samer@39: function y=groupby_ord(f,x)
samer@39: % groupby_ord - collect rows of x into equivalence classes based f
samer@4: %
samer@39: % groupby_ord :: ([[1,M]]->A), [[N,M]] -> {[K]->[[L,M]]} :- equality(A).
samer@4: %
samer@4: % IMPORTANT: this version assumes that data is ordered by f, that is,
samer@4: % only consecutive rows will be grouped together.
samer@4: 
samer@4: 	N=size(x,1);
samer@4: 	y={}; 
samer@4: 	
samer@4: 	i=1; j=1;
samer@4: 	while j<=N
samer@4: 		if i==j, z=f(x(i,:)); end
samer@37: 		if j==N || any(f(x(j+1,:))~=z)
samer@4: 			y=vertcat(y,{x(i:j,:)});
samer@4: 			i=j+1; j=i;
samer@4: 		else j=j+1; 
samer@4: 		end
samer@4: 	end
samer@4: