interpp.c

math library from gnu

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_spline.h>

int
main (void)
{
  int N = 4;
  double x[4] = {0.00, 0.10,  0.27,  0.30};
  double y[4] = {0.15, 0.70, -0.10,  0.15}; /* Note: first = last 
                                               for periodic data */

  gsl_interp_accel *acc = gsl_interp_accel_alloc ();
  const gsl_interp_type *t = gsl_interp_cspline_periodic; 
  gsl_spline *spline = gsl_spline_alloc (t, N);

  int i; double xi, yi;

  printf ("#m=0,S=5\n");
  for (i = 0; i < N; i++)
    {
      printf ("%g %g\n", x[i], y[i]);
    }

  printf ("#m=1,S=0\n");
  gsl_spline_init (spline, x, y, N);

  for (i = 0; i <= 100; i++)
    {
      xi = (1 - i / 100.0) * x[0] + (i / 100.0) * x[N-1];
      yi = gsl_spline_eval (spline, xi, acc);
      printf ("%g %g\n", xi, yi);
    }
  
  gsl_spline_free (spline);
  gsl_interp_accel_free (acc);
  return 0;
}