Description of slposteriori

Home > sltoolbox > stat > slposteriori.m

slposteriori

PURPOSE

SLPOSTERIORI Computes the posterioris

SYNOPSIS

function posteriori = slposteriori(condprops, priori, op)

DESCRIPTION

SLPOSTERIORI Computes the posterioris 

 $ Syntax $
   - posteriori = slposteriori(condprops, priori)
   - posteriori = slposteriori(condprops, priori, 'log')

 $ Arguments $
   - condprods:      the conditional probabilities of classes
   - priori:         the prior probabilities of classes
   - posteriori:     the resulting posterior probabilities

 $ Description $
   - posteriori = slposteriori(condprops, priori) Computes the posterior
     probability according to the given conditional probabilities of all
     samples to all classes and the priori of the classes. If the number
     of classes is C and the number of samples is n, then the size of 
     condprops should be k * n, the size of priori should be k-dim vector. 
     And the resultant posteriori matrix will be of size k * n.

   - posteriori = slposteriori(condprops, priori, 'log') where condprops
     are given by its logarithm. And the computation is based on logarithm
     in a stable manner.

 $ Remarks $
   - If priori is not specified, then they are assumed to be equal.

 $ History $
   - Created by Dahua Lin on Dec 21st, 2005
   - Modified by Dahua Lin on Apr 22, 2006
       - fix some header comments
       - fix some places to increase efficiency
   - Modified by Dahua Lin on Sep 10, 2006
       - replace sladd by sladdvec and slmul by slmulvec to increase 
         efficiency.

CROSS-REFERENCE INFORMATION

This function calls:

• sladdvec SLADDVEC adds a vector to columns or rows of a matrix
• slmulvec SLMULVEC multiplies a vector to columns or rows of a matrix

This function is called by:

• sllogistreg SLLOGISTREG Performs Multivariate Logistic Regression
• slfmm SLFMM Learns a Finite Mixture Model (FMM)

SOURCE CODE

0001 function posteriori = slposteriori(condprops, priori, op)
0002 %SLPOSTERIORI Computes the posterioris
0003 %
0004 % $ Syntax $
0005 %   - posteriori = slposteriori(condprops, priori)
0006 %   - posteriori = slposteriori(condprops, priori, 'log')
0007 %
0008 % $ Arguments $
0009 %   - condprods:      the conditional probabilities of classes
0010 %   - priori:         the prior probabilities of classes
0011 %   - posteriori:     the resulting posterior probabilities
0012 %
0013 % $ Description $
0014 %   - posteriori = slposteriori(condprops, priori) Computes the posterior
0015 %     probability according to the given conditional probabilities of all
0016 %     samples to all classes and the priori of the classes. If the number
0017 %     of classes is C and the number of samples is n, then the size of
0018 %     condprops should be k * n, the size of priori should be k-dim vector.
0019 %     And the resultant posteriori matrix will be of size k * n.
0020 %
0021 %   - posteriori = slposteriori(condprops, priori, 'log') where condprops
0022 %     are given by its logarithm. And the computation is based on logarithm
0023 %     in a stable manner.
0024 %
0025 % $ Remarks $
0026 %   - If priori is not specified, then they are assumed to be equal.
0027 %
0028 % $ History $
0029 %   - Created by Dahua Lin on Dec 21st, 2005
0030 %   - Modified by Dahua Lin on Apr 22, 2006
0031 %       - fix some header comments
0032 %       - fix some places to increase efficiency
0033 %   - Modified by Dahua Lin on Sep 10, 2006
0034 %       - replace sladd by sladdvec and slmul by slmulvec to increase
0035 %         efficiency.
0036 %
0037 
0038 %% parse and verify
0039 k = size(condprops, 1);
0040 if nargin < 2 || isempty(priori)
0041     priori = ones(k, 1) / k;
0042 else
0043     if numel(priori) ~= k
0044         error('sltoolbox:sizmismatch', ...
0045             'The size of priori is not consisitent with that of condprops');
0046     end
0047     priori = priori(:);
0048 end
0049 if nargin < 3 || isempty(op)
0050     logstyle = false;
0051 else 
0052     if strcmpi(op, 'log')
0053         logstyle = true;
0054     else
0055         error('sltoolbox:invalidoption', ...
0056             'Invalid option %s', op);
0057     end
0058 end
0059 
0060 %% compute
0061 if logstyle
0062     prods = sladdvec(condprops, log(priori), 1);
0063     prods = sladdvec(prods, -max(prods, [], 1), 2);
0064     prods = exp(prods);
0065 else
0066     prods = slmulvec(condprops, priori);
0067 end
0068 
0069 tprods = sum(prods, 1);
0070 posteriori = slmulvec(prods, 1 ./ tprods);
0071 
0072 
0073 
0074

---

Generated on Wed 20-Sep-2006 12:43:11 by m2html © 2003