#include <stdio.h>
#include <gsl/gsl_rstat.h>

int
main(void)
{
  double data[5] = {17.2, 18.1, 16.5, 18.3, 12.6};
  double mean, variance, largest, smallest, sd,
         rms, sd_mean, median, skew, kurtosis;
  gsl_rstat_workspace *rstat_p = gsl_rstat_alloc();
  size_t i, n;

  /* add data to rstat accumulator */
  for (i = 0; i < 5; ++i)
    gsl_rstat_add(data[i], rstat_p);

  mean     = gsl_rstat_mean(rstat_p);
  variance = gsl_rstat_variance(rstat_p);
  largest  = gsl_rstat_max(rstat_p);
  smallest = gsl_rstat_min(rstat_p);
  median   = gsl_rstat_median(rstat_p);
  sd       = gsl_rstat_sd(rstat_p);
  sd_mean  = gsl_rstat_sd_mean(rstat_p);
  skew     = gsl_rstat_skew(rstat_p);
  rms      = gsl_rstat_rms(rstat_p);
  kurtosis = gsl_rstat_kurtosis(rstat_p);
  n        = gsl_rstat_n(rstat_p);

  printf ("The dataset is %g, %g, %g, %g, %g\n",
         data[0], data[1], data[2], data[3], data[4]);

  printf ("The sample mean is %g\n", mean);
  printf ("The estimated variance is %g\n", variance);
  printf ("The largest value is %g\n", largest);
  printf ("The smallest value is %g\n", smallest);
  printf( "The median is %g\n", median);
  printf( "The standard deviation is %g\n", sd);
  printf( "The root mean square is %g\n", rms);
  printf( "The standard devation of the mean is %g\n", sd_mean);
  printf( "The skew is %g\n", skew);
  printf( "The kurtosis %g\n", kurtosis);
  printf( "There are %zu items in the accumulator\n", n);

  gsl_rstat_reset(rstat_p);
  n = gsl_rstat_n(rstat_p);
  printf( "There are %zu items in the accumulator\n", n);

  gsl_rstat_free(rstat_p);

  return 0;
}

The dataset is 17.2, 18.1, 16.5, 18.3, 12.6
The sample mean is 16.54
The estimated variance is 5.373
The largest value is 18.3
The smallest value is 12.6
The median is 16.5
The standard deviation is 2.31797
The root mean square is 16.6694
The standard devation of the mean is 1.03663
The skew is -0.829058
The kurtosis -1.2217
There are 5 items in the accumulator
There are 0 items in the accumulator

#include <stdio.h>
#include <stdlib.h>
#include <gsl/gsl_rstat.h>
#include <gsl/gsl_statistics.h>
#include <gsl/gsl_rng.h>
#include <gsl/gsl_randist.h>
#include <gsl/gsl_sort.h>

int
main(void)
{
  const size_t N = 10000;
  double *data = malloc(N * sizeof(double));
  gsl_rstat_quantile_workspace *work_25 = gsl_rstat_quantile_alloc(0.25);
  gsl_rstat_quantile_workspace *work_50 = gsl_rstat_quantile_alloc(0.5);
  gsl_rstat_quantile_workspace *work_75 = gsl_rstat_quantile_alloc(0.75);
  gsl_rng *r = gsl_rng_alloc(gsl_rng_default);
  double exact_p25, exact_p50, exact_p75;
  double val_p25, val_p50, val_p75;
  size_t i;

  /* add data to quantile accumulators; also store data for exact
   * comparisons */
  for (i = 0; i < N; ++i)
    {
      data[i] = gsl_ran_rayleigh(r, 1.0);
      gsl_rstat_quantile_add(data[i], work_25);
      gsl_rstat_quantile_add(data[i], work_50);
      gsl_rstat_quantile_add(data[i], work_75);
    }

  /* exact values */
  gsl_sort(data, 1, N);
  exact_p25 = gsl_stats_quantile_from_sorted_data(data, 1, N, 0.25);
  exact_p50 = gsl_stats_quantile_from_sorted_data(data, 1, N, 0.5);
  exact_p75 = gsl_stats_quantile_from_sorted_data(data, 1, N, 0.75);

  /* estimated values */
  val_p25 = gsl_rstat_quantile_get(work_25);
  val_p50 = gsl_rstat_quantile_get(work_50);
  val_p75 = gsl_rstat_quantile_get(work_75);

  printf ("The dataset is %g, %g, %g, %g, %g, ...\n",
         data[0], data[1], data[2], data[3], data[4]);

  printf ("0.25 quartile: exact = %.5f, estimated = %.5f, error = %.6e\n",
          exact_p25, val_p25, (val_p25 - exact_p25) / exact_p25);
  printf ("0.50 quartile: exact = %.5f, estimated = %.5f, error = %.6e\n",
          exact_p50, val_p50, (val_p50 - exact_p50) / exact_p50);
  printf ("0.75 quartile: exact = %.5f, estimated = %.5f, error = %.6e\n",
          exact_p75, val_p75, (val_p75 - exact_p75) / exact_p75);

  gsl_rstat_quantile_free(work_25);
  gsl_rstat_quantile_free(work_50);
  gsl_rstat_quantile_free(work_75);
  gsl_rng_free(r);
  free(data);

  return 0;
}

The dataset is 0.00645272, 0.0074002, 0.0120706, 0.0207256, 0.0227282, ...
0.25 quartile: exact = 0.75766, estimated = 0.75580, error = -2.450209e-03
0.50 quartile: exact = 1.17508, estimated = 1.17438, error = -5.995912e-04
0.75 quartile: exact = 1.65347, estimated = 1.65696, error = 2.110571e-03