function X=rvgen(f, M, N, seed) 
% rvgen(f, M, N, seed)  Generates a matrix of N samples of M experiments 
%    from a given probability or cumulative function.  Each row of the  
%    matrix represent an experiment, and each column represents a sample.   
%    The seed sets the random number generator seed.  So the resulting 
%    matrix is M by N.  Trials may be duplicated by using the same seed. 
% 
%    rvgen would rather have a cumulative function. 
if iscf(f)==0 & ispf(f)==0 
   error('invalid probability function') 
end 
if ispf(f)==1 
   f=getcf(f); 
end 
x=xvals(f); 
p=pvals(f); 
rand('seed', seed); 
slope=1; 
X=zeros(M, N); 
for t=1:M, 
   for i=1:N, 
      R=rand; 
      j=round(R*length(x)+0.5); 
      if j>length(x) 
         j=length(x); 
      end 
      if R>=p(j) 
         while j<length(p) & R>=p(j) 
            if j<=length(p)-10 
               if R>p(j+10) 
                  j=j+10; 
               else 
                  j=j+1; 
               end 
            else 
               j=j+1; 
            end 
         end 
         X(t, i)=x(j); 
      else 
         j=j-1; 
         while j>0 & R<p(j+1) 
            if j>10  
               if R<p(j-9) 
                  j=j-10; 
               else 
                  j=j-1; 
               end 
            else 
               j=j-1; 
            end 
         end 
         if j<length(x)-1 & j>0 
            j=j+1; 
         elseif j==0 
            if R>p(1) 
               j=1; 
            else 
               j=0; 
            end 
         end 
         X(t, i)=x(j+1); 
      end 
   end 
end