123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125 |
- /* randist/hyperg.c
- *
- * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 James Theiler, Brian Gough
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 3 of the License, or (at
- * your option) any later version.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- * General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
- */
- #include "gsl__config.h"
- #include <math.h>
- #include "gsl_rng.h"
- #include "gsl_randist.h"
- #include "gsl_sf_gamma.h"
- /* The hypergeometric distribution has the form,
- prob(k) = choose(n1,t) choose(n2, t-k) / choose(n1+n2,t)
- where choose(a,b) = a!/(b!(a-b)!)
- n1 + n2 is the total population (tagged plus untagged)
- n1 is the tagged population
- t is the number of samples taken (without replacement)
- k is the number of tagged samples found
- */
- unsigned int
- gsl_ran_hypergeometric (const gsl_rng * r, unsigned int n1, unsigned int n2,
- unsigned int t)
- {
- const unsigned int n = n1 + n2;
- unsigned int i = 0;
- unsigned int a = n1;
- unsigned int b = n1 + n2;
- unsigned int k = 0;
- if (t > n)
- {
- t = n ;
- }
- if (t < n / 2)
- {
- for (i = 0 ; i < t ; i++)
- {
- double u = gsl_rng_uniform(r) ;
-
- if (b * u < a)
- {
- k++ ;
- if (k == n1)
- return k ;
- a-- ;
- }
- b-- ;
- }
- return k;
- }
- else
- {
- for (i = 0 ; i < n - t ; i++)
- {
- double u = gsl_rng_uniform(r) ;
-
- if (b * u < a)
- {
- k++ ;
- if (k == n1)
- return n1 - k ;
- a-- ;
- }
- b-- ;
- }
- return n1 - k;
- }
- }
- double
- gsl_ran_hypergeometric_pdf (const unsigned int k,
- const unsigned int n1,
- const unsigned int n2,
- unsigned int t)
- {
- if (t > n1 + n2)
- {
- t = n1 + n2 ;
- }
- if (k > n1 || k > t)
- {
- return 0 ;
- }
- else if (t > n2 && k + n2 < t )
- {
- return 0 ;
- }
- else
- {
- double p;
-
- double c1 = gsl_sf_lnchoose(n1,k);
- double c2 = gsl_sf_lnchoose(n2,t-k);
- double c3 = gsl_sf_lnchoose(n1+n2,t);
- p = exp(c1 + c2 - c3) ;
- return p;
- }
- }
|