p_spectrum.m

《模式分析的核方法》一书中的源代码

function [result, K] = p_spectrum(s,t,p)
%P_SPECTRUM
%        -Finds the contiguous subsequence match count between strings s and t
%         by using a dynamic programming implementation,
%         where the length of the subsequence is p.
%         *(There is also a brute force implementation of this algorithm.
%           Type help p_spectrum_bf for info.)
%
%        -Simply prompting the function will return the value K(s,t), however
%         using the function as [result,K] = K(s,t) will also return the matrix K.
%
%        -The following algorithm is used:
%         K[p](sa,t) = K[p](s,t) + [Summation of i from 1 to |t|] G[p-1](s,t(1:i-1)) [t(i) == a]
%           K[p](s,t) = 0 if |s| < p  or |t| < p
%         G[p](sa, tb) = G[p-1](s,t)[a==b]
%           G[0](s,t) = 1 for all s,t
%           G[p](s,t) = 0 if |s| == 0  or |t| == 0
%         
%
%        -Example: p_spectrum('abccc','abc', 3) returns a value of 1.
%            (Note that p_spectrum('abccc','abc',3)=p_spectrum('abc','abccc',3) since K(s,t,p) = K(t,s,p) ).
%        -Example: p_spectrum('a','a', 1) returns a value of 1.
%        -Example: p_spectrum('a','b', 1) returns a value of 0.
%        -Example: p_spectrum('ab','ab', 2) returns a value of 1.
%         
%
%
%USAGE:   scalar = p_spectrum('string1','string2', p);    (where p is the length of the substring)
%
%         [scalar, matrix] = p_spectrum('string1,'string2', p);
%
%For more information, visit http://www.kernel-methods.net/

%Written and tested in Matlab 6.0, Release 12.
%Copyright 2003, Manju M. Pai 4/2003
[e-mail]
%------------------------------------------------------------------------------------------

%Obtain lengths of strings
[num_rows_s, n] = size(s);
[num_rows_t, m] = size(t);

%Initially set every matrix index to -1 to show value has not yet been found
K = repmat(-1, [n, m]);                %The main kernel
G = repmat(-1, [n, m, p]);             %The suffix kernel

%Error checking statements:
  %Make sure input vectors are horizontal.
  if (num_rows_s ~= 1 | num_rows_t ~= 1)  
     error('Error: s and t must be horizontal vectors.');
  end;
  
  %If p is less than zero or not a number, program should quit due to faulty variable input.
  if p <= 0 | ischar(p)
      error('Error: p needs to be a number greater than 0.');
  end;
  
%End of error checking

%Fill in the rest of the matrix using the function p_spectrum_kernel()
for i=1:n
    for j=1:m      
        [K(i,j), G] = p_spectrum_kernel(s(1:i), t(1:j), K, G, p);
    end;
end;

result = K(n,m);

%------------------------------------------------------------------------------------------

function [ans, G] = p_spectrum_kernel(sa, t, K, G, p)
%This function is called by p_spectrum(s,t,p).
%Type 'help p_spectrum' for a description of the program.
%

%------------------------------------------------------------------------------------------

%Obtain lengths of both strings
n = length(sa);
m = length(t);

%truncate last character of string and obtain length of new string
s = sa(1:n-1);
length_s = length(s);

%Start algorithm:
  % 1) Split main algorithm into two parts:
    % a) K(s,t)
       if (length(s) < p) | (length(t) < p)
         %This is a base case where 0 is returned if either string has length 0
         ans = 0;
       elseif( K( length(s), length(t) ) == -1 )
         % Value has not yet been calculated
         ans = p_spectrum_kernel(s, t, K, G, p);
       else
         % Value has already been calculated
         ans = K( length(s), length(t) );
       end;

    % b) Summation of G[p-1](s,t(1:i-1))[t(i) == a] for  i = 1:(length(t) - p)
    
      %this is the letter (a) that was truncated off the string
      letter = sa(n);

      %We need this 'for' loop as a cursor that iterates through the t string.
      pos_array = find(t(1:(m)) == letter);  %array which consists of all indices of t where t(i) == a
      for index = 1:length(pos_array)
        i = pos_array(index);
        length_t = length(t(1:(i-1)));
        if ( (p-1) == 0 )
          result = 1;
        elseif (length_s == 0 | length_t == 0)
          %This is a base case where 0 is returned if either string has length 0
          result = 0;
        elseif ( G( length_s, length_t, (p-1)) == -1 )
          % Value has not yet been calculated
          [result, G] = suffix_kernel(s, t(1:(i-1)), G, (p-1));
        else
          % Value has already been calculated
          result = G( length_s, length_t, (p-1));
        end;
        ans = ans + result;
      end;
            
      return
% End of algorithm



%------------------------------------------------------------------------------------------

function [ans, G] = suffix_kernel(sa, tb, G, p)
%This function is called by p_spectrum(s,t,p).
%Type 'help p_spectrum' for a description of the program.
%

%------------------------------------------------------------------------------------------


%Obtain lengths of both strings
n = length(sa);
m = length(tb);

%if last characters of both strings do not match, return 0
if ~(strcmpi( sa(n), tb(m) ) )
    ans = 0;
    return
end;

%truncate last character of string
s = sa(1:n-1);
t = tb(1:m-1);

%Obtain lengths of truncated strings
length_s = length(s);
length_t = length(t);


%Start algorithm: G(sa,tb) = (1 + lambda^2)*G-1(s,t)[a==b]
if ((p-1) == 0)
    %This is a base case where 1 is returned if G[p] = G[0]
    ans = 1;
elseif (length_s == 0) | (length_t == 0)
    %This is a base case where 0 is returned if either string has length 0
    ans = 0;
elseif( G( length_s, length_t, (p-1) ) == -1 )
    % Value has not yet been calculated
    [ans, G] = suffix_kernel(s, t, G, (p-1));
    G( length_s, length_t, (p-1)) = ans;
else
    % Value has already been calculated
    ans = G( length_s, length_t, (p-1) );
end;

return