/* Support for order statistics.
- This data structure supplies infrastructure for higher-level statistics that
- rely on order statistics. It is a kind of "abstract base class" that is not
- useful on its own. The common pattern for using the statistics based on it
+ The kth order statistic of a statistical sample is equal to its kth-smallest
+ value. The minimum is the first order statistic and the maximum is the
+ largest. This code and data structure supplies infrastructure for
+ higher-level statistics that rely on order statistics. It is a kind of
+ "abstract base class" that is not useful on its own.
+
+ This is implemented here as a kind of "partial frequency table". The
+ order_stats_accumulate() and order_stats_accumulate_idx() functions
+ effectively generate all of the frequency table entries for the variable,
+ one by one, and pass them to the "accumulate" function, if any. They can
+ also record pairs of frequency tables entries surrounding desired target
+ cumulative weights in 'k' data structures.
+
+ Client use
+ ==========
+
+ The common pattern for clients to use statistics based on order statistics
is this:
- Create the higher-level statistic with, for example, percentile_create().
struct casereader;
struct variable;
-/*
- cc <= tc < cc_p1
+/* A pair of adjacent frequency table entries.
+
+ cc <= tc < cc_p1
*/
struct k
{
+ /* Target cumulative weight.
+ Set by the client before invoking order_stats_accumulate{,_idx}. */
double tc;
- double cc;
- double cc_p1;
- double c;
- double c_p1;
- double y;
- double y_p1;
+
+ /* Lower order statistics. */
+ double cc; /* Largest cumulative weight <= tc. */
+ double c; /* Weight for data values equal to 'y'. */
+ double y; /* Data value. */
+
+ /* Upper order statistics. */
+ double cc_p1; /* Smallest cumulative weight > tc. */
+ double c_p1; /* Weight for data values equal to 'y_p1'. */
+ double y_p1; /* Data value. */
};
/* Order statistics calculation data structure. See the comment at the top of
struct order_stats
{
struct statistic parent;
- int n_k;
- struct k *k;
- double cc;
+ void (*accumulate) (struct statistic *, const struct ccase *, double c, double cc, double y);
+
+ struct k *k;
+ size_t n_k;
};
void order_stats_accumulate_idx (struct order_stats **os, size_t n_os,