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logistic_inv.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:  logistic_inv (x)
##
## For each component of x, compute the quantile (the inverse of the
## CDF) at x of the logistic distribution.

## Author:  KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description:  Quantile function of the logistic distribution

function inv = logistic_inv (x)

  if (nargin != 1)
    usage ("logistic_inv (x)");
  endif

  [r, c] = size (x);
  s = r * c;
  x = reshape (x, 1, s);
  inv = zeros (1, s);

  k = find ((x < 0) | (x > 1) | isnan (x));
  if any (k)
    inv(k) = NaN * ones (1, length (k));
  endif

  k = find (x == 0);
  if any (k)
    inv(k) = (-Inf) * ones (1, length (k));
  endif

  k = find (x == 1);
  if any (k)
    inv(k) = Inf * ones (1, length (k));
  endif
  
  k = find ((x > 0) & (x < 1));
  if any (k)
    inv (k) = - log (1 ./ x(k) - 1);
  endif
  
  inv = reshape (inv, r, c);
  
endfunction

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