
Gaussian(3) User Contributed Perl Documentation Gaussian(3)
NAME
PDL::Gaussian  Gaussian distributions.
SYNOPSIS
$a = new PDL::Gaussian([3],[5]);
$a>set_covariance(...)
DESCRIPTION
This package provides a set of standard routines to handle sets gaussian distributions.
A new set of gaussians is initialized by
$a = new PDL::Gaussian(xdims,gdims);
Where xdims is a reference to an array containing the dimensions in the space the gaussian
is in and gdimslist is a reference to an array containing the dimensionality of the gauss
ian space. For example, after
$a = new PDL::Gaussian([2],[3,4]);
$b = new PDL::Gaussian([],[]);
The variable $a contains set of 12 (="3*4") 2Dimensional gaussians and $b is the simplest
form: one 1D gaussian. Currently, xdims may containe either zero or one dimensions due to
limitations of PDL::PP.
To set the distribution parameters, you can use the routines
$a>set_covariance($cv); # covariance matrices
$a>set_icovariance($icv); # inverse covariance matrices
$a>set_mu($mu); # centers
The dimensions of $cv and $icv must be "(@xdims,@xdims,@gdims)" and the dimensions of $mu
must be "(@xdims,@gdims)".
Alternatively you can use the routines
$cv = $a>get_covariance(); # cv = reference to covariance matrix
... # Fuzz around with cv
$a>upd_covariance(); # update
and similarly for "icovariance" (inverse covariance). The last sub call is important to
update the other parts of the object.
To get a string representation of the gaussians (most useful for debugging) use the rou
tine
$string = $a>asstr();
It is possible to calculate the probability or logarithm of probability of each of the
distributions at some points.
$a>calc_value($x,$p);
$a>calc_lnvalue($x,$p);
Here, $x must have dimensions "(ndims,...)" and $p must have dimensions "(gdimslist, ...)"
where the elipsis represents the same dimensions in both variables. It is usually advis
able to work with the logarithms of probabilities to avoid numerical problems.
It is possible to generate the parameters for the gaussians from data. The function
$a>fromweighteddata($data,$wt,$small_covariance);
where $data is of dimensions "(ndims,npoints)" and $wt is of dimensions "(npoints,gdim
slist)", analyzes the data statistically and gives a corresponding gaussian distribution.
The parameter $small_covariance is the smallest allowed covariance in any direction: if
one or more of the eigenvalues of the covariance matrix are smaller than this, they are
automatically set to $small_covariance to avoid singularities.
BUGS
Stupid interface.
Limitation to 1 xdimensions is questionable (although it's hard to imagine a case when
more is needed). Note that this does not mean that you can only have 1dimensional gaus
sians. It just means that if you want to have a 6dimensional gaussian, your xs must be
structured like (6) and not (2,3). So clumping the dimensions should make things work
able.
Also, it limits you so that even if you have one variable, you need to have the '1' dimen
sions explicitly everywhere.
Singular distributions are not handled. This should use SVD and be able to handle both in
finitely narrow and wide dimensions, preferably so that infinitely narrow dimensions can
be queried like "$a"relations()> or something like that.
The routines should, if the user requests for it, check all the dimensions of the given
arguments for reasonability.
AUTHOR
Copyright (C) 1996 Tuomas J. Lukka (lukka@fas.harvard.edu) All rights reserved. There is
no warranty. You are allowed to redistribute this software / documentation under certain
conditions. For details, see the file COPYING in the PDL distribution. If this file is
separated from the PDL distribution, the copyright notice should be included in the file.
perl v5.8.0 20000429 Gaussian(3) 
