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weibull_rnd.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:  weibull_rnd (alpha, sigma [, r, c])
##
## weibull_rnd (alpha, sigma) returns a matrix of random samples from
## the Weibull distribution with parameters alpha and sigma. The size of
## the matrix is the common size of alpha and sigma.
##
## weibull_rnd (alpha, sigma, r, c) returns an r by c matrix of random
## samples from the Weibull distribution with parameters alpha and sigma
## which must be scalar or of size r by c.
  
## Author:  KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description:  Random deviates from the Weibull distribution

function rnd = weibull_rnd (shape, scale, r, c)

  if (nargin == 4)
    if ( !(is_scalar (r) && (r > 0) && (r == round (r))) )
      error ("weibull_rnd:  r must be a positive integer");
    endif
    if ( !(is_scalar (c) && (c > 0) && (c == round (c))) )
      error ("weibull_rnd:  c must be a positive integer");
    endif
    [retval, shape, scale] = common_size (shape, scale, zeros (r, c));
    if (retval > 0)
      error (strcat("weibull_rnd:  ",
                "alpha and sigma must be scalar or of size ",
                sprintf ("%d by %d", r, c)));
    endif
  elseif (nargin == 2)
    [retval, shape, scale] = common_size (shape, scale);
    if (retval > 0)
      error (strcat("weibull_rnd:  ",
                "alpha and sigma must be of common size or scalar"));
    endif
  else
    usage ("weibull_rnd (alpha, sigma [, r, c])");
  endif

  [r, c] = size (shape);
  s = r * c;
  shape = reshape (shape, 1, s);
  scale = reshape (scale, 1, s);

  rnd = NaN * ones (1, s);
  k = find ((shape > 0) & (shape < Inf) & (scale > 0) & (scale < Inf));
  if any (k)
    rnd(k) = (scale(k) 
            .* (- log (1 - rand (1, length (k)))) .^ (1 ./ shape(k)));
  endif
  
  rnd = reshape (rnd, r, c);
  
endfunction

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