+
+static int
+compare_doubles (const void *a_, const void *b_)
+{
+ const double *ap = a_;
+ const double *bp = b_;
+ double a = *ap;
+ double b = *bp;
+
+ /* Sort SYSMIS to the end. */
+ return (a == b ? 0
+ : a == SYSMIS ? 1
+ : b == SYSMIS ? -1
+ : a > b ? 1 : -1);
+}
+
+double
+median (double *a, size_t n)
+{
+ /* Sort the array in-place, sorting SYSMIS to the end. */
+ qsort (a, n, sizeof *a, compare_doubles);
+
+ /* Drop SYSMIS. */
+ n = count_valid (a, n);
+
+ return (!n ? SYSMIS
+ : n % 2 ? a[n / 2]
+ : (a[n / 2 - 1] + a[n / 2]) / 2.0);
+}