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## Copyright (C) 1996, 1997 Kurt Hornik
##
## 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 2, 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 file. If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
## usage: hypergeometric_pdf (x, m, t, n)
##
## Compute the probability density function (PDF) at x of the
## hypergeometric distribution with parameters m, t, and n. This is the
## probability of obtaining x marked items when randomly drawing a
## sample of size n without replacement from a population of total size
## t containing m marked items.
##
## The arguments must be of common size or scalar.
## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description: PDF of the hypergeometric distribution
function pdf = hypergeometric_pdf (x, m, t, n)
if (nargin != 4)
usage ("hypergeometric_pdf (x, m, t, n)");
endif
[retval, x, m, t, n] = common_size (x, m, t, n);
if (retval > 0)
error (["hypergeometric_pdf: ", ...
"x, m, t, and n must be of common size or scalar"]);
endif
[r, c] = size (x);
s = r * c;
x = reshape (x, 1, s);
m = reshape (m, 1, s);
t = reshape (t, 1, s);
n = reshape (n, 1, s);
pdf = zeros * ones (1, s);
## everything in i1 gives NaN
i1 = ((m < 0) | (t < 0) | (n <= 0) | (m != round (m)) |
(t != round (t)) | (n != round (n)) | (m > t) | (n > t));
## everything in i2 gives 0 unless in i1
i2 = ((x != round (x)) | (x < 0) | (x > m) | (n < x) | (n-x > t-m));
k = find (i1);
if any (k)
pdf (k) = NaN * ones (size (k));
endif
k = find (!i1 & !i2);
if any (k)
pdf (k) = (bincoeff (m(k), x(k)) .* bincoeff (t(k)-m(k), n(k)-x(k))
./ bincoeff (t(k), n(k)));
endif
endfunction
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