c-exercises/av-variance.c

#include <stdio.h>
#include <stdbool.h>
#include <assert.h>
#include <math.h>

double mean(double set[], int set_n) {
  double sum = 0.0;
  for(int i=0; i<set_n; i++) {
    sum += set[i];
  }

  return sum/set_n;
}

bool isalmost(double x, double c, double epsilon) {
  assert(epsilon >= 0.0);
  return fabs(x-c) < epsilon;
}

double sample_variance(double set[], int set_n) {
  double m = mean(set, set_n);

  double ss = 0.0;
  for (int i=0; i<set_n; i++) {
    ss += pow((set[i]-m), 2.0);
  }

  // Divide by (n-1), as this is *sample* variance (Bessel's correction).
  ss /= (set_n-1);

  return ss;
}

int main() {
  // Some data
  double d[] = {7, 8, 5, 1, 6, 2};
  const int d_n = 6;

  double e[] = {-10, 0, 10, 11, 12, 13, 14, 8, 9, 10, 11, 10, 10};
  const int e_n = 13;

  // Some tests
  assert(isalmost(mean(d, d_n), 4.833, 0.001));
  assert(isalmost(mean(e, e_n), 8.307, 0.001));

  // XXX assert(isalmost(median(d, d_n), 5.5, 0.001));
  // XXX assert(isalmost(median(e, e_n), XXX, 0.001));

  assert(isalmost(sample_variance(d, d_n),  7.766, 0.001));
  assert(isalmost(sample_variance(e, e_n), 41.564, 0.001));

  // XXX Stddev
  // XXX Range
  // XXX Mode
  // XXX Percentile
}