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binomial_pdf.m

## Copyright (C) 1995, 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:  binomial_pdf (x, n, p)
##
## For each element of x, compute the probability density function (PDF)
## at x of the binomial distribution with parameters n and p.

## Author:  KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description:  PDF of the binomial distribution

function pdf = binomial_pdf (x, n, p)
  
  if (nargin != 3)
    usage ("binomial_pdf (x, n, p)");
  endif

  [retval, x, n, p] = common_size (x, n, p);
  if (retval > 0)
    error (["binomial_pdf:  ", ...
          "x, n and p must be of common size or scalar"]);
  endif
  
  [r, c] = size (x);
  s = r * c;
  x   = reshape (x, 1, s);
  n   = reshape (n, 1, s);
  p   = reshape (p, 1, s);
  cdf = zeros (1, s);

  k = find (isnan (x) | !(n >= 0) | (n != round (n)) ...
      | !(p >= 0) | !(p <= 1));
  if any (k)
    pdf(k) = NaN * ones (1, length (k));
  endif
  
  k = find ((x >= 0) & (x <= n) & (x == round (x)) ...
      & (n == round (n)) & (p >= 0) & (p <= 1));
  if any (k)
    pdf(k) = bincoeff (n(k), x(k)) .* (p(k) .^ x(k)) ...
      .* ((1 - p(k)) .^ (n(k) - x(k)));
  endif
 
  pdf = reshape (pdf, r, c);
  
endfunction

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