1e-8, &result, &abserr);
  cout << "x = 2.0" << endl;
  cout << "f'(x) = " << setprecision(11)<< result << " +/- " << abserr << endl;
  cout << "exact = " << setprecision(11) << 1.5 * sqrt(2.0) << endl << endl;
  
  gsl_deriv_forward (&F, 0.0, 1e-8, &result, &abserr);
  printf ("x = 0.0\n");
  printf ("f'(x) = %.10f +/- %.10f\n", result, abserr);
  printf ("exact = %.10f\n", 0.0);
  
  return 0;
}