function mat2libsvm(X,nclass,outFname) 
% Usage: mat2libsvm(X,nclass,outFname)
%
% previously named: f_toLibsvmFormat.m 
% convert the traditional training/testing data format (X) 
% to that required by libsvm and write the results (converted  
% pattern data) to the file. 
% nclass: number of classes.  
% outFname: output (ascii) file name. 
% An example: % Traditional format (nclass = 3): 
%----------------------------------- 
% 		Features			 |	Labels 
%----------------------------------- 
% 0.101 1.2 0 3.4 4.5 | 1 0 0 % 0 0 3.2 1.9 2.7 | 0 1 0 
% 4.5 2.3765 4.6 0 9.3 | 0 0 1 
% ... 
%----------------------------------- 
% 
% LIBSVM format 
%----------------------------------- 
% Labels |		Features 
%----------------------------------- 
%   1 	1:0.101 2:1.2 4:3.4 5:4.5  
%   2 	3:3.2 4:1.9 5:2.7  
%   3 	1:4.5 2:2.3765 3:4.6 5:9.3  
% ... 
%----------------------------------- 
%
% Note that "0" entries in the feature field will not  
% indexed and listed. 
% 
% by Dan Li, 8/16/01  
% convert the label field 
% modified by Yu Hen Hu on 10/13/01
% if nclass = 0, it translate into a model file. The first column of X 
% will be the lagrange multipliers and remain at the first column

[npattern, N] = size(X); 
if nclass > 0, 
   nfeature = N-nclass;  
else
   nfeature = N-1;
end

if ~isempty(outFname)    
   fid = fopen(outFname,'wt');     
   if fid ~= -1 , % if file open is successful
      for i = 1:npattern       
         if nclass > 0, 
            Z = num2str(X(i,end-nclass+1:end)*(1:nclass)');
            for j = 1:nfeature       	
               if X(i,j)~=0          	
                  Z = [Z ' ' num2str(j) ':' num2str(X(i,j))];       	
               end       
            end        
         elseif nclass==0,
            Z = num2str(X(i,1));
            for j = 2:N,       	
               if X(i,j)~=0          	
                  Z = [Z ' ' num2str(j) ':' num2str(X(i,j))];       	
               end       
            end        
         end
         fprintf(fid,'%s\n',Z);     
      end 
      fclose(fid);
   else % report error
      error(['cannot open ' outFname '!'])
   end % if 
end