Calculate sample variance

Makefile

@@ -60,3 +60,6 @@ mtrace-test.txt: mtrace-test.trace
 
 uiowa-threads-example: uiowa-threads.o uiowa-threads-example.c
 	$(CC) $(CFLAGS) -o $@ $^
+
+av-variance: av-variance.c
+	$(CC) $(CFLAGS) -o $@ $< -lm

av-variance.c

@@ -17,6 +17,20 @@ bool isalmost(double x, double c, double epsilon) {
   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};
@@ -31,7 +45,10 @@ int main() {
 
   // XXX assert(isalmost(median(d, d_n), 5.5, 0.001));
   // XXX assert(isalmost(median(e, e_n), XXX, 0.001));
-  // XXX Variance
+
+  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